USER'S GUIDE

Vaisala Transmissometer
LT31

M210667EN-C
PUBLISHED BY

Vaisala Oyj Phone (int.): +358 9 8949 1

P.O. Box 26 Fax: +358 9 8949 2227
FI-00421 Helsinki
Finland

Visit our Internet pages at http://www.vaisala.com/

© Vaisala 2011

No part of this manual may be reproduced in any form or by any means, electronic or
mechanical (including photocopying), nor may its contents be communicated to a third
party without prior written permission of the copyright holder.

The contents are subject to change without prior notice.

Please observe that this manual does not create any legally binding obligations for
Vaisala towards the customer or end user. All legally binding commitments and
agreements are included exclusively in the applicable supply contract or Conditions of
Sale.

N17728
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Table of Contents

CHAPTER 1
GENERAL INFORMATION.......................................................................... 15
About This Manual ................................................................. 15
Contents of This Manual ..................................................... 15
Version Information ............................................................. 16
Related Manuals ................................................................. 16
Feedback............................................................................. 16
Safety....................................................................................... 16
General Safety Considerations ........................................... 16
Product Related Safety Precautions ................................... 17
ESD Protection.................................................................... 19
Recycling ................................................................................ 20
Regulatory Compliances ....................................................... 21
Patent Notice .......................................................................... 21
Trademarks ............................................................................. 21
Warranty.................................................................................. 21

CHAPTER 2
PRODUCT OVERVIEW................................................................................ 23
Description and Purpose....................................................... 23
LT31 Key Features.............................................................. 23
Range Extension............................................................ 23
Auto-calibration .............................................................. 23
Auto-alignment ............................................................... 24
Window Contamination Measurement ........................... 24
Contamination Prevention.............................................. 24
Alignment Quality Control .............................................. 24
Double-mast Construction ............................................. 24
LED Light Source ........................................................... 24
Self-diagnostics.............................................................. 24
Compatibility................................................................... 25
Introduction to LT31............................................................... 25
Mast with Support Unit ........................................................ 30
Measurement Unit............................................................... 32
Interface Unit....................................................................... 34
Equipment Nomenclature...................................................... 35

CHAPTER 3
FUNCTIONAL DESCRIPTION..................................................................... 39
Operating Principles .............................................................. 39
Concept of Visibility............................................................. 39
Meteorological Optical Range ............................................. 40
Visibility Measurement ........................................................ 41
Auto-calibration ................................................................... 41
Auto-alignment .................................................................... 42

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Alignment Quality Control....................................................43

Preventing Contamination ...................................................44
Measuring Window Contamination ................................44
Hardware Descriptions ..........................................................45
Measurement Unit LTM112/212..........................................47
Optics Unit LTO112/212.................................................48
Main Transmitter Module LTL112 ..................................49
Main Receiver Module LTD112......................................50
Window Transmitter Module LTL212 .............................52
Measurement Head CPU LTC112 .................................53
Interface Unit LTI111/211....................................................55
Power Supply FSP102 ...................................................56
Master CPU LTC212 ......................................................58
Present Weather Detector Forward Scatter Sensor............59
Software Descriptions............................................................60
Master CPU .........................................................................60
Measurement CPUs ............................................................60
Optional Equipment Descriptions.........................................61
Background Luminance Sensor LM21 ................................61
TERMBOX-1200/TERMBOX-9000 .....................................62
Obstruction Light LT31OBS ................................................62
Battery Backup LTBB111 ....................................................63

CHAPTER 4
INSTALLATION............................................................................................67
Preparing Installation.............................................................67
Selecting Location ...............................................................70
Unpacking Instructions ........................................................70
Unpacking Procedure.....................................................71
Storage ...........................................................................71
Equipment Grounding and Lightning Protection .................72
Cable Selection ...................................................................73
Line Power Cable ...........................................................74
Communication Cable ....................................................76
Communication Options........................................................78
Serial Communications Settings .........................................78
Serial Transmission with RS-232 ...................................78
Serial Multipoint Transmission with RS-485...................79
Modem DMX501 (optional) .................................................81
Multipoint Modem Connection........................................85
Installation Procedure............................................................85
Foundation...........................................................................86
Dowel mounting..............................................................87
Molded Screw Mounting.................................................89
Preparing Assembly of LT31 .................................................92
On-site Receiver Installation ...............................................92
Setting up Receiver Mast to Foundation ........................94
Setting Up Weather Protection Hood to Support Unit....95
Mounting Optional LM21 ................................................98
On-site Transmitter Installation .........................................109
Setting up Transmitter Mast to Foundation ..................110
Setting up Weather Protection Hood to Support Unit ..110
Foundation Level Differences ......................................111
Tilting Transmitter Mast...........................................111
Mounting Optional Obstruction Light LT31OBS...........112

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Mounting Transmitter Measurement Unit .................... 114

Mounting PWD to Support Arm.................................... 118
Connecting Signal Lines to Transmitter Interface Unit 119
Mounting and Connecting Optional TERMBOX-9000
(Junction Box) .............................................................. 121
Connecting Power Cable to Transmitter Interface Unit 122
Finalizing On-site Receiver Assembly .............................. 123
Orientation of Support Unit .......................................... 123
Foundation Level Differences ...................................... 123
Tilting the Receiver Mast ........................................ 123
Mounting Optional Obstruction Light LT31OBS........... 124
Mounting Receiver Measurement Unit......................... 124
Connecting Signal Lines to Receiver Interface Unit .... 125
Mounting and Connecting Optional TERMBOX-1200
(Junction Box) .............................................................. 127
Connecting Communication and Power Cables .......... 128
Connecting Communication Cable to Receiver
Interface Unit........................................................... 128
Connecting Power Cable to Receiver
Interface Unit........................................................... 128
Switching on LT31 System .......................................... 128
Double-checking Transmitter Alignment...................... 131
Startup Procedure ................................................................ 132
LT Maintenance Interface ................................................. 132
Connection to Maintenance Terminal .......................... 132
Settings for Maintenance Terminal .............................. 134
Entering and Exiting Command Mode ......................... 134
OPEN Command .................................................... 135
CLOSE Command .................................................. 135
Initial Settings .................................................................... 135
Checking Operation .......................................................... 138
Final Alignment ................................................................. 139
Calibration ......................................................................... 139
Finalizing Installation........................................................... 140
Mounting Measurement Head Cover ................................ 140
Closing Interface Units ...................................................... 142
Mounting Interface Unit Covers ........................................ 142
Disassembly for Transportation ......................................... 144

CHAPTER 5
OPERATION............................................................................................... 145
LT31 Commands .................................................................. 145
Entering and Exiting Command Mode .............................. 145
OPEN Command ......................................................... 145
CLOSE Command ....................................................... 147
Available Commands ........................................................ 147
Command Completion ................................................. 147
Terminal Line Adjustment ................................................. 153
Command Polling.............................................................. 153
CRC16 Checksum ....................................................... 155
Menu-driven Operation ........................................................ 156
Submenus ......................................................................... 157
Configuration................................................................ 157
Item List .................................................................. 158
Free-form Input ....................................................... 158
System Status .............................................................. 158

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Alignment .....................................................................159
Calibration ....................................................................159
Linearity Test................................................................159
Simulation.....................................................................159
PWD Sensor.................................................................160
Service..........................................................................160
Events...........................................................................160
Message Formats .................................................................161
MESSAGE Command .......................................................162
Message 1 - LT31 Standard Message ..............................162
Status Codes................................................................164
LT31 System ...........................................................164
Message 2 - LT31 Message Including PW Data
(optional)............................................................................174
Message 3 - Status Message............................................175
Message 4 - Vaisala System Standard Message .............176
Message 5 and 6 - FD12 Emulation..................................177
Message 5 - FD12 Message 2 .....................................177
Message 6 - FD12 Message 7 .....................................178
Message 7 - MITRAS Emulation .......................................179
Message 8 - Flamingo Emulation......................................180
Message 9 - Service Raw Values .....................................181
Message Sending Modes..................................................181
Automatic Mode ...........................................................181
Polled Mode .................................................................182
METAR Messaging............................................................183
System Configuration ..........................................................184
Default Factory Settings ....................................................185
External Sensors (Optional) ..............................................186
Background Luminance Sensor LM21 .........................186
RVR System Test Commands .............................................186
Fixed Test Messages ........................................................187
Message 1....................................................................187
Message 2....................................................................187
Message 4....................................................................187
Message 5 - FD12 Message 2 Emulation ....................187
Message 6 - FD12 Message 7 Emulation ....................188
Message 7 - MITRAS Emulation..................................188
Message 8 - SKOPOGRAPH II Flamingo ....................188
Manual Simulation Message .............................................188
Simulation Sequences.......................................................189
Message 1...............................................................191
Message 2...............................................................191
Message 4...............................................................191
Message 7 (MITRAS)..............................................191
Message 8 (SKOPO 2) ...........................................191

CHAPTER 6
MAINTENANCE..........................................................................................193
Periodic Maintenance...........................................................193
Cleaning Instructions.........................................................194
Cleaning LT31 Windows and Weather Protection Hoods .194
Cleaning PWD Forward Scatter Sensor............................195
Cleaning Lenses and Hoods ........................................195
Cleaning RAINCAP® ....................................................197
Cleaning Window of Optional LM21 ..................................197

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Inspection and Cleaning Winter Dirt ................................. 198

Blower System .................................................................. 198
Checking Operation ..................................................... 199
Replacing Particle Filter ............................................... 199
Optional Heated Weather Protection Hoods..................... 200
Checking Operation ..................................................... 201
Repair Instructions............................................................... 201
Replacing Fuses ............................................................... 201
Replacing Power Supply FSP102..................................... 203
Replacing Master CPU LTC212........................................ 203
Replacing Optional Modem DMX501................................ 204
Replacing Receiver Measurement Unit LTM212 .............. 205
Replacing Transmitter Measurement Unit LTM112 .......... 206
Replacing Receiver Optics Unit LTO212 .......................... 207
Replacing Transmitter Optics Unit LTO112 ...................... 209
Replacing Measurement CPU/Window Receiver
LTC112.............................................................................. 211
Replacing Window Transmitter Module LTL212 ............... 213
Replacing Main Transmitter Module LTL112 .................... 216
Replacing Main Receiver Module LTD112 ....................... 218
Replacing Blower Unit LTB111 ......................................... 220
Replacing Battery in Case of Optional LTBB111 .............. 222
Replacing PWD Sensor .................................................... 223
Replacing optional LM21 Sensor ...................................... 224
Alignment.............................................................................. 225
Raw and Fine Alignment ................................................... 226
Fine Alignment .................................................................. 231
Calibration............................................................................. 232
Calibration Menu ............................................................... 233
Offset and Visibility Calibration ......................................... 234
Visibility Calibration ........................................................... 238
Command Line Calibration................................................ 240
Visibility Calibration ...................................................... 240
PWD Sensor.......................................................................... 241
CLEAN Command............................................................. 241
ZERO Command............................................................... 242
CAL Command.................................................................. 243
CHEC Command .............................................................. 243
Other Commands.............................................................. 244
Spare Unit Init ................................................................... 244
Full Command Set Access................................................ 244
Linearity Test ........................................................................ 245
Service Menu ........................................................................ 250
Displayed Values .............................................................. 251
Transmitter/Receiver Measurement Unit ..................... 251
Blow ........................................................................ 251
Heat......................................................................... 252
Beep........................................................................ 252
Status ...................................................................... 252
Freq (Advanced Command Level only) .................. 252
mRdc (Receiver only)
(Advanced Command Level only)........................... 252
mTint (Transmitter only)
(Advanced Command Level only)........................... 252
mTdera (Transmitter only)
(Advanced Command Level only)........................... 252

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wRdc (Advanced Command Level only).................252

wTint (Advanced Command Level only) .................252
wTdera (Advanced Command Level only)..............253
ver (Advanced Command Level only).....................253
hor (Advanced Command Level only).....................253
mRac (Receiver only)
(Advanced Command Level only) ...........................253
wRac (Advanced Command Level only).................253
PWD Sensor.................................................................253
Visi...........................................................................253
WMO1min ...............................................................253
LM Sensor ....................................................................253
BGL .........................................................................253
Commands ........................................................................254
Blower...........................................................................254
Heater...........................................................................255
Beeper ..........................................................................255
Frequency (Advanced Command Level only) ..............256
Movement (Advanced Command Level only) ..............258
Scanning (Advanced Command Level Only) ...............259
Receiver Response Time
(Advanced Command Level Only) ...............................260

CHAPTER 7
TROUBLESHOOTING................................................................................261
Status Messages ..................................................................261
Miscellaneous Problems......................................................273
Technical Support ................................................................275
Return Instructions ..............................................................276

CHAPTER 8
TECHNICAL DATA ....................................................................................277
Operational Specifications ..................................................277
Optical Specifications ..........................................................278
Electrical Specifications ......................................................279
Mechanical Specifications...................................................279
Environmental Specifications .............................................279
Electromagnetic Compatibility............................................280

APPENDIX A
OPTIONS INSTALLATION.........................................................................281
Extension Poles Installation................................................281
LTEP100/200 Extension Pole Unit ....................................281
Prerequisites......................................................................281
Installing LTEP100/200 .....................................................282
Drilling Instructions .......................................................282
Setting Up Receiver/Transmitter Mast to Foundation .......284
INDEX .........................................................................................................291

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List of Figures
Figure 1 Vaisala Transmissometer LT31 ............................................... 26
Figure 2 Receiver Unit LTR111 with LM21 Background Luminance
Sensor ...................................................................................... 27
Figure 3 Transmitter Unit LTT111 with PWD Forward Scatter Sensor .. 28
Figure 4 Measurement Head.................................................................. 29
Figure 5 Support Unit ............................................................................. 31
Figure 6 Measurement Unit .................................................................... 32
Figure 7 Measurement Unit Opened ...................................................... 33
Figure 8 Interface Unit ............................................................................ 34
Figure 9 Alignment Mechanism .............................................................. 43
Figure 10 Window Contamination Prevention Principle ........................... 44
Figure 11 Window Contamination Measurement Principle ...................... 45
Figure 12 Communication Principle.......................................................... 46
Figure 13 Measurement Unit .................................................................... 47
Figure 14 Optics Unit Transmitter LTO112 .............................................. 48
Figure 15 Optics Unit Receiver LTO212 .................................................. 49
Figure 16 Main Transmitter Module LTL112 ............................................ 49
Figure 17 Block Diagram LTL112............................................................. 50
Figure 18 Main Receiver Module LTD112................................................ 51
Figure 19 Block Diagram LTD112 ............................................................ 52
Figure 20 Window Transmitter Module LTL212 ....................................... 52
Figure 21 Block Diagram LTL212............................................................. 53
Figure 22 Measurement Head CPU LTC112 ........................................... 53
Figure 23 Block Diagram LTC112 ............................................................ 55
Figure 24 Interface Unit Receiver LTI211 ................................................ 56
Figure 25 Power Supply FSP102 ............................................................. 57
Figure 26 Master CPU LTC212 ................................................................ 58
Figure 27 PWD Forward Scatter Sensor.................................................. 59
Figure 28 Installed Background Luminance Sensor LM21 ...................... 61
Figure 29 Installed Obstruction Light LT31OBS....................................... 63
Figure 30 Installed Battery Charger Module QBR101.............................. 64
Figure 31 Installed Backup Battery........................................................... 65
Figure 32 LT31 Equipment Grounding ..................................................... 73
Figure 33 TERMBOX-1200/Junction Box for Receiver ............................ 75
Figure 34 TERMBOX-9000/Junction Box for Transmitter ........................ 75
Figure 35 Cable Grounding Instruction .................................................... 77
Figure 36 RS-232 Communication Option ............................................... 79
Figure 37 RS-485 Communication Option ............................................... 80
Figure 38 RS-485 Option.......................................................................... 81
Figure 39 Modem Wiring .......................................................................... 82
Figure 40 AC and Battery Backup Switch ................................................ 83
Figure 41 Installing DMX501 .................................................................... 84
Figure 42 Casting Concrete Foundation .................................................. 86
Figure 43 Positioning Drilling Holes.......................................................... 88
Figure 44 Foundation Screws................................................................... 89
Figure 45 LT31 Foundation Construction................................................. 91
Figure 46 Receiver Installation Parts........................................................ 93
Figure 47 Foundation Screws................................................................... 94
Figure 48 Vertical Mast Alignment............................................................ 94
Figure 49 Tightening Nuts ........................................................................ 95
Figure 50 Mounting Weather Protection Hood ......................................... 96
Figure 51 Cable Routing for Optional Heated Hood................................. 96
Figure 52 Fastening Screws of Support Unit............................................ 97
Figure 53 Aligning with Sight Hole............................................................ 98
Figure 54 LT31 with LM21 Option Installed.............................................. 99

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Figure 55 Routing LM21 Cable through Mounting Bracket ....................100

Figure 56 Routing LM21 Cable through LM21 Support Arm ..................100
Figure 57 Installing LM21 to Mounting Bracket and Attaching
Connector ...............................................................................101
Figure 58 Mounting LM21 to Support Arm..............................................102
Figure 59 Coarse Horizontal LM21 Alignment........................................103
Figure 60 Securing Support Arm ............................................................103
Figure 61 LM21 Fine Horizontal Alignment ............................................104
Figure 62 LM21 Vertical Alignment.........................................................106
Figure 63 Opening Interface Unit Box ....................................................107
Figure 64 Wiring of LM21 to Receiver Interface Unit..............................108
Figure 65 Parts for Transmitter Installation.............................................109
Figure 66 Front Nut Adjustment to Tilt Mast ...........................................111
Figure 67 LT31OBS Mounting Preparation ............................................112
Figure 68 Mounting LT31OBS ................................................................113
Figure 69 Cable Routing of LT31OBS ....................................................113
Figure 70 Cable Hole below Interface Unit .............................................114
Figure 71 Cable Comb for Securing LT31OBS Cable............................114
Figure 72 Measurement Unit Cover Screws...........................................115
Figure 73 Cutting Transportation Lock ...................................................115
Figure 74 Fastening Screws of Measurement Unit ................................116
Figure 75 Height of Measurement Unit...................................................116
Figure 76 Parallel Position of Measurement Unit ...................................117
Figure 77 Final Connector Position for Measurement Unit.....................117
Figure 78 Fastening Measurement Unit .................................................118
Figure 79 Mounting PWD........................................................................118
Figure 80 Connecting PWD ....................................................................119
Figure 81 Transmitter Unit LTT111 Wiring .............................................120
Figure 82 Grounding Cable for TERMBOX ............................................121
Figure 83 LT31 System Wiring with TERMBOX Option .........................122
Figure 84 Receiver Unit LTR111 Wiring .................................................126
Figure 85 Power Supply Switches ..........................................................129
Figure 86 Power Supply LEDs................................................................129
Figure 87 Master CPU LTC212 LEDs ....................................................130
Figure 88 Measurement Unit LEDs ........................................................131
Figure 89 External Maintenance Line Connector ...................................132
Figure 90 LT Maintenance Interface.......................................................133
Figure 91 Maintenance Line connector on LTC212 ...............................134
Figure 92 Main Menu ..............................................................................136
Figure 93 Configuration Menu ................................................................137
Figure 94 System Status.........................................................................138
Figure 95 Lock-Screws at Measurement Head Cover............................140
Figure 96 Receptacle Inside the Measurement Head Cover..................140
Figure 97 Using Lock Screws at Measurement Head Cover..................141
Figure 98 Interface Unit Locks................................................................142
Figure 99 Lower Interface Unit Cover.....................................................143
Figure 100 Hooking-in Upper Interface Unit Cover ..................................143
Figure 101 Upper Interface Unit Cover.....................................................144
Figure 102 Main Menu ..............................................................................156
Figure 103 Configuration Menu ................................................................157
Figure 104 Configuration Menu: Data Port Speed ...................................158
Figure 105 Configuration Message Interval..............................................158
Figure 106 Status Menu............................................................................159
Figure 107 Event Log................................................................................160
Figure 108 Event Log: Reduced List ........................................................161
Figure 109 Simulation Menu .....................................................................189
Figure 110 Summary of Sequence Data ..................................................190
Figure 111 LT31 Windows ........................................................................194

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Figure 112 PWD Hardware Structure....................................................... 195

Figure 113 PWD Sensor Lenses .............................................................. 196
Figure 114 Particle Filter Holder............................................................... 200
Figure 115 Particle Filter .......................................................................... 200
Figure 116 Replacing Fuses..................................................................... 202
Figure 117 LTC212................................................................................... 204
Figure 118 DMX501.................................................................................. 204
Figure 119 LTC212 with Optional Modem DMX501................................. 205
Figure 120 Receiver Measurement Unit Opened..................................... 208
Figure 121 Transmitter Measurement Unit Opened................................. 210
Figure 122 Fastening Screws of LTC112................................................. 212
Figure 123 Fastening Screws of LTL212 ................................................. 214
Figure 124 Securing Screw of LTL112 ..................................................... 217
Figure 125 Securing Screws of LTD112 .................................................. 219
Figure 126 Fastening Screws of Blower Unit ........................................... 221
Figure 127 Backup Battery ....................................................................... 223
Figure 128 Alignment Menu ..................................................................... 225
Figure 129 Moving the Axes to Mid-position ............................................ 227
Figure 130 Scanning Axes during Raw and Fine Alignment.................... 230
Figure 131 Scanning Axes during Fine Alignment ................................... 232
Figure 132 Calibration Menu .................................................................... 233
Figure 133 Optical Blocker Mounted on Receiver Support Unit............... 235
Figure 134 Offset Calibration.................................................................... 236
Figure 135 Visibility Calibration ................................................................ 237
Figure 136 Choosing MOR Reading ........................................................ 238
Figure 137 Visibility Calibration ................................................................ 239
Figure 138 Choosing MOR Reading ........................................................ 239
Figure 139 PWD Sensor Menu................................................................. 241
Figure 140 PWD Sensor Menu, CLEAN Command................................. 242
Figure 141 PWD Maintenance Interface .................................................. 245
Figure 142 Filter Holder Mounted to the Receiver Support Unit .............. 246
Figure 143 Linearity Test Menu................................................................ 246
Figure 144 Linearity Test Menu, Filter Applied......................................... 249
Figure 145 Linearity Test Menu, Reference Update ................................ 250
Figure 146 Service Menu on User Command Level ................................ 251
Figure 147 Service Menu on Advanced Command Level ........................ 251
Figure 148 Switching Off Receiver Unit/Blower ....................................... 255
Figure 149 Decreasing Main Transmitter Modulation Frequency ............ 257
Figure 150 Setting Main Transmitter Modulation Frequency to 994 Hz... 257
Figure 151 Setting Main Receiver Response Time to Fast...................... 260
Figure 152 Positioning Drilling Holes........................................................ 283
Figure 153 Frangible Mounting Rod and Foundation Screws.................. 284
Figure 154 Extension Pole Mounting........................................................ 285
Figure 155 Vertical Extension Pole Alignment ......................................... 286
Figure 156 Tightening Nuts ...................................................................... 287
Figure 157 LTEP100/200 Installation ....................................................... 288

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List of Tables
Table 1 Manual Revisions ..................................................................... 16
Table 2 Related Manuals ...................................................................... 16
Table 3 LT31 Transmissometer Nomenclature..................................... 35
Table 4 LT31 Nomenclature for the Options ......................................... 36
Table 5 LT31 Spare Parts ..................................................................... 36
Table 6 LT31 Spare Parts for Options .................................................. 37
Table 7 AC Cable Selection for LT31.................................................... 76
Table 8 Communication Cable Lengths ................................................ 78
Table 9 Default Settings ...................................................................... 136
Table 10 User Level Commands........................................................... 148
Table 11 Advanced Level Commands .................................................. 150
Table 12 Event Log Entry...................................................................... 161
Table 13 Status Codes.......................................................................... 164
Table 14 LT31 Overall Status (Byte Number 21) .................................. 164
Table 15 LT31 MOR Status (Byte Number 22) ..................................... 165
Table 16 LT31 Power Supply (Byte Number 23) .................................. 166
Table 17 LT31 Auto-calibration (Byte Number 24)................................ 166
Table 18 LT31 Window Contamination (Byte Number 25) ................... 166
Table 19 LT31 Alignment (Byte Number 26)......................................... 167
Table 20 LT31 Future Extension 1 (Byte Number 27) .......................... 167
Table 21 LT31 Future Extension 2 (Byte Number 28) .......................... 167
Table 22 LT31 Transmitter Status (Byte Number 29) ........................... 168
Table 23 LT31 Transmitter: Main Transmitter Module
(Byte Number 30) ................................................................... 168
Table 24 LT31 Transmitter: Window Transmitter Module
(Byte Number 31) ................................................................... 169
Table 25 LT31 Transmitter: Measurement CPU (Byte Number 32)...... 169
Table 26 LT31 Transmitter: Miscellaneous (Byte Number 33) ............. 170
Table 27 LT31 Receiver Status (Byte Number 34) ............................... 170
Table 28 LT31 Receiver: Master CPU Status (Byte Number 35) ......... 170
Table 29 LT31 Receiver: Main Receiver Module (Byte Number 36) .... 171
Table 30 LT31 Receiver: Window Transmitter Module
(Byte Number 37) ................................................................... 171
Table 31 LT31 Receiver: Measurement CPU (Byte Number 38).......... 172
Table 32 LT31 Receiver: Miscellaneous (Byte Number 39) ................. 172
Table 33 LT31 PWD Status (Byte Number 40) ..................................... 173
Table 34 Background Luminance Sensor LM21 (optional)
Alarm Status Information (Byte Number 54) .......................... 173
Table 35 Default Factory Parameter Settings ....................................... 186
Table 36 Events..................................................................................... 262
Table 37 Miscellaneous Problems ........................................................ 273
Table 38 LT31 Operational Specifications ............................................ 277
Table 39 LT31 Transmitter Optical Specifications ................................ 278
Table 40 LT31 Receiver Optical Specifications .................................... 278
Table 41 LT31 Electrical Specifications ................................................ 279
Table 42 LT31 Mechanical Specifications............................................. 279
Table 43 LT31 Environmental Specifications........................................ 279
Table 44 LT31 CE Compliance ............................................................. 280

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Chapter 1 ________________________________________________________ General Information

CHAPTER 1
GENERAL INFORMATION

This chapter provides general notes for the product.

About This Manual

This manual provides information for installing, operating, and
maintaining Vaisala Transmissometer LT31 (hereinafter LT31).

Contents of This Manual

This manual consists of the following chapters:

- Chapter 1, General Information, provides general notes for the

product.
- Chapter 2, Product Overview, introduces the features, advantages, and
the product nomenclature.
- Chapter 3, Functional Description, describes the functionality of the
product.
- Chapter 4, Installation, provides you with information that is intended
to help you install this product.
- Chapter 5, Operation, contains information that is needed to operate
this product.
- Chapter 6, Maintenance, provides information that is needed in basic
maintenance of the product.
- Chapter 7, Troubleshooting, describes common problems, their
probable causes and remedies, and contact information.
- Chapter 8, Technical Data, provides the technical data of the product.

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Version Information
Table 1 Manual Revisions
Manual Code Description
M210667EN-A First manual
M210667EN-B February 2010. Previous version.
M210667EN-C This manual. Updated content and figures due to
new LM21 option and software changes.

Related Manuals
Table 2 Related Manuals
Manual Code Manual Name
M210283EN LM21 User's Guide
M210541EN PWD10/20 User's Guide
M210542EN PWD12 User's Guide
M210543EN PWD22 User's Guide
M211276EN LT31 Enhanced Weather Protection Hoods
Installation Guide

Feedback
Vaisala Customer Documentation Team welcomes your comments and
suggestions on the quality and usefulness of this publication. If you find
errors or have other suggestions for improvement, please indicate the
chapter, section, and page number. You can send comments to us by e-
mail: manuals@vaisala.com.

Safety

General Safety Considerations

Throughout the manual, important safety considerations are highlighted
as follows:

WARNING Warning alerts you to a serious hazard. If you do not read and follow
instructions very carefully at this point, there is a risk of injury or even
death.

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Chapter 1 ________________________________________________________ General Information

CAUTION Caution warns you of a potential hazard. If you do not read and follow
instructions carefully at this point, the product could be damaged or
important data could be lost.

NOTE Note highlights important information on using the product.

Product Related Safety Precautions

Vaisala Transmissometer LT31 delivered to you has been tested for
safety and approved as shipped from the factory. Failure to comply with
these precautions or with specific warnings elsewhere in this manual
violates safety standards of design, manufacture, and intended use of the
instrument or equipment. Vaisala Oyj and its subsidiaries assume no
liability for the consequences of customer's failure to comply with these
requirements. Note the following precautions:

WARNING To minimize the electrical shock hazard, the Vaisala Transmissometer

LT31 chassis must be connected to a good electrical earth. The
instrument must be equipped with a three-conductor AC power cable.
Be sure that the earth wire of the cable is connected to an electrical
ground.

WARNING There is also a grounding plug at the bottom of the interface enclosure
of the LT31 Transmissometer. Good grounding with a 16 mm2 cable
must be provided. Besides improving safety, this protects the LT31
Transmissometer against lightning induced voltages.

WARNING To prevent operator injury or damage to the LT31 Transmissometer,

verify that the LINE VOLTAGE SETTING is correct BEFORE
connecting the line power. Also ensure that the line power outlet is
provided with a protective ground contact.

WARNING Do not operate the equipment in the presence of flammable gases or

fumes. Operation of any electrical instrument in such an environment
constitutes a definite safety hazard.

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WARNING Do not attempt internal service or adjustment unless another person,

capable of rendering first aid and resuscitation, is present.

WARNING Operating personnel must not remove instrument covers. Component

replacement or internal adjustments must be made by qualified
maintenance personnel. Do not replace components with the power
cable connected. Under certain conditions, dangerous voltages may
exist even with the power cable disconnected. To avoid injuries
disconnect the power and discharge all circuits before touching them.

CAUTION The component boards including CMOS microchips should be

transported and stored in conductive packages. Although new CMOS
devices are protected against overvoltage damages caused by static
electric discharge of the operator, careful handling is recommended: the
operator should be properly grounded. Avoid unnecessary handling of
component boards.

CAUTION Because of the danger of introducing additional hazards, do not modify

or substitute parts in the instrument. Contact Vaisala or its authorized
representative for repairs to ensure that safety features are maintained.

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RADIO FREQUENCY INTERFERENCY STATEMENT (USA)

The United States Federal Communications Commission (in 47 CFR

15.838) has specified that the following notice be brought to the attention
of users of this kind of product in the USA:

Federal communications commission radio frequency interference

statement

This equipment generates and uses radio frequency energy and if not
installed and used properly, that is in strict accordance with the
manufacturer's instructions, may cause interference to radio and
television reception. The Visibility Sensor is designed to provide
reasonable protection against such interference in an airport
installation. However, there is no guarantee that interference will not
occur in a particular installation. If this equipment causes interference to
radio or television reception, which can be determined by turning the
equipment off and on, the user is encouraged to try to correct the
interference by one or more of the following measures:

- reorient the receiving antenna

- relocate the device with respect to the receiver
- move the device away from the receiver

If necessary, consult the dealer or an experienced radio/television

technician for additional suggestions.

ESD Protection
Electrostatic Discharge (ESD) can cause immediate or latent damage to
electronic circuits. Vaisala products are adequately protected against
ESD for their intended use. However, it is possible to damage the product
by delivering electrostatic discharges when touching, removing, or
inserting any objects inside the equipment housing.

To make sure you are not delivering high static voltages yourself:

- Handle ESD sensitive components on a properly grounded and

protected ESD workbench. When this is not possible, ground yourself
to the equipment chassis before touching the boards. Ground yourself
with a wrist strap and a resistive connection cord. When neither of the
above is possible, touch a conductive part of the equipment chassis
with your other hand before touching the boards.
- Always hold the boards by the edges and avoid touching the
component contacts.

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Recycling
Recycle all applicable material.

To separate waste for the LT31 system, do the following:

1. Remove the optional LM21 and LT31OBS, if installed. They have

to be disposed of separately as electronics waste material.
2. Remove the measurement head covers and measurement units
LTM112 and LTM212.
3. Cut/remove all the cables from the structure. They have to be
disposed of separately as electronics waste material.
4. Remove the blowers and the junction boxes. They have to be
disposed of separately as electronics waste material.
5. Remove the complete optics units LTO112 and LTO212 from the
measurement units. They have to be disposed of separately as
electronics waste material.
6. Remove the complete mounting plate including all electrical
components from the interface units LTI111 and LTI211. They
have to be disposed of separately as electronics waste material.
7. In case TERMBOXes are in use, remove the complete mounting
plate including all electrical components from the boxes. They have
to be disposed of separately as electronics waste material.

After this strip down work is carried out, the LT31 system is prepared for
material recycling:

- The upper and lower interface unit covers and the air ducts of the
measurement units are plastic parts and fully recyclable (>ASA<).
- The interface unit enclosures LTI111 and LTI211 (to be removed) are
stainless steel parts and fully recyclable.
- The remaining mechanical structure is exclusively manufactured from
aluminum and fully recyclable.

Dispose of batteries and the unit according to statutory regulations.

Do not dispose of with regular household refuse.

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Regulatory Compliances
For information about the performance and environmental test standards
that Transmissometer LT31 complies with, refer to Chapter 8, Technical
Data on page 277.

Patent Notice
Major aspects of Transmissometer LT31 are protected by associated
patents.

Trademarks
Microsoft®, Windows®, and Windows NT® are registered trademarks
of Microsoft Corporation in the United States and/or other countries.

Warranty
For certain products Vaisala normally gives a limited one-year warranty.
Please observe that any such warranty may not be valid in case of
damage due to normal wear and tear, exceptional operating conditions,
negligent handling or installation, or unauthorized modifications. Please
see the applicable supply contract or Conditions of Sale for details of the
warranty for each product.

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CHAPTER 2
PRODUCT OVERVIEW

This chapter introduces the features, advantages, and the product

nomenclature.

Description and Purpose

Vaisala Transmissometer LT31 requires a 115/230 VAC power supply
and an RS-232 or RS-485 cable, or a single pair modem line for
communication. LT31 sends visibility and status messages to a remote
display unit or data processing system.

A set of built-in commands and test routines are provided for the
configuration and monitoring of the multiple LT31 functions. During
assembly and maintenance, a laptop with a VT100 emulation terminal
program is needed for checking the system parameters and changing their
values.

LT31 Key Features

The key features of LT31 are listed in this section.

Range Extension
The signal conditioning and A/D converter technology specific to
Transmissometer LT31 allows coverage for the whole runway visual
range (RVR), as well the range required for aeronautical visibility with a
single baseline system.

Auto-calibration
A forward scatter sensor is an integral part of the LT31 system. It is used
to fully calibrate the transmittance measurement automatically. This
allows compensation for window contamination and misalignment in the
medium and long term.

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Auto-alignment
A precise alignment mechanism inside the measurement unit is used to
carry out the fine alignment of the LT31 system automatically.

Window Contamination Measurement

Separate transmitters and receivers in combination with a V-shaped
window arrangement are used to measure the real window transparency.
This enables compensation for the window contamination in short term
and detection of possible blocking situations.

Contamination Prevention
Air stream driven by a blower creates an air curtain in front of the LT31
windows to reduce the amount of window contamination caused by
precipitation.

Alignment Quality Control

Based on the auto-calibration and window contamination measurement,
the alignment quality of the transmissometer is permanently observed
and reported in the standard serial data message of LT31.

Double-mast Construction
The double-mast construction prevents the optical parts of the
transmissometer from being influenced by solar radiation or wind.

LED Light Source

A modern, white, high-power LED light source is used for the main
transmitter. The spectral response of the LT31 main receiver is adapted
to the spectral sensitivity of the human eye, which fulfills the related
international requirements.

Self-diagnostics
A comprehensive self-diagnostics leads to detailed status information for
all functional units in the standard serial data message of LT31.

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Compatibility
LT31 is compatible to the mounting hole pattern of Vaisala MITRAS and
Impulsphysik SKOPOGRAPH. The emulation of related serial data
messages is also a standard feature.

Introduction to LT31
The Transmissometer LT31 measurement system consists of one LT31
transmitter unit (which includes the transmissometer light transmitter)
and one LT31 receiver unit (which incorporates the transmissometer light
receiver). Figure 1 on page 26 shows the main parts of the Vaisala
Transmissometer LT31.

VAISALA _______________________________________________________________________ 25
User's Guide ______________________________________________________________________

0401-016

Figure 1 Vaisala Transmissometer LT31

The following numbers refer to Figure 1 above.

1 = LT31 receiver unit
2 = Receiver measurement head
3 = Mast
4 = Receiver interface unit
5 = LT31 transmitter unit
6 = PWD forward scatter sensor
7 = Transmitter measurement head
8 = Mast
9 = Transmitter interface unit

Receiver Unit LTR111 is shown in Figure 2 on page 27 and Transmitter

Unit LTT111 in Figure 3 on page 28.

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1104-087

Figure 2 Receiver Unit LTR111 with LM21 Background

Luminance Sensor

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User's Guide ______________________________________________________________________

1104-088

Figure 3 Transmitter Unit LTT111 with PWD Forward Scatter

Sensor

Both these units contain a similar mast construction with an inner and
outer mast tube. The measurement unit of the transmitter and the receiver
is mounted to the inner mast tube. All other parts of the measurement
head are attached to the Support Unit which is mounted to the outer mast
tube. The parts can be seen in Figure 4 on page 29.

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0401-020

Figure 4 Measurement Head

The following numbers refer to Figure 4 above.

1 = Measurement head cover
2 = Measurement unit
3 = Support unit
4 = Outer mast tube

This special setup protects the optical parts which are located inside the
measurement unit, from mechanical influences due to solar radiation or
wind load. This results in optimized alignment stability.

The transmitter and receiver units are equipped with an interface unit that
contains the power supply and all needed terminals. In case of the
receiver interface unit, the master CPU is installed additionally.

Furthermore, a PWD forward scatter sensor is an integral part of the

measurement system. This sensor is mounted to the measurement head of
the LT31 transmitter unit.

The LT31 transmitter unit consists of the following parts:

- Mast
- Transmitter interface unit including power supply and terminals

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User's Guide ______________________________________________________________________

- Support unit including the blower unit, weather protection hood,

support arm, measurement head cover, and an optional obstruction
light
- Transmitter measurement unit includes:
- Enclosure with a window heater and an enclosure cover with an
air duct
- Transmitter optics unit with an alignment mechanism,
measurement CPU (including the window receiver), main
transmitter module, and window transmitter module
- PWD forward scatter sensor

The LT31 receiver unit consists of the following parts:

- Mast
- Transmitter interface unit including power supply, terminals, and
master CPU
- Support unit including the blower unit, weather protection hood,
support arm, cover, and an optional obstruction light
- Receiver measurement unit includes:
- Enclosure with window heater and enclosure cover with air duct
- Receiver optics unit with an alignment mechanism,
measurement CPU (including the window receiver), main
receiver module, and window transmitter module
- Vaisala Background Luminance Sensor LM21 (optional)

Mast with Support Unit

The LT31 mast has a double-tube construction to prevent alignment
quality loss caused by single-sided solar radiation and wind load. The
measurement unit is mounted to the inner mast tube. All other parts are
mounted to the outer mast tube.

The frangibility of LT31 is achieved by using special mounting bolts

equipped with a predetermined breaking point.

All connection cables between the measurement head and the interface
unit are routed between the inner and the outer mast tube.

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The parts of the support unit are shown in Figure 5 below.

0401-021

Figure 5 Support Unit

The following numbers refer to Figure 5 above.

1 = Blower
2 = Support arm
3 = Inner mast tube
4 = Weather protection hood
5 = Mounting frame
6 = Outer mast tube

The support unit is mounted to the outer mast tube of the LT31 mast
construction. It consists of the mounting frame, weather protection hood,
support arm, and blower.

The only difference between the support unit of the LT31 transmitter unit
and the LT31 receiver is that the support arm of the transmitter is
equipped with a mounting flange for the PWD forward scatter sensor.

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Measurement Unit
The measurement unit (see Figure 6 below) consists of the following
parts:

- Measurement head enclosure with a cover and air duct

- Window heater
- Optics unit with
- Measurement CPU board (including a window receiver)
- Window transmitter module
- Main transmitter module for the transmitter measurement head
or main receiver module for the receiver measurement head
- Optics tube
- Alignment mechanism

0401-022

Figure 6 Measurement Unit

The following numbers refer to Figure 6 above.

1 = Optics unit
2 = Enclosure
3 = Windows

For the parts inside the measurement unit, see Figure 7 on page 33.

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0401-023

Figure 7 Measurement Unit Opened

The following numbers below refer to Figure 7 above.

1 = Window heater
2 = Window transmitter module
3 = Optics unit
4 = Optics tube
5 = Measurement CPU
6 = Mounting position for the main transmitter module
7 = Main receiver module

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User's Guide ______________________________________________________________________

Interface Unit
The interface unit is attached to the mast construction. An optional
junction box with a protection circuitry for rough overvoltage can be
mounted below the interface unit. The weather protection cover of the
interface unit is split into two pieces. The lower part covers the optional
junction box and the upper part protects the interface unit electronics
box.

The interface unit contains all the needed terminals and power supply. In
the LT31 receiver, the interface unit also contains the master CPU which
controls the entire measurement system.

If the battery backup option is installed, the related components are also
located inside the interface unit.

Figure 8 below shows an opened interface unit.

0806-035

Figure 8 Interface Unit

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Equipment Nomenclature
Table 3 LT31 Transmissometer Nomenclature
Code Common Name Description
LTT111 Transmitter Unit
LTM112 Measurement unit Enclosure with windows, heating, optics
transmitter unit and air duct
LTO112 Optics unit Incorporates optics tube, alignment
transmitter mechanism and all electronic modules
LTC112 Measurement CPU
LTL112 Main transmitter module
LTL212 Window transmitter module
LTS111 Support unit Incorporates hoods, blower and
transmitter measurement head terminal box
LTB111-115 Blower unit 115 VAC version
LTB111-230 Blower unit 230 VAC version
LTI111 Interface unit Incorporates the power supply
transmitter
FSP102 Power supply
LTAM250 LT31 mast
PWD22 Forward Scatter Sensor
LTR111 Receiver Unit
LTM212 Measurement unit Enclosure with windows, heating, optics
receiver unit and air duct
LTO212 Optics unit Incorporates optics tube, alignment
transmitter mechanism and all electronic modules
LTC112 Measurement CPU
LTD112 Main receiver module
LTL212 Window transmitter module
LTS211 Support unit Incorporates hoods, blower and
receiver measurement head terminal box
LTB111-115 Blower unit 115 VAC version
LTB111-230 Blower unit 230 VAC version
LTI211 Interface unit Incorporates the power supply and the
receiver Master CPU
FSP102 Power supply
LTC212 Master CPU
LTAM250 LT31 mast

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Table 4 LT31 Nomenclature for the Options

Code Common Name Description
LTMK111 Mounting kit To be used when using an existing
concrete pad
LTMK211 Mounting kit with extended To be used when casting the screws
foundation screws into new pads
DMX501 Modem For remote communication (leased line)
LM21LT Background Luminance LM21 sensor with LT31 specific support
Sensor arm, mounting bracket and cable
LTBB111 Battery Backup QBR101 battery charger and battery
2Ah
LT31OBS Obstruction Light
LTTB111 TERMBOX option Junction box, AC, signal connections,
and enhanced lightning protection
LTOF111 Optical Filter Set Filter holder device with set of optical
filters for LT31 linearity check
LTSH111 Weather protection hood Standard weather protection hood
LTSH211 Weather protection hood Standard weather protection hood for
with PWD hood heaters cold climates with snow and frost
LTEH211-115 Enhanced weather Enhanced weather protection hood for
protection hood with cold, demanding climates with snow or
heating (115VAC) ice storms
LTEH211-230 Enhanced weather Enhanced weather protection hood for
protection hood with cold, demanding climates with snow or
heating (230VAC) ice storms

Table 5 LT31 Spare Parts

Code Common Name Description
LTM112 Transmitter measurement Enclosure with windows, heating, optics
unit unit, and air duct.
LTM212 Receiver measurement Enclosure with windows, heating, optics
unit unit, and air duct.
LTO112 Transmitter optics unit Incorporates optics tube, alignment
mechanism, and all electronic modules.
LTO212 Receiver optics unit Incorporates optics tube, alignment
mechanism, and all electronic modules.
LTC112 Measurement CPU
LTL112 Main transmitter module
LTD112 Main receiver module
LTL212 Window transmitter module
LTC212SP Master CPU
LTC212PWSP Master CPU Including PW-data option
FSP102 Power supply
LTB111-115SP Blower unit 115 VAC
version
LTB111-230SP Blower unit 230 VAC
version
214686 Blower particle filter
PWD22 Forward scatter sensor Complete spare unit
PWD spare Refer to PWD22 User's Guide
parts
QMZ101 Maintenance cable

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Table 6 LT31 Spare Parts for Options

Code Common Name Description
DMX501 Modem
LM21F Background luminance Complete spare unit
sensor
LM21 spare Refer to LM21 User's Guide
parts
OBS24DSSP Obstruction light One spare unit
TERMBOX- Junction box receiver
1200
TERMBOX- Junction box transmitter
9000
QBR101BSP Battery charger One spare unit
4592SP Backup battery One spare unit

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CHAPTER 3
FUNCTIONAL DESCRIPTION

This chapter describes the functionality of the Vaisala Transmissometer

LT31.

Operating Principles
This section describes the basic principles behind the operation of the
Vaisala Transmissometer LT31. The most direct and accurate method for
assessing visibility is the measurement of transmittance. Vaisala
Transmissometer LT31 directly measures the atmospheric transmission
between the light transmitter and the light receiver.

Concept of Visibility
Visibility is a complex psycho-physical phenomenon, closely tied to the
factors involving human sight. The estimation of visibility is subject to
variations in individual perceptive and interpretative abilities, as well as
the light source characteristics and transmission factors. Any visual
estimation of visibility is therefore subjective.

Human observations of visibility are more difficult to define and control

in daylight, as human observation is strongly dependent upon the
selection of lighted visibility markers. It is possible, however, to define
and estimate night-time visibility in terms of equivalent day-time
visibility to ensure that no artificial changes occur in the estimate at dawn
and at twilight. Although this approach has the advantage of permitting
the use of instrumental means to measure visibility, it does not always
meet the requirements of some users.

The concept of visibility is mostly used in meteorology in the following

ways:

- As one of the parameters identifying air-mass characteristics,

especially for the needs of synoptic meteorology and climatology.
For this purpose, visibility must be representative of the optical state
of the atmosphere.

VAISALA _______________________________________________________________________ 39
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- As an operational parameter which corresponds to specific criteria for

special applications.
In this case, the concept of visibility is applied directly in terms of the
distance of visibility.

To simplify this problem and to enable quantitative physical

measurement, the factors involved in estimating visibility distance can be
separated as follows:

- Photometric and dimensional characteristics of the object which are,

or should be, perceived.
- Conditions of visual perception, including the effects of extraneous
lightning and observer location.
- Optical state of the atmosphere between the object and the observer.

The photometric and dimensional characteristics of visibility objects are

readily defined and measured in specific instances. The conditions of
visual perception have been investigated, and statistical quantitative
evidence of these factors is available.

The above factors include the visual threshold of illuminance (εt) and the
threshold value of the luminance contrast.

The optical state of the atmosphere depends directly on meteorological

conditions, its basic parameter being the visual extinction coefficient

Of the factors described above, only the optical state of the atmosphere
directly depends on the meteorological conditions. It is, therefore,
important that this basic visibility parameter express the optical state of
the atmosphere objectively, and not be influenced by extra-
meteorological conditions.

This objectively-expressed visibility parameter forms a basis for the

concept of visibility.

Meteorological Optical Range

The meteorological optical range (MOR) is a conveniently derived
parameter. MOR assumes that in practical circumstances the eye can only
recognize patterns at a luminance contrast threshold of 5 per cent. From
MOR, one can infer specialized visibility parameters. Conversely, MOR
can be estimated from particular visibility observations.

Since the World Meteorological Organization (WMO) has defined

"visual range" in units of the distance within which a black object can be
detected and recognized on the horizon in daylight, visual range as
defined by WMO is calculated using a term developed by Koschmieder
for the visual range of a black object against the horizon-level sky.

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In terms of transmittance, this can be written as follows:

ln (t)
VN = xB
ln(T)

where:
VN = So-called standard visibility.
t = The contrast threshold, still just perceivable by the observer.
T = Transmittance, meaning the section of the incoming light
which remains within a focused beam after having passed the
distance B.
B = Distance.

The WMO recommends the value 0.05 for t. The standard visibility VN
is called meteorological optical range (MOR) for this case:

ln (0.05)
MOR = Bx
ln (T)

Visibility Measurement
The LT31 directly measures the atmospheric transmission between the
light transmitter and the light receiver. It makes an assessment of the
mean extinction coefficient, including both the scattering and absorption
contributions to the measurement. The LT31 provides a reliable method
for assessing visibility or MOR.

MOR values can be further used to assess the runway visual range
(RVR).

The transmittance is measured using an effective baseline which is the

distance between the light transmitter protective window surface and
light receiver protective window. (For information about the
measurement unit's windows, see sections Introduction to LT31 on page
25 and Measurement Unit on page 32.)

Auto-calibration
The calibration of a transmissometer is traditionally based on human
observation.

Vaisala Transmissometer LT31 does no longer need to be calibrated

manually. LT31 uses an integrated forward scatter sensor of the Vaisala
PWD series to fully calibrate the system automatically.

VAISALA _______________________________________________________________________ 41
User's Guide ______________________________________________________________________

Even though transmissometers are known as the most accurate visibility

sensors for the low and medium visibility ranges, the advantage of using
a forward scatter sensor is that there is no accuracy limit to the
measurement principle for high visibility readings. Furthermore, window
dirt contamination has only a minor influence on the accuracy.

The LT31 uses the auto-calibration method patented by Vaisala in order

to utilize the advantages of the forward scatter sensor measurement
principle to calibrate the transmissometer measurement. In practice the
PWD forward scatter sensor is used to indicate auto-calibration
situations, which are high visibility situations were a calibration factor is
calculated and applied to the transmissometer measurement.

An auto-calibration situation is recognized when all of the following

conditions apply:

- The forward scatter sensor detects a visibility situation with visibility

readings above the auto-calibration threshold, typically 10 km.
- The atmospheric situation is homogenous, meaning that the deviation
of the visibility reading is below a certain threshold for the past
observation period.
- No precipitation.
- No hardware failure detected in the transmissometer and forward
scatter sensor system.

The integration of the PWD forward scatter sensor into the LT31
transmissometer system enables auto-calibration at the installation site.
The transmittance measurement is recalibrated whenever the system
detects an auto-calibration situation.

The auto-calibration functionality compensates for all medium and long-

term drift effects due to window contamination and quality imperfections
of alignment.

Auto-alignment
The alignment quality of a transmissometer can lead to reduced
measurement performance. The fine alignment of classic
transmissometer products, in particular, is sometimes difficult.

A decreased quality in alignment due, for example, to a foundation drift,

leads to reduced receiver signal. The amount of signal loss creates
visibility reading deviations similar to the effect of window
contamination.

The Transmissometer LT31 includes an automatic fine-alignment

mechanism. To enable automatic fine-alignment, the transmitter as well

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Chapter 3 ______________________________________________________ Functional Description

as the receiver optics tube of the Vaisala Transmissometer LT31 are

mounted in a cardanic frame so that the optics tube can be moved around
the lens position.

When the auto-alignment feature is activated, the optics tubes are

automatically moved in horizontal or vertical directions with gear motor
driven eccentric elements, while a spring is used to pull the optics tube
toward the eccentric elements. The mechanical position is monitored
using variable resistors. See Figure 9 below.

0401-026

Figure 9 Alignment Mechanism

The user must manually initiate the auto-alignment sequence after the
installation, when the realignment is requested or after maintenance
activities. For more information, see section Alignment on page 159.

Alignment Quality Control

The Transmissometer LT31 includes an integrated alignment quality
monitoring mechanism which is based on auto-calibration and the
window contamination measurement.

In general, a reduction of the alignment quality as well as increased

window contamination lead to the same effect, a decreased
transmissometer receiver signal strength. In the Transmissometer LT31,
the auto-calibration feature delivers the information about the total signal
loss since the initial calibration and in parallel, the window
contamination measurement indicates the signal reduction due to
contamination.

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Based on the information the auto-calibration feature delivers, the

alignment quality control algorithm evaluates the signal loss due to
reduced alignment quality whenever an auto-calibration situation occurs.

To a certain degree, the signal loss due to reduced alignment quality is

compensated for by the auto-calibration mechanism. The status of the
alignment quality evaluation is given in the frame of the LT31 serial data
message and a realignment request is generated in case the alignment
quality loss is too high. For more information, see section Message
Formats on page 161.

Preventing Contamination
In general, precipitation increases the window contamination when the
precipitation particles reach the transmissometer windows.

The long and narrow weather protection hoods of LT31 reduce window
contamination. However, it is possible that wind-driven precipitation
particles reach the windows and cause increased contamination.
Therefore, a powerful blower is used in combination with a specially
shaped air duct (mounted on top of the measurement unit) to create an air
curtain in front of the windows. This principle is described in Figure 10
below.

0810-075

Figure 10 Window Contamination Prevention Principle

The following numbers refer to Figure 10 above.

1 = Air stream
2 = Precipitation particle

As the air stream is directed downwards, wind-driven particles are forced

to change their direction.

Measuring Window Contamination

LT31 provides a unique window contamination measurement for the
windows of the transmitter and receiver measurement units.

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A separate window transmitter module emits a light beam which crosses

the V-shaped windows of the instrument. Using a specially shaped part
of the measurement unit enclosure as a reflecting surface, the light beam
is directed to the corresponding window receiver optics and electronics
which are part of the measurement CPU board. This principle is
described in Figure 11 below.

LT31 uses the measurement CPU board’s measurement for the

following:

- Measuring the real transparency of the windows.

- Compensating for window contamination in the short term.
- Detecting possible blocking situations.
- Creating related warning/error messages.

0401-028

Figure 11 Window Contamination Measurement Principle

The following numbers refer to Figure 11 above.

1 = Reflecting surface
2 = Windows
3 = Window transmitter
4 = Window receiver
5 = Main board

Hardware Descriptions
The Vaisala Transmissometer LT31 system contains the following
functional units:

VAISALA _______________________________________________________________________ 45
User's Guide ______________________________________________________________________

- Transmitter interface unit (power supply and terminals)

- Receiver interface unit (power supply and terminals, master CPU)
- Transmitter measurement unit (measurement CPU, main transmitter
module, and window transmitter module)
- Receiver measurement unit (measurement CPU, main receiver
module, and window transmitter module)
- PWD forward scatter sensor
- Background luminance sensor LM21 (optional)

At least four of the intelligent units of the measurement system (master

CPU, transmitter measurement CPU, receiver measurement CPU, PWD)
are connected by the RS-485, the LT31 internal communication bus
(called the module bus). Furthermore, the modulation clock of the main
transmitter module is transmitted to the receiver measurement unit using
RS-422 technology.

Figure 12 below gives an overview of the location of the functional units

and how they are connected.

0401-029

Figure 12 Communication Principle

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Measurement Unit LTM112/212

The measurement unit of the LT31 transmitter unit is named LTM112
and consists of the following:

- Measurement enclosure with cover and air duct

- Window heater
- Transmitter optics unit (main transmitter module, window transmitter
module, and measurement CPU board including window receiver)

The measurement unit of the LT31 receiver unit is named LTM212 and
consists of the following:

- Measurement enclosure with cover and air duct

- Window heater
- Optics unit with optics tube, alignment mechanism, main receiver
module, window transmitter module, and measurement CPU board
(including window receiver)

Measurement unit is shown in Figure 13 below.

0912-231

Figure 13 Measurement Unit

The following numbers refer to Figure 13 above.

1 = Air duct
2 = Measurement enclosure
3 = Fastening screws

VAISALA _______________________________________________________________________ 47
User's Guide ______________________________________________________________________

Optics Unit LTO112/212

The optics unit of the LT31 transmitter is named LTO112 (see Figure 14
below).

1104-089

Figure 14 Optics Unit Transmitter LTO112

The following numbers refer to Figure 14 above:

1 = Mounting chassis with an alignment mechanism, optics tube
and optics suspension
2 = Window transmitter module
3 = Measurement CPU (including window receiver)
4 = Main transmitter module

The optics unit of the LT31 receiver is named LTO212 (see Figure 15 on
page 49).

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1104-090

Figure 15 Optics Unit Receiver LTO212

The following numbers refer to Figure 15 above:

1 = Mounting chassis with an alignment mechanism, optics tube
and optics suspension
2 = Window transmitter module
3 = Measurement CPU (including window receiver)
4 = Main receiver module

Main Transmitter Module LTL112

The main transmitter module LTL112 is mounted to the optics tube of
the optics unit and is secured by a screw. It is connected to the
measurement CPU using two connectors. The main transmitter module is
shown in Figure 16 below.

1104-091

Figure 16 Main Transmitter Module LTL112

The main transmitter module contains a white, high-power LED as a

light source and an optical diffuser foil for homogenous light distribution.

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Under the control of the measurement CPU, the light source is modulated
with a frequency of approximately 1 kHz. A monitor photodiode
measures the emitted light intensity and the main transmitter module uses
a closed loop regulator in combination with a LED driver to achieve a
normalized, constant transmitter intensity.

The intensity signal and a derating signal, proportional to the applied

LED forward current, are connected to the ADC of the measurement
CPU for self-diagnostics.

The main transmitter module is also equipped with a temperature

measurement and EEPROM. These components are connected to the
measurement CPU through the IIC-Bus interface. Individual calibration
parameters are stored in the EEPROM and are used to adapt individual
settings, especially after module exchange.

The basic principle of LTL112 is shown in Figure 17 below.

CPU
Derating

Transmitter Regulator
LED CPU
+Driver Transmitter Clock

Intensity
Measurement CPU
Transmitter Intensity

Temperature
CPU
Measurement

EEPROM
CPU
0401-034

Figure 17 Block Diagram LTL112

Main Receiver Module LTD112

The main receiver module LTD112 (see Figure 18 on page 51) is
mounted to the optics tube of the optics unit and is secured by a screw. It
is connected to the measurement CPU using two connectors.
Additionally, a BNC connection is used to forward the main receiver
signal to the measurement CPU.

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1104-092

Figure 18 Main Receiver Module LTD112

The main receiver module uses a PIN photodiode as a light receiver. An

optical filter is mounted in front of the photodiode to adapt the spectral
receiver sensitivity to the spectral response of the human eye.

The raw receiver signal, which is generated by the detector and the
preamplifier is amplified using a narrow-band amplifier. Afterwards the
signal is directed to a synchronous demodulator, which is controlled by
the original main transmitter modulation clock, transmitted to the
measurement CPU of the receiver using RS-422 technology. The
measurement CPU circuitry converts the signal to TTL and then it is sent
to the main receiver module. With this technology it is possible to
achieve an optimal signal to noise ratio. The resulting main receiver
signal is sent to one channel of the 20-bit ADC of the measurement CPU
through a coaxial cable.

In parallel to the AC signal processing, the DC component of the receiver

signal is amplified and brought to the ADC of the measurement CPU.
This main receiver DC signal is used for self-diagnostic purposes, to
detect possible saturation.

The main receiver module is also equipped with a test LED that the
measurement CPU controls.

The main receiver module contains a temperature measurement and an

EEPROM. These components are connected to the measurement CPU
via the IIC-Bus interface. Individual calibration parameters are stored in
the EEPROM and are used to adapt individual settings, especially after
module exchange.

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The basic principle of LTD112 is shown in Figure 19 below.

DC
CPU
Measurement Main Receiver DC

Main Transmitter
Main Transmitter Clock

Detector Narrow Band Synchronous

20bit ADC
Amplifier Demodulator Main Receiver Signal

Test LED
CPU
Test LED

Temperature
CPU
Measurement

EEPROM
CPU

0401-036

Figure 19 Block Diagram LTD112

Window Transmitter Module LTL212

The window transmitter module LTL212 (see Figure 20 below) is
mounted to the optics unit and is secured by two screws. It is connected
to the measurement CPU using two connectors.

1104-093

Figure 20 Window Transmitter Module LTL212

The window transmitter module contains a white, high-power LED as a

light source. Under the control of the measurement CPU, the light source
is modulated with a frequency of approximately 250 Hz. A monitor
photodiode measures the emitted light intensity, and a closed loop
regulator in combination with a LED driver achieves a normalized,
constant transmitter intensity.

The intensity signal and a derating signal, proportional to the applied

LED forward current, are connected to the ADC of the measurement
CPU for self-diagnostics.

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The window transmitter module is also equipped with a temperature

measurement and EEPROM. The temperature measurement and
EEPROM components are connected to the measurement CPU through
the IIC-Bus interface. Individual calibration parameters are stored in the
EEPROM and are used to adapt individual settings, especially after
module exchange.

The basic principle of LTL212 is shown in Figure 21 below.

CPU
Derating

Transmitter Regulator
LED CPU
+Driver Transmitter Clock

Intensity
Measurement CPU
Transmitter Intensity

Temperature
CPU
Measurement

EEPROM
CPU
0401-034

Figure 21 Block Diagram LTL212

Measurement Head CPU LTC112

The measurement head CPU LTC112 (see Figure 22 below) is used for
the optics unit of the LT31 transmitter and as well for the LT31 receiver.
The hardware and software for transmitter and receiver are identical. The
CPU detects automatically if it is active in an LT31 transmitter or
receiver environment and selects the software configuration accordingly.

1104-094

Figure 22 Measurement Head CPU LTC112

The CPU section of the LTC112 is equipped with the following major
components:

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- 80C552 CPU (including 8-channel 10-bit ADC)

- RAM and Flash memory
- 2-channel 20-bit ADC
- EEPROM
- RS-485 interface

The functional units of the measurement head CPU LTC112 are the
following:

- Power supply
- RS-485 module interface
- Alignment unit control
- Heater control
- Piezo beeper
- Automatic transmitter/receiver mode detection
- Temperature measurement near the windows
- Temperature measurement far away from the heater elements
- Modulation frequency generation for window transmitter
(approximately 250 Hz)
- Modulation frequency generation for the main transmitter (in case of
transmitter configuration; approximately 1 kHz)
- Window receiver (including the optics tube and adapted optical filter)
including temperature measurement
- CPU with all necessary peripherals and software download through
serial data interface capability
- 2-channel 20-bit ADC for main receiver signal (in case of receiver
configuration) and window receiver signal
- A/D conversion of the window transmitter intensity measurement
- A/D conversion of the main transmitter intensity measurement (in
case of transmitter configuration)
- A/D conversion of the main receiver DC signal measurement to detect
possible saturation situations due to solar radiation and/or to
compensate for solar radiation influence (in case of the receiver
configuration)

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The basic principle of LTC112 is shown in Figure 23 below.

Window Transmitter
Module

Window Receiver
Detector

DC
Measurement

Window
Temperature
Measurement 2-channel
20bit ADC Main
Enclosure
Temperature
CPU Receiver
Module
Measurement including 8-channel or

10bit ADC Main

WinReceiver Transmitter
Temperature Module
Measurement

EEPROM

Power Interface Alignment Heater Blower Beeper

Supply Control Control Control
0401-038

Figure 23 Block Diagram LTC112

Interface Unit LTI111/211

The interface unit LTI111/211 (see Figure 24 on page 56) consists of the
AC power supply (FSP102, transformer, and switch), master CPU
LTC212 in the case of the receiver interface unit, and the optional backup
battery LTBB111.

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0806-036

Figure 24 Interface Unit Receiver LTI211

The interface unit supplies the power required for the measurement unit
and the optional heater and the optional Obstruction Light LT31OBS.
The receiver interface unit LTI211 contains an interface and power
supply connection for the optional Background Luminance Sensor LM21
and the transmitter interface unit LTI111 provides an interface and power
supply for the PWD Forward Scatter Sensor.

The external power inputs and the communication interfaces of LT31 are
located in the interface units. All external interfaces are equipped with
surge protection circuitry.

Power Supply FSP102

The power supply board (see Figure 25 on page 57) includes AC and
secondary fuses, the AC voltage level switch and the transient protection
circuitry. The operation of the AC power supply is indicated with two
LEDs. The green LED indicates that the non-stabilized power supply
output of nominal 24VDC is available and the yellow LED indicates that
the 28VAC output is functional.

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0808-002

Figure 25 Power Supply FSP102

The transient protection circuitry has been tested to sustain 2 kV level

transient surges generated according to the EN 61000-4-5 testing
standard. According to the test results, 2 kV level transient surges should
not affect the operation of the circuitry. With higher level and repeated
transient surges, the high surge current will blow the main fuses F1 and
F2. The circuit board is thus protected against high surge currents and the
unit is operational after fuses are replaced.

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Master CPU LTC212

The master CPU LTC212 (see Figure 26 below) is located in the
receiver interface unit LTI211.

0808-003

Figure 26 Master CPU LTC212

The CPU module is equipped with:

- H8 CPU
- RAM & Flash memory
- LT Maintenance Interface (RS-232)
- Module interface (RS-485); to control the LT31 system and to collect
raw data

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- Configurable host interface

- RS-232 with RTS/CTS
- RS-485
- Socket for optional modem
- Power supply

The master CPU software can be updated using the maintenance or the
host interface.

Present Weather Detector Forward

Scatter Sensor
A Present Weather Detector (PWD) forward scatter sensor (see Figure 27
below) is mounted to the support arm of the LT31 transmitter unit. It is
used for auto-calibration and for present weather detection (optional).

0401-043

Figure 27 PWD Forward Scatter Sensor

The key functions of the PWD forward scatter sensor include the
following:

- Accurate measurement of the prevailing visibility based on proven

forward scatter principle
- Scientifically valid chain of calibration
- Extensive set of self-diagnostics and automatic detection of
contamination ensures that false values are not reported
- Easy calibration
- Weather-proof mechanical design

The PWD forward scatter sensor is connected to the power supply and to
the module bus terminals (RS-485) of the interface unit of the LT31
transmitter.

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For maintenance purposes, the PWD provides a user interface that can be
accessed in the following ways:

- Selected commands are accessible through the LT Maintenance

Interface (see section LT Maintenance Interface on page 132).
- The full PWD command set can be used when connected to the PWD
Maintenance Interface.

Software Descriptions
Besides the PWD forward scatter sensor and the optional LM21
background luminance sensor, LT31 contains the following intelligent
units:

- The master CPU

- The measurement CPU of the transmitter
- The measurement CPU of the receiver

Master CPU
The master CPU is located in the interface unit of the LT31 receiver
limit.

The main tasks of the master CPU are as follows:

- Collect raw data from all connected intelligent units

- Carry out all calculations and algorithms
- Self-check and alarm/status report
- Host interfacing
- Maintenance/user interfacing

Measurement CPUs
The CPU board of the measurement units of LT31 transmitter and
receiver are similar and contain exactly the same software. During the
power on procedure the operation mode is automatically set to
transmitter or receiver mode depending on the connected hardware.

The main tasks of the measurement CPU and the associated software are
the following:

- Collecting raw data and calculate 5-second average values

- Distributing data on the request of the master CPU
- Carrying out actions on the request of the master CPU

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- Controlling the window heater

- Downloading software

Optional Equipment Descriptions

Background Luminance Sensor LM21

The LM21 sensor offers the means for measuring ambient light level or
background luminance in RVR applications. The sensor is mounted to
the LM21 support arm, which is attached to the mast construction of the
LT31 receiver unit.

The support arm can be rotated for coarse horizontal orientation of the
LM21 while the mounting bracket allows fine horizontal and vertical
alignment.

The installed background luminance sensor is shown in Figure 28 below.

1104-095

Figure 28 Installed Background Luminance Sensor LM21

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The background luminance sensor is used for measuring the background

against which the pilot views the runway lights or runway markings. For
details on Background Luminance Sensor LM21, refer to LM21 User's
Guide (see section Related Manuals on page 16).

TERMBOX-1200/TERMBOX-9000
The termination box (TERMBOX) acts as the connection point and surge
protector device for AC power and signal line cables. An AC power
cable with a maximum diameter of 25 mm can be taken inside the
termination box. The connector contacts are proper for cable leads up to
10 mm2.

The Termbox-1200 (LT31 receiver unit) offers connection points and

surge protectors for two signal lines.

Surge protectors are available for the following signal line types:

- Leased-line modem
- RS-485

The TERMBOX -9000 (LT31 transmitter unit) offers surge protection for
the AC power only.

Obstruction Light LT31OBS

The obstruction light is equipped with a mounting clamp (see Figure 29
on page 63). The mounting clamp is mounted to the support arm of the
LT31 transmitter and receiver support unit.

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0401-089

Figure 29 Installed Obstruction Light LT31OBS

The obstruction light is powered using 24VDC generated by the FSP102

power supply.

Battery Backup LTBB111

The optional battery backup LTBB111 (see Figure 30 on page 64) is
installed in the interface units of LT31 transmitter and receiver unit. A
proper and fully charged battery ensures operation for a period of
approximately 60 minutes (at 20 C) if power failure occurs.

In case AC voltage is available, the QBR101 passes the 24VDC voltage

generated by the power supply to the direct current loads. The backup
battery is charged in parallel.

In case of AC power failure, the QBR101 switches over to the battery

supply without interruption.

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0401-041

Figure 30 Installed Battery Charger Module QBR101

The following numbers refer to Figure 30 above.

1 = Battery charger module QBR101
2 = DC switch

The battery is mounted below the master CPU cover and is part of
LTI111 and LTI211. An installed backup battery is shown in Figure 31
on page 65.

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0810-074

Figure 31 Installed Backup Battery

The DC switch is used to interrupt the battery-based power supply for

storage, transportation, and maintenance of the instrument.

CAUTION Do not short circuit the battery.

NOTE A valve-regulated lead battery is used for the battery backup. According
to the ISO 14001, the battery must be recycled.

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CHAPTER 4
INSTALLATION

This chapter provides you with information that is intended to help you
install this product.

NOTE If LT31 is equipped with enhanced weather protection hoods, follow the
instructions given in the document LT31 Enhanced Weather Protection
Hoods Installation Guide (see section Related Manuals on page 16).

Preparing Installation
Before you begin to install Vaisala Transmissometer LT31, make a plan
of the installation steps. The following is an exemplary plan describing
how to organize the installation process.

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1. Select the location and perform site survey:

- Check that the installation area is in accordance with the ICAO
recommendations.
- Check the ground level at the recommended safety distance of
120 m from the runway. It should have approximately runway
surface level.
- Select the final foundation positions.
- Determine the transmitter and receiver positions.
2. Prepare cabling plan:
- Grounding cabling layout and cable type.
- Power supply cabling layout and cable type.
- Intercommunication signal cabling between transmitter and
receiver. Layout and cable type.
- Consider communication needs, generate modem/signal cabling
layout and cable type.
3. Order the construction materials and cables.
4. Do the digging for the cables and foundation.
5. Cast the concrete foundations.
6. Prepare the cables:
- Connect the AC and signal cables of the site to a separate
junction box and have them ready for direct connection to the
optional TERMBOX or the instruments.
7. Set the foundation screws.
8. Install the receiver mast:
- Mount the receiver mast and align it vertically.
- Assemble the weather protection hood.
- Align the support unit horizontally.
- Mount the optional LM21.
9. Install the transmitter mast:
- Mount the receiver mast and align it vertically.
- Assemble the weather protection hood.
- Align the support unit horizontally.
- Mount the optional LT31OBS.
10. Finalize the transmitter installation:
- Install the transmitter measurement unit.
- Mount the PWD Forward Scatter Sensor.
- Connect the optional LT31OBS.
- Make intercommunication connections to the receiver.

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- Connect the AC voltage to the power supply of the interface

unit.
11. Finalize the receiver installation:
- Install the receiver measurement unit.
- Mount the optional LT31OBS.
- Connect the optional LT31OBS.
- Make intercommunication connections to the transmitter.
- Connect the communication line to the host system.
- Connect the AC voltage to the power supply of the interface
unit.
12. Perform startup tests, final alignment, and calibration for the LT31
system.

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Selecting Location
The recommended location for installing LT31 is clearly defined in the
related ICAO Manual of Runway Visual Range Observing and Reporting
Practices, DOC 9328-AN/908. This ensures that the MOR measurements
for the air traffic are as representative as possible and that all safety
related matters are sufficiently taken into account.

The main requirements for the location of the transmissometer are

presented in the following:

1. LT31 must be sited according to the related ICAO requirements.

- LT31 should be located at a distance of usually up to 120 m in
parallel to the runway.
- The recommended standard distance between LT31 transmitter
and receiver is 30 m (50 m and 75 m are optionally possible).
25 m baseline is available as a purchased option.
- Depending on the touch-down or mid-position, the related
location along the runway is defined by ICAO as well.
- The typical measurement height must meet approximately
2 ... 3 m above the runway surface level. LT31 is prepared for
this measurement height above the foundation level.
2. The site should be free of obstacles that can disturb the optical
measurement, as well as obvious sources of contamination.
- There should be no bright, flashing lights near the LT31 receiver
or in the receiver's field of view.
- If a Background Luminance Sensor LM21 will be installed, it
needs a clear view of the sky in its preferred viewing direction
(see section Background Luminance Sensor LM21 on page 61).
3. Power supply and communication lines must be available.
- When siting LT31, consideration must be given to the available
power supply and communication lines, as this influences the
amount of work and accessories needed, and hence the actual
cost of the installation.

Unpacking Instructions
The contents of the delivery in question are specified in the packing list
included in the delivery documents. The LT31 equipment is normally
delivered in three cases containing the following parts:

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First box:

- Transmitter mast including interface and support units

- Weather protection hood, measurement head cover
- Upper and lower interface unit cover
- Measurement unit
- Mounting material (optional)

Second box:

- Receiver mast including interface and support units

- Protection hood, measurement head cover
- Upper and lower interface unit cover
- Measurement unit
- Mounting material (optional)

Third box:

- PWD forward scatter sensor

Optional material is packed separately.

NOTE The case containing the optical parts should be handled carefully. Do
not drop either end of the case for more than five centimeters.

Unpacking Procedure
1. Read the packing list supplied with the delivery documents.
Compare the packing list with the purchase order to ensure that the
shipment is complete.
2. Open the covers.
3. In case of any discrepancies or damage, contact the supplier.
4. Place the packing materials and covers back in the cases and store
them for possible reshipment.

Storage
Store LT31 in its packages in dry conditions, not in the open air. The
storage conditions are:

- Temperature: -50 C to 70 C
- Relative humidity: lower than 95 %

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NOTE If LT31 contains a battery backup option and if the unit is stored in an
extremely cold environment, make sure that the battery is fully charged.

Equipment Grounding and Lightning

Protection
The power supply cable with its protective earth conductor (PE) provides
a standard protective ground for the AC power supply inside the interface
unit. A 3-conductor AC cable with a proper earth conductor grounding
must be used.

Equipment grounding protects the electrical modules of LT31 against, for

example, lightning and prevents radio frequency interference. Equipment
grounding for LT31 is done with a jacketed grounding cable and
conductive grounding rod(s). A copper grounding wire with a minimum
cross section of 16 mm2, respectively AWG 5, is to be used to connect
the LT31 equipment grounding rod to the local grounding rods.

Depending on the need, one to four copper-sheathed steel rods are driven
into the ground. In case several rods are needed, the alignment from the
foot of the base plate must be radial.

The grounding principles are the following:

- Install the grounding rod as close to the pole mast as possible, to

minimize the length of the grounding cable. The grounding cable can
also be cast inside the concrete base.
- The length of the grounding rod depends on the local ground water
level. The lower end of the grounding rod should continuously touch
moist soil.

The quality of the grounding can be checked with a georesistance meter.

The resistance must be:

- less than 10  to guarantee electrical safety and the intended

functionality of LT31 fuses and overvoltage protection circuitry.
- less than 1  to achieve maximum possible overvoltage protection.

Additionally a ground connection with a minimum cross section of

4 mm2, respectively AWG 11 is to be installed between the equipment
grounding of transmitter and receiver unit. This connection ensures that
no potential differences can evolve.

Equipment grounding is shown in Figure 32 on page 73.

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0401-046

Figure 32 LT31 Equipment Grounding

WARNING A lightning strike through the communication wire can cause a

dangerous voltage surge at the remote sites if the remote units are not
properly grounded.

Cable Selection
This section describes the power and signal cabling of LT31. The
customer is responsible for supplying the power and signal cables, and
conduits for the cables. In all field cabling, the following should be
remembered:

- Use armored field cables.

- Cables must be suitable for underground use.
- Check the cable core diameter according to maximum allowable drop,
refer to Table 7 on page 76.
- Lead the cables to the equipment through conduits.
- Check the cable conduit diameters or use additional termination
boxes.
- Ground the cable shield at both ends.
- Use spike and overvoltage protection devices at both ends of the field
cables.

It is recommended to protect the cables by putting a brightly colored,

warning plastic tape into the ground about 0.5 m above the cables and
0.2 m below the ground level.

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An armored tube or similar should be used to mechanically protect the

AC cable and the signal cable from the termination box down into the
ground (0.7 m below the ground).

This equipment is designed for continuous 24-hour operation. The AC

power supply should be continuous and without spikes or blackouts. If
the AC voltage fluctuates outside the given tolerance, AC voltage
stabilizers are recommended.

Line Power Cable

LT31 should be equipped with a power cable of sufficient length with
free wire ends. If a local terminal with the power supply of 115/230 VAC
is not available, an extended AC cable from LT31 to the nearest power
source is needed. This cable should be armored and suitable for
underground use. The armored reinforcing acts as a mechanical shield
and also provides protection against lightning. The cable screen must be
grounded at both ends.

NOTE A separate junction box, where the extended AC cable is terminated, is

highly recommended at the installation site. A junction box
(TERMBOX-1200/TERMBOX-9000) that fits inside the LT31 radiation
shield is available from Vaisala. See Figure 33 and Figure 34 on page
75. These junction boxes also contains extra lightning protection which
is recommended at sites where thunderstorms are common.

NOTE The local safety regulations and practices concerning electrical systems
must be complied with.

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0210-026

Figure 33 TERMBOX-1200/Junction Box for Receiver

0210-021

Figure 34 TERMBOX-9000/Junction Box for Transmitter

The distance between the equipment and the AC distribution transformer,

and the power consumption of the equipment, dictate the thickness of the
cable. The minimum requirement for the power cable is 3 x min 1.5 mm2
(AWG 15) 3-conductor power cable.

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The recommended AC wire cross sections and typical cable diameters for
AC voltage 230VAC are shown in Table 7 below. A copper cable with
a 5 % voltage drop has been used. For the voltage of 115VAC, the
maximum distances should be divided by four. The maximum power
consumption of LT31 is 800 W.

Table 7 AC Cable Selection for LT31

Maximum One Wire Cross-section Nearest AWG
Distance from Area Gauge
Voltage Source
2 km 10 mm2 No 7 AWG
3 km 16 mm2 No 5 AWG
4.5 km 25 mm2 No 3 AWG

NOTE Cables with diameters more than 15 mm require a separate junction box.
Vaisala TERMBOX-1200/TERMBOX-9000 is recommended.

Communication Cable
The customer must provide cables and conduits. The cables used for the
transmission of digital and low-level analogue signals should be of the
following kind. All field cables should be armored, suitable for
underground use, and led to the equipment through conduits. Unless the
cables are armored, they have to be led through pipes. The signal cable
shields should be grounded at both ends.

For the intercommunication between the LT31 transmitter and receiver

two screened 2 x 0.22 mm2 (approximately AWG 23) twisted pair signal
cables with a shield and minimum diameter of 8 mm are recommended.

For modem and RS signal cable, use a screened 2 x 0.22 mm2

(approximately AWG 23) twisted pair signal cable with a shield and
minimum diameter of 8 mm. If the modem line length exceeds 50 km,
consult Vaisala.

If RS-485 connections are used, pay special attention to the quality of the
cable. The shield should be continuous, in other words, also connected to
the distribution frames.

The interface enclosure has a data cable outlet for cable diameters of 8 to
11 mm. This is reserved for signal or modem cables. The optional
AC/data cable outlet can also be used for data cables. The cable should
be led out from the unit inside the lower mast section. Grounding must be
done within the cable gland in order to keep the EMI levels within
specifications.

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NOTE If there is a junction box installed, the cable shield should be grounded
at both the input and output cable glands.

Detailed instructions for a proper RF-grounding of any jacketed cables

are given in the following:

1. Lead the signal cable through the cable inlet. See Figure 35 below.
2. Strip 50 cm of the cable sheath leaving approximately 2 cm of
shield.
3. Remove the cap of the cable gland, including the plastic cylinder.
Slide the cap with the plastic cylinder onto the cable.
4. Add shrinkable tubes to increase the cable diameter of thin cables
(diameters less than 5 mm).
5. Slide the plastic cylinder to the edge of the cable sheath. Turn the
cable shield over the cylinder; see Figure 35 below.
6. Tighten the cable with the cable gland and proceed with the wiring.
7. Ground the signal cable with the same method at both ends.
8. Wire the communication cable according to the instructions in
section Communication Options on page 78.

0201-070

Figure 35 Cable Grounding Instruction

The following number refers to Figure 35 above.

1 = Cable shield

NOTE Cables with diameters more than 15 mm require a separate junction box.
Vaisala TERMBOX-1200/TERMBOX-9000 is recommended.

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Communication Options
LT31 provides RS-232, RS-485, and modem transmission interfaces for
data messages. A separate RS-232 interface is provided for maintenance
purposes. Consider your communication needs before the installation.
The method of communication depends on the distance between the host
computer system and LT31. The possibilities are described in Table 8
below.

Table 8 Communication Cable Lengths

Cable Length One LT31 Several LT31 on the line
< 50 m RS-232; RS-485 RS-485; modem (only 300 bps)
< 1200 m RS-485; modem RS-485; modem (only 300 bps)
> 1200 m Modem Modem (only 300 bps)

Serial Communications Settings

NOTE The default serial communications port settings of LT31 are the
following: 9600 bps, no parity, 8 data bits, and 1 stop bit.

Serial Transmission with RS-232

For the RS-232 communication method, connect the signal wires to the
screw terminal at the CPU board LTC212. See Figure 36 on page 79. The
flow control lines RTS and CTS are not required but they can be used.
The hardware flow control can be enabled in LT31. As a default, the RTS
and CTS lines are not used. For details, see Chapter 5, Operation, on
page 145.

NOTE The cable shield of the RS-232 cable must be properly grounded in the
cable gland of the interface enclosure, see Figure 35 on page 77.

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Reset
Status
RXD
TXD

Reset Button

1
2
3
4
5 +12V Out
6
7
8 GND
9 VB+
Sensor PC
10 VB-
line 11 RS-485+
12 RS-485- D-9 pin

13 CTS RTS
Data 14 RTS CTS 2 RXD
line RTS 7
15 RXD TXD 3 TXD
RS-232 CTS 8
16 TXD RXD
17 GND GND 5 GND
Data 18 RS-485+ RS-232
line 19 RS-485- Communication
line
Maint. 20 RXD
line 21 TXD
RS-232 22 GND
23 MOD1
Module
24 MOD2
(modem) 25 MOD3
(modem) 26 MOD4
27 MOD5

0401-055

Figure 36 RS-232 Communication Option

Serial Multipoint Transmission with RS-485

The RS-485 transmission standard allows several LT31s to communicate
(half-duplex) with the host computer using a single twisted pair. The RS-
485 interface is potential separated. For the RS-485 communication
method, connect the signal wires to the screw terminal at the CPU board.
See Figure 37 on page 80.

NOTE The cable shield of RS-485 cable must be properly grounded in the
cable gland of interface enclosure, see Figure 35 on page 77.

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Reset
Status
RXD
TXD

Reset Button

1
2
3
4
5 +12V Out
6
7
8 GND
9 VB+
Sensor 10 VB-
line 11 RS-485+
12 RS-485-
13 CTS
Data 14 RTS
line
15 RXD
RS-232
16 TXD Remote computer
17 GND
Data 18 RS-485+
RS-485 + RS-485
line 19 RS-485- - transceiver
Maint. 20 RXD
line 21 TXD
RS-232 22 GND
Other system
23 MOD1
Module
24 MOD2
(modem) 25 MOD3 RS-485
receiver
(modem) 26 MOD4
27 MOD5
Driver

0401-159

Figure 37 RS-485 Communication Option

Terminate the RS-485 chain with a 120  resistor. This is especially

important with long cables. If LT31 is at the end of the RS-485 chain, the
120  resistor can be connected across the screw terminal pins. The
120  resistors are available in a plastic bag inside the interface
enclosure.

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Figure 38 below illustrates an RS-485 application using a half-duplex

data transfer over a single twisted pair.

9502-061

Figure 38 RS-485 Option

Modem DMX501 (optional)

The DMX501 modem is a leased line modem module and it is compliant
with 300 bps V.21, 1200 bps V.22, 2400 bps V.22 bis, and 1200/75 bps
V.23. It is applicable to long distance communications (~10 km) using a
leased telephone grade cable. Since the modem is not approved by the
PTT (Post, Telephone, and Telegraph), it should not be used in public
telephone networks. However, most of the commercial brands following
the CCITT standards, as well as the DMX50 and DMX55 modems for
the Vaisala MILOS500, can be used with the DMX501 modem.

If the modem is installed (LT31 was ordered with the modem option),
connect the incoming signal wires to the MOD3 and MOD4 screw
terminals 25 and 26 at the CPU board LTC212. See Figure 39 on page
82.

NOTE The cable shield of the modem cable must be properly grounded in the
cable gland of the interface enclosure, see Figure 35 on page 77.

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User's Guide ______________________________________________________________________

Reset
Status
RXD
TXD

Reset Button

1
2
3
4
5 +12V Out
6
7
8 GND
9 VB+
Sensor 10 VB-
line 11 RS-485+
12 RS-485-
13 CTS
Data 14 RTS
line
15 RXD
RS-232
16 TXD
17 GND
Data 18 RS-485+
line 19 RS-485-
Maint. 20 RXD
line 21 TXD
MILOS 500 or other
RS-232 22 GND
host computer
23 MOD1
Module Modem
24 MOD2 Modem
connection
(modem) 25 MOD3
(modem) 26 MOD4
27 MOD5

0401-070

Figure 39 Modem Wiring

If the DMX501 modem is not installed, it can also be retrofitted to the

LTC212 unit. This is shown in Figure 40 on page 83.

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To do this, follow the instructions below:

1. Turn the AC power switch off and also the battery backup switch,
if installed.

0401-146

Figure 40 AC and Battery Backup Switch

2. Disconnect all the connectors from the LTC212 board.

3. Open the four screws holding the cover plate and remove it (see
Figure 41 on page 84).
4. Open the two screws holding the LTC212 board in place and
withdraw the board carefully.
5. Remove the DMX501 modem module from its protective package
and install it by pressing it gently into the module place of the
LTC212 board (see Figure 41 on page 84).

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0201-071

Figure 41 Installing DMX501

The following number refers to Figure 41 above.

1 = DMX501 Modem module

NOTE Handle the boards carefully. Do not touch the components on the boards
to avoid ESD damage to them. Ground yourself before attempting to
touch the boards.

NOTE Do not let the boards get wet.

NOTE Note the correct orientation of the DMX501 module.

6. After installing the DMX501 modem, push the LTC212 board back
to its place and attach the two screws securing it into the correct
position.
7. Attach the cover plate by tightening the four screws holding it.
8. Insert the connectors onto the LTC212 board and note their correct
order (refer to the numbers on the cover plate).
9. Make the necessary signal wiring as described above and turn on
the AC switch and also the battery backup switch if installed).

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Multipoint Modem Connection

If a multipoint connection (several pieces of equipment on the same line)
is used, the only available connection standard is 300 bps V.21. In this
configuration, the data collection computer or the weather station acts as
a master that polls the LT31 Transmissometers and the other possible
sensors.

The master modem must be in the Originate mode. All other modems on
the line must be in the Answer mode and the carrier signal must be
disconnected by default. In this mode the modem turns the carrier signal
on only when it gets the appropriate poll signal and turns it off when the
message has been sent. If there are several transmissometers on the line,
they must have different identifiers. For details, see Chapter 5, Operation,
on page 145.

When using the multipoint modem connection mode, have the limited
transfer capability of the 300 bps V.21 mode in mind. For an example,
see the following:

- LT31 message 2 (option; includes present weather data) is configured.

- Polling interval is 15 seconds.

A maximum number of three transmissometers can be used on one line.

Installation Procedure
The installation of LT31 has several separate procedures which are
described in the following sections. The procedures should be followed
in the given order to make installation as easy as possible.

NOTE Direct sunlight into the LT31 transmissometer light transmitter included
in LTM112 and light receiver included in LTM212 should be avoided.

In case this recommendation cannot be followed due to the runway setup

that dictates an LT31 orientation where sunlight can unavoidably enter
one of the LT31 measurement units for a few days of the year during
sunrise or sunset, it is recommended that the LT31 receiver unit LTR111
is installed pointing towards the low sun.

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Foundation
The very first procedure is the preparation of the foundations. For more
detailed instructions, see the document LT31 Enhanced Weather
Protection Hoods Installation Guide in section Related Manuals on page
16.

This preparation work includes the construction of the needed concrete

foundations as well as the setting of the foundation screws for secure
fixing of the equipment.

Two concrete foundations are required. The recommended dimensions

for the foundations are illustrated in Figure 42 below. This size generates
sufficient space to place a ladder safely near the transmissometer
transmitter and receiver. The distance between foundation centers should
meet the desired baseline length plus 0.5 m for LT31 (typically 30 m +
0.5 m).

0401-054

Figure 42 Casting Concrete Foundation

If a new foundation needs to be cast, it is recommended that a duct for

power and signal cables is mounted into the cast. The tube with the
cables should come out from the casting near the bottom line of the
triangle formed by the foundation screws which will be positioned later.
The concrete foundation should be cast directly on site. An appropriate
reinforcement must be foreseen to stabilize the structure.

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When casting a new foundation, use the optional foundation kit

LTMK211 that contains the necessary hardware.

It is recommended to construct an extension for the surface around the

foundation. A safe positioning of a ladder for maintenance purposes can
be assured hereby. See Figure 42 on page 86.

Dowel mounting
The optional foundation kit (LTMK111) for LT31 dowel mounting
contains the required equipment for mounting to an existing surface. The
triangle-shaped drilling template should be used to set the drilling holes
as precisely as possible. One drilling hole at each position must point
towards the opposite foundation.

NOTE Set the foundation screws to ensure secure fixing of the equipment.

For the foundation screws, the following tools are required:

- (Torque) wrench, size 24

- Hammer, 1.5 kg
- Drilling machine/hammer drill, preferably with a stand for
perpendicular drills
- Mounting kit with LT31 drilling template
- Mandrel bar 14 mm (HILTI tool HSD-G M16 x 65 recommended)
- Silicon rubber
- Tape measure, length min. 30 m
Proceed as follows:

1. Orientate the drilling template in such a manner, that one of the

drilling holes points exactly towards the opposite installation place.
On both foundations, the instrument should afterwards be mounted
in the center of the foundation surface. The distance between the
single holes which point towards the opposite place, must match
exactly the desired baseline length of typically 30 m. Refer to
Figure 43 on page 88. 50 m and 75 m baselines are optional. 25 m
baseline is available as a purchased option.

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0401-068

Figure 43 Positioning Drilling Holes

2. Drill three 20 mm holes with a depth of 70 mm using the drilling

template. Check carefully that the holes are drilled perfectly
perpendicular. Remove the template. Clean the holes.
3. Insert the dowels into the holes and hammer them down until their
edges are on the foundation level. Recommended dowels: HILTI
HKD-S M16 x 65.
4. Use the HILTI tool HSD-G M16 x 65 respectively a 14 mm 
mandrel bar and hammer the expanding mechanism of the wedge
bolts down until the bolts are completely fixed inside the holes.
5. Fasten the foundation screws to the inserted dowels by hand. Check
the right orientation of the foundation screws.
The breaking area (area with smaller diameter) just above the
welded nut on the optional foundation screws must point upwards.
Place a washer between the foundation surface and the welded nut.
Refer to Figure 44 on page 89.

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0401-069

Figure 44 Foundation Screws

6. Insulate the area between the foundation screw and foundation

surface below the washer with silicon rubber, so that no water can
access the drilling hole.
7. Tighten the foundation screws using the welded nut (60 Nm;
maximum 80 Nm).
8. Check using the drilling template that the screw positions fit.

Molded Screw Mounting

The optional foundation kit for LT31 molded screw mounting contains
the required equipment for foundation screw insertion into the wet
concrete.

Foundation construction is shown in Figure 45 on page 91. To mount the

LT31 foundation screws while casting the foundation, proceed as
follows:

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User's Guide ______________________________________________________________________

1. Screw on the three reinforcing plates to the lower end of the

foundation screws as in Figure 45 on page 91. To prevent the plates
from rotating after their correct position has been found, destroy
the threads above and below them by hammering.
2. Fix the drilling template to the upper ends of the foundation screws
with six M16 nuts.
3. Embed the assembly in the concrete foundation as shown in Figure
45 on page 91. Be sure to align the assembly taking the optical axis
of LT31 into account as shown in the figure (orientate the assembly
in such a manner, that one of the foundation screws points exactly
towards the opposite installation place).
4. After the concrete has set, remove the template.

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0906-095

Figure 45 LT31 Foundation Construction

The following numbers and letter refer to Figure 45 above:

1 = Drilling template
2 = Foundation screw
3 = Reinforcing plate
A = Direction of optical axis of LT31

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User's Guide ______________________________________________________________________

Preparing Assembly of LT31

Vaisala Transmissometer LT31 is delivered in a disassembled state to
protect the parts during transportation.

To assemble and install LT31 on site in the most efficient way, follow
the steps below.

If the instruments parts are already unpacked, transport them carefully to

their installation places. Use some stuffing to protect the parts during
transportation. If possible, use the original transportation boxes for the
transportation of LT31.

When assembling LT31, the following tools are required:

- Screwdrivers, sizes 3, 1.6 x 10

- Crosstip screwdrivers, sizes 1 x 3, 1 x 4
- Wrenches, sizes (40), 24, 20, 18, 17
- Allen key hex-socket, sizes 3, 4, 5
- Flat pliers
- Edge cutter
- Hex-socket drivers
- Laptop with terminal program, VT100 emulation
- Maintenance cable
- Level
- Torque wrench, size 24
- A-ladder
- Cable tabs

On-site Receiver Installation

The foundation of the receiver installation is the place, where the data
cabling for the long distance communication with the host computer is
accessible.

The following parts have to be unpacked and stored for a short time
safely near the installation site.

For the receiver installation, the following parts are needed:

- Receiver mast (type LTR111) including the mounted interface unit

(type LTI211) at the mast base and support unit at the mast top.
- Receiver measurement unit (type LTM212).

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- Weather protection hood

- Measurement head cover
- Upper interface unit cover
- Lower interface unit cover

Later on these parts are also needed:

- Maintenance cable
- Optical blocker

See Figure 46 below.

0401-071

Figure 46 Receiver Installation Parts

For the receiver installation, the following parts are optional:

- Background Luminance Sensor LM21.

- Support arm, mounting bracket and cable for LM21.
- Junction box with enhanced overvoltage protection for signal and AC
lines (type TERMBOX-1200).
- Obstruction Light (type LT31OBS) .

- Enhanced weather protection hood (for details, refer to LT31

Enhanced Weather Protection Hoods Installation Guide in section
Related Manuals on page 16)

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User's Guide ______________________________________________________________________

Setting up Receiver Mast to Foundation

1. Screw one nut on the three foundation screws (see Figure 47
below), until the distance between the foundation surface and the
nut top is 5 cm.
2. Place a washer on top of the nuts.

0401-049

Figure 47 Foundation Screws

3. Place the receiver mast on the prepared foundation screws. The

interface unit at the mast base must be oriented backwards from the
baseline between the two installation places.
4. Secure this preliminary mast position. Use a washer and nut on
each of the tree foundation screws, fix it only hand-tight.
5. Ensure that the base plate of the mast has settled on the nuts and
washers mounted below.
6. Use the level and find a perfect vertical alignment for the mast by
loosening the respective securing nut and aligning the complete
mast construction by turning the nut below the base plate. See
Figure 48 below.

0401-051

Figure 48 Vertical Mast Alignment

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7. When the mast is aligned perpendicular, secure the base plate

again, fix the nuts to ensure a stable mast position.
8. If the level of receiver and transmitter foundation surfaces is
identically in the frame of ± 10 cm, tighten the nuts as tight as
possible (see Figure 49 below). A torque of 60 ... 80 Nm is
recommended for the final fixture.

0401-053

Figure 49 Tightening Nuts

Setting Up Weather Protection Hood to Support Unit

To set up the weather protection hood to the support unit, follow the
instructions below. Alternatively, the enhanced weather protection hood,
if ordered (part of the optional material), is mounted according to a
separate instruction. For details, see the document LT31 Enhanced
Weather Protection Hoods Installation Guide in section Related Manuals
on page 16. Both hood types have identical mechanical fixtures.

When LT31 is ordered with heated hoods, the PWD forward scatter
sensor is also equipped with hood heating. In this case, the heating power
wires of the PWD connection cable are connected to the FSP102 power
supply inside the LTI111 transmitter interface unit as follows:

Connect the heating power wires of the PWD22 cable to FSP102 power
supply of LTI111 Transmitter Interface Unit:

1. Connect wire YEL/BRN and wire WHT/YEL to FSP102 terminal

8 of X4.
2. Connect wire BRN/GRN and wire WHT/GRN to FSP102 terminal
11 of X4.

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To set up the weather protection unit:

1. Place the ladder in front of the receiver mast.

2. Then install the weather protection hood.
3. Fix the hoods with two screws (hex socket head, size 3) at each rear
end corner of the hood. See Figure 50 below.

0401-073

Figure 50 Mounting Weather Protection Hood

WARNING Death or serious injury may occur!

The AC power voltage must be disconnected from the instrument.

Otherwise, switch the AC power off.

4. In case a heated weather protection hood is used, lay the heater

cable inside the support unit as shown in Figure 51 below.

0401-075

Figure 51 Cable Routing for Optional Heated Hood

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5. Now place the ladder behind the receiver mast and check the
orientation of the support unit.
6. Loosen the two fastening screws (hex socket, size 4) at the right
side (operator's view) of the mast top. The complete support unit
must now be turnable. See Figure 52 below.

0401-076

Figure 52 Fastening Screws of Support Unit

7. Align the support unit horizontally to the position where the

transmitter mast will be located in the future. Use the sight hole in
the rear plate together with the sight. See Figure 53 on page 98.

VAISALA _______________________________________________________________________ 97
User's Guide ______________________________________________________________________

0401-077

Figure 53 Aligning with Sight Hole

8. Fix the two fastening screws of the support unit at the mast top.

Mounting Optional LM21

Background Luminance Sensor LM21 is provided together with LM21
cable, support arm and mounting bracket for attachment to the LT31
receiver mast. The Figure 54 on page 99 shows the LT31 with installed
LM21 option.

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1104-096

Figure 54 LT31 with LM21 Option Installed

The following numbers refer to Figure 54 above.

1 = Transmissometer LT31
2 = Background Luminance Sensor LM21F
3 = Mounting bracket
4 = Support arm
5 = Fastener assembly

To install the optional LM21 proceed as follows:

1. Route the LM21 cable (with open wire ends) through the hole in
the rotation plate of the mounting bracket so that the connector is
finally located as shown in Figure 55 on page 100.

VAISALA _______________________________________________________________________ 99
User's Guide ______________________________________________________________________

1104-097

Figure 55 Routing LM21 Cable through Mounting Bracket

The following numbers refer to Figure 55 above.

1 = Rotation plate of the mounting bracket
2 = Connector of LM21 cable

2. Insert the LM21 support arm into the guide which is located behind
the interface unit.
3. Route the open wire ends of the LM21 cable through the upper end
of the LM21 support arm. See Figure 56 below.

1104-098

Figure 56 Routing LM21 Cable through LM21 Support Arm

4. Canalize the cable end through the entire LM21 support arm and
then through the cable hole in the outer mast tube below the
interface unit box.
5. Attach the connector to the associated receptacle of the LM21,
secure the connection with the connector’s lock nut and install the
LM21 to the mounting bracket using the LM21 fastening screws
which come with the LM21 (M5x12 DIN912) using a hex socket
wrench (size 4). See Figure 57 on page 101.

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1104-099

Figure 57 Installing LM21 to Mounting Bracket and Attaching

Connector

The following numbers refer to Figure 57 above.

1 = LM21
2 = LM21 fastening screws
3 = mounting bracket
4 = connector of LM21 cable

6. Install the LM21 mounting bracket assembly to the LM21 support

arm using two fastening screws size M5x25 DIN912 (which are
part of the delivery) and the fastener assembly according to Figure
58 on page 102 using a hex socket wrench (size 4).

NOTE Ensure that the LM21 cable is not pinched during this procedure.

VAISALA ______________________________________________________________________ 101

User's Guide ______________________________________________________________________

1104-100

Figure 58 Mounting LM21 to Support Arm

The following numbers refer to Figure 58 above.

1 = LM21
2 = Support arm
3 = LM21 fastening screws
4 = Fastener assembly
5 = Mounting bracket

7. Carry out the horizontal alignment of the LM21.

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NOTE The recommended orientation is preferably towards north in the

northern hemisphere and towards south in the southern hemisphere.
Depending on the superior RVR system layout, also an alignment
towards the runway end, in approach direction is possible. Direct
sunlight into the sensor must be avoided.

Verify that no artificial light sources illuminate the LM21 in its viewing
direction. For details, refer to Background Luminance Sensor LM21
User's Guide.

o Rotate the support arm to achieve the intended coarse

orientation. See Figure 59 below.

1104-101

Figure 59 Coarse Horizontal LM21 Alignment

o Secure the support arm in its final position with the respective
hex socket size 4 screw located slightly above the interface
unit. See Figure 60 below.

1104-102

Figure 60 Securing Support Arm

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User's Guide ______________________________________________________________________

o Horizontal fine alignment is done using the rotation plate of the

mounting bracket. Loosen the two hex socket size 4 screws of
the rotation plate, rotate the LM21 as intended and tighten the
rotation plate screws afterwards. See Figure 61 below.

1104-103

Figure 61 LM21 Fine Horizontal Alignment

The following numbers refer to Figure 61 above:

1 = Rotation plate
2 = Rotation plate screws

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8. Carry out the vertical alignment of the LM21.

NOTE The final vertical orientation of the LM21 is slightly tilted. The
elevation angle should meet approximately 30° above the horizon.

Verify that no artificial light sources illuminate the LM21 in its viewing
direction. For details, refer to Background Luminance Sensor LM21
User's Guide.

Slightly loosen all four vertical alignment screws of the mounting

bracket using a hex socket wrench size 4. Adjust the vertical
alignment of the LM21 as intended and tighten all four vertical
alignment screws of the mounting bracket. See Figure 62 on page
106.

VAISALA ______________________________________________________________________ 105

User's Guide ______________________________________________________________________

1104-104

Figure 62 LM21 Vertical Alignment

The following numbers refer to Figure 62 above.

1 = Mounting bracket
2 = Vertical alignment screws

9. Open the interface unit box. Figure 63 on page 107.

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0401-162

Figure 63 Opening Interface Unit Box

10. Wire the LM21 cable through one fitting cable feed-through at the
bottom of the box.
11. The cable shield of the Background Luminance Sensor cable must
be properly grounded into the cable gland. See Figure 35 on page
77.
12. Tighten the cable gland carefully and wind the rest of the cable into
a ring. Secure it with standard cable tabs.

NOTE LT31 must be powered down when connecting LM21

13. The cable connections have to be done according to Figure 64 on

page 108.

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User's Guide ______________________________________________________________________

1104-105

Figure 64 Wiring of LM21 to Receiver Interface Unit

The following numbers refer to Figure 64 above.

1 = Pink wire – connected to Master CPU pin 9
2 = Gray wire – connected to Master CPU pin 10
3 = Green wire – connected to Master CPU pin 11
4 = Yellow wire – connected to Master CPU pin 12
5 = Brown & red wire – connected to power supply pin 8
6 = White & blue wire – connected to power supply pin 11

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On-site Transmitter Installation

The transmitter installation foundation is the place, where only the data
cabling for the communication with the receiver interface unit and AC
power is accessible.

The following parts have to be unpacked and stored for a short time
safely near the installation place.

For the installation of the transmitter, the following parts are required:

- Transmitter mast (type LTT111) including the mounted interface unit

(type LTI111) at the mast base and support unit at the mast top.
- Transmitter measurement unit (type LTM112)
- Weather protection hood
- Measurement head cover
- Lower interface unit cover
- Upper interface unit cover
- PWD Forward Scatter Sensor

See Figure 65 below.

0401-080

Figure 65 Parts for Transmitter Installation

The following parts are optional for the transmitter:

- Junction box with enhanced overvoltage protection for signal and AC

lines (type TERMBOX-9000)
- Obstruction Light (type LT31OBS)

- Enhanced weather protection hood (for details, refer to LT31

Enhanced Weather Protection Hoods Installation Guide in section
Related Manuals on page 16)

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User's Guide ______________________________________________________________________

Setting up Transmitter Mast to Foundation

To set up the transmitter mast to the foundation, do the following:

1. Screw one nut each on the three foundation screws until the
distance between the foundation surface and the nut top is 5 cm.
Place a washer on top of the nuts. See Figure 47 on page 94.
2. Place the transmitter mast on the prepared foundation screws. The
interface unit at the mast base must be orientated backwards from
the baseline between the two installation places.
3. Secure this preliminary mast position. Use a washer and a nut on
each of the three foundation screws, fix it only hand-tight.
4. Ensure that the base plate of the mast has settled on the nuts and
washers that are mounted below.
5. Use the level and find a perfect vertical alignment for the mast. For
that purpose loosen the respective fastening nut and align the
complete mast construction by turning the nut below the base plate.
See Figure 48 on page 94.
6. When the mast is aligned perpendicular, secure the base plate
again, fix the nuts to ensure a stable mast position.
7. If the level of receiver and transmitter foundation surfaces is
identically in the frame of ± 10 cm, the nuts can be finally
tightened now as tight as possible. A torque of 60 … 80 Nm is
recommended for the final fixture. See Figure 49 on page 95.

Setting up Weather Protection Hood to Support Unit

To set up the weather protection hood to the support unit, follow the
instructions below. Alternatively, the heated weather protection hood can
be mounted if ordered (part of the optional material). Both the hood types
have identical mechanical fixtures.

1. Place the ladder in front of the transmitter mast.

2. Then install the weather protection hood.
3. Fix the hoods with two screws (hex-socket, size 3) at each rear end
corner of the hood. See Figure 50 on page 96.

WARNING Death or serious injury may occur!

The AC voltage must be disconnected from the instrument. Switch the

AC off.

4. In case a heated weather protection hood is used, lay the heater

cable inside the support unit as shown in Figure 51 on page 96.

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5. Now place the ladder behind the transmitter mast and check the
orientation of the support unit.
6. Loosen the two fastening screws (hex socket, size 4) at the right
side (operator's view) of the mast top. The complete support unit
must now be turnable. See Figure 52 on page 97.
7. Align the support unit horizontally onto the receiver mast. Use the
sight hole in the rear plate together with the sight pin. See Figure
53 on page 98.
8. Fix the two fastening screws for the support unit at the mast top
carefully. The support unit will not be moved again.
9. The sight must show now the measurement head of the receiver.
Otherwise the support unit position and/or the vertical alignment of
the mast have to be checked. Probably a slight tilting of the mast is
necessary due to foundation level differences.

Foundation Level Differences

Tilting Transmitter Mast

If the level of the transmitter foundation is significantly different, that is,
more than ± 20 cm, from that of the receiver foundation, the sight will
image an area below or above the receiver measurement head. In that
case, the transmitter mast has to be slightly tilted off the perpendicular
alignment.

1. Move the mast by turning the nuts above and below the base plate
on the front foundation screw. See Figure 66 below.

0401-088

Figure 66 Front Nut Adjustment to Tilt Mast

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NOTE The two other securing nuts have to be loose enough to allow the slight
movement of the base plate.

2. After this alignment is finished, the foundation nuts must be fixed

as tight as possible to allow for a safe positioning of the mast. A
torque of 60 … 80 Nm is recommended.

Mounting Optional Obstruction Light LT31OBS

1. Place the ladder behind the transmitter mast.
2. Fix the optional obstruction light at the rear bow of the support
arm. Pay attention that the sight hole is still accessible.
3. Favorably the fixture should be prepared in an upside down turned
position for the LT31OBS. See Figure 67 below.

0401-082

Figure 67 LT31OBS Mounting Preparation

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4. Check the upright position of LT31OBS before the hex-socket

screws size 4 are finally tightened. See Figure 68 below.

0401-089

Figure 68 Mounting LT31OBS

5. LT31OBS must be electrically connected to the power supply

voltage inside the interface unit.
6. Route the obstruction light cable inside the support unit, below the
blower system, through the hole in the blower mounting plate, and
then between the inner and outer mast tube down to the interface
unit. See Figure 69 below.

0401-090

Figure 69 Cable Routing of LT31OBS

7. Canalize the cable end through the cable hole in the outer mast tube
below the interface unit box. See Figure 70 on page 114.

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0401-091

Figure 70 Cable Hole below Interface Unit

8. Fix the cable to the cable comb inside the support unit using
standard cable tabs. See Figure 71 below.

0401-092

Figure 71 Cable Comb for Securing LT31OBS Cable

9. Open the interface unit box. See Figure 63 on page 107.

10. Wire the cable through one cable feed-through at the bottom of the
box.
11. Do the connection as shown in Figure 81 on page 120.
12. Tighten the cable feed-through carefully and wind the rest of the
cable into a ring. Secure it with standard cable tabs.
13. LT31OBS switches on automatically when the illumination is
approximately below 400 lux.

Mounting Transmitter Measurement Unit

CAUTION Before the measurement unit can be installed, the transportation lock for
the optics unit must be removed! This should be done on a clean
workbench in a dry room.

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1. Open the measurement unit carefully. Loosen four hex-socket

screws size 5 and remove the cover with air duct. See Figure 72
below.

0401-083

Figure 72 Measurement Unit Cover Screws

2. Carefully cut the transportation lock (see Figure 73 below) and

remove it from the measurement unit. Do not damage any of the
module cables!

0401-084

Figure 73 Cutting Transportation Lock

3. Finally remove the transportation lock warning sheet and close the
measurement unit carefully. Pay attention about the position of the
cover seal.
4. Place the ladder to the side of the transmitter mast and select the
side where the weather protection hood points left.
5. Loosen the two fastening screws at the side of the measurement
unit flange (hex-socket, size 4). See Figure 74 on page 116.

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0808-004

Figure 74 Fastening Screws of Measurement Unit

6. Place the measurement unit on top of the inner mast tube (windows
pointing left). Do not use the air duct on top of the unit as a handle,
carry the measurement unit at the enclosure sides!
7. The connection cable must be routed below the small junction box
near the blower.
8. Make sure that the measurement unit has completely settled on the
inner mast tube of the transmitter mast. The air duct inlet on top of
the measurement unit must be placed in the same level than the
outlet of the blower. See Figure 75 below.

0810-077

Figure 75 Height of Measurement Unit

9. Turn the measurement unit roughly into a parallel position with the
support unit (housing edges parallel to the outline of the support
unit). The measurement unit must have a distance of approximately
8 mm to all surrounding parts of the support unit. See Figure 76 on
page 117.

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0401-096

Figure 76 Parallel Position of Measurement Unit

10. Connect the measurement unit cable plug with the cable socket
from the support unit.
11. Place the connectors at the rear of the support unit right side from
the small junction box. For an illustration, see Figure 77 below.

0401-097

Figure 77 Final Connector Position for Measurement Unit

12. Place the ladder behind the transmitter and check the parallel
position again.
13. Then tighten the fastening screws (hex-socket, size 4) on the right
side of the measurement unit which are now accessible through the
mast adapter of the support unit. See Figure 78 on page 118.

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0401-085

Figure 78 Fastening Measurement Unit

Mounting PWD to Support Arm

1. The “ear” on the support arm is foreseen for the mounting of the
PWD Forward Scatter Sensor. See Figure 79 below.

0401-098

Figure 79 Mounting PWD

2. Use two hex-socket screws size 5 with washers to mount the PWD
Forward Scatter Sensor to the support arm.
3. Connect the prepared cable plug to PWD. Make sure that the plug
is carefully and completely screwed on the PWD receptacle. See
Figure 80 on page 119.

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0401-099

Figure 80 Connecting PWD

4. All necessary electrical connections in the interface unit are already

prepared.
5. Remove the ladder.

Connecting Signal Lines to Transmitter Interface

Unit
1. Open the interface unit box.
2. Wire the cables through the cable feed-throughs at the bottom of
the box.
3. Make the connection of cables that are shown circled in Figure 81
on page 120.
4. The interface unit of the transmitter must be connected to the
two-wire RS-485 module bus (terminals 8 and 10) which connects
all intelligent modules within the LT31 system with each other. In
addition, a two-wire synchronization line (terminals 6 and 7)
connects the modulation clock between the transmitter and
receiver.

NOTE The cable shield must be properly grounded in the cable gland of the
interface unit enclosure. See Figure 35 on page 77.

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1104-106

Figure 81 Transmitter Unit LTT111 Wiring

5. Tighten the cable feed-through carefully, wind the rest of the cable
length to a ring and secure it with standard cable tabs. For details,
see section Communication Cable on page 76.

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Mounting and Connecting Optional

TERMBOX-9000 (Junction Box)
TERMBOX-9000 enables a separate connection of the underground
cables and incorporates enhanced overvoltage protection. The
TERMBOX is to be mounted below the interface unit enclosure and
enables short cableways between foundation and cable feed-throughs.

WARNING The AC power must be switched off until all power line wiring is
finalized.

1. Open the TERMBOX-9000 cover plate.

2. Connect a short grounding cable (4 mm2) to the grounding clamp at
the left bottom side of the box. See Figure 82 below.

0401-102

Figure 82 Grounding Cable for TERMBOX

3. Screw the box onto the transmitter mast below the interface unit.
Use the screws from the delivery and hex socket driver size 5.
4. Connect the grounding cable carefully to the grounding bolt at the
base plate below the box. The wrench size is 17. See Figure 82
above.
5. Use a short 3-conductor power cable (3 x 1.5 mm ^2) with free wire
ends. Connect this power cable to the corresponding terminals
inside the TERMBOX. Use a fitting cable feed-through.

CAUTION Double-check that the AC power is switched off.

No voltage must be detected at the end of the underground power cable.

6. Route the underground cable through a fitting cable

feed-through and connect the power lines carefully.

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7. Tighten the cable feed-through carefully and close the cover plate.

Connecting Power Cable to Transmitter Interface

Unit
The power cable should be either connected into the Vaisala
TERMBOX-9000, see section Mounting and Connecting Optional
TERMBOX-9000 (Junction Box) on page 121 or routed straight through
the opening in the base plate of the mast and further into a local power
distribution box. Use a 3-conductor power cable (3 x 1.5 mm2) of
sufficient length with free wire ends. Make sure to connect the wires in
the correct way, especially the ones to the protective ground (usually
yellow/green). See Figure 81 on page 120 for wiring instructions.

0808-031

Figure 83 LT31 System Wiring with TERMBOX Option

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Finalizing On-site Receiver Assembly

Orientation of Support Unit

1. Place the ladder behind the receiver mast and check the orientation
of the support unit.
2. Loosen the two fastening screws (hex socket, size 4) on the right
side (operator's view) of the mast top. The complete support unit
must now be turnable. See Figure 52 on page 97.
3. Align the support unit horizontally onto the transmitter mast. Use
the sight hole in the rear plate together with the sight pin. See
Figure 53 on page 98.
4. Fix the two fastening screws of the support unit at the mast top.
The support unit will not be moved again.
5. The sight must show now the measurement head of the transmitter.
Otherwise the support unit position and/or the vertical alignment of
the mast have to be checked. Probably a slight tilting of the mast is
necessary due to foundation level differences.
6. If the sight shows the measurement head of the transmitter, the
foundation nuts can be fixed as tight as possible to allow for a safe
positioning of the mast. A torque of 60 ... 80 Nm is recommended.

Foundation Level Differences

Tilting the Receiver Mast

If the level of the receiver foundation is significantly different, that is,
more than ± 20 cm, from that of the transmitter foundation, the sight will
image an area below or above the transmitter measurement head. In that
case, the receiver mast has to be slightly tilted off the perpendicular
alignment.

1. Move the mast by turning the nuts above and below the base plate
on the front foundation screw. See Figure 66 on page 111.

NOTE The two other securing nuts have to be loose enough to allow the slight
movement of the base plate.

2. After this alignment is finished, the foundation nuts must be fixed

as tight as possible to allow for a safe positioning of the mast. A
torque of 60 … 80 Nm is recommended.

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Mounting Optional Obstruction Light LT31OBS

1. Place the ladder behind the receiver mast.
2. Then follow the instructions in section Mounting Optional
Obstruction Light LT31OBS on page 112.

Mounting Receiver Measurement Unit

CAUTION Before the measurement unit can be installed, the transportation lock for
the optics unit must be removed!

1. Open the measurement unit carefully. Loosen four hex-socket

screws size 5 and remove the cover with air duct. See Figure 72 on
page 115.
2. Carefully cut the transportation lock and remove it from the
measurement unit. Do not damage any of the module cables! See
Figure 73 on page 115.
3. Finally remove the transportation lock warning sheet and close the
measurement unit carefully. Pay attention to the position of the
cover seal.
4. Place the ladder to the side of the receiver mast. Select the side
where the protection hood points left.
5. Loosen the two fastening screws at the side of the measurement
unit flange (hex-socket, size 4). See Figure 74 on page 116.
6. Place the measurement unit on top of the inner mast tube (windows
pointing left). Do not use the air duct on top of the unit as a handle,
carry the measurement unit at the enclosure sides! Be aware of the
connection cable to be routed below the small junction box near the
blower.
7. Make sure that the measurement unit has completely settled on the
inner mast tube of the receiver mast. The air duct inlet on top of the
measurement unit must be placed in the same level than the outlet
of the blower. See Figure 75 on page 116.
8. Turn the measurement unit roughly into a parallel position with the
support unit (housing edges parallel to support unit outline). The
measurement unit must have a distance of approximately 8 mm to
all surrounding support unit parts. See Figure 76 on page 117.
9. Connect the measurement unit cable plug with the cable socket
from the support unit. Place the connectors at the rear of the
support unit right hand from the small junction box. See Figure 77
on page 117.
10. Place the ladder behind the receiver, check the parallel position
again and tighten the fastening screws (hex-socket, size 4) on the

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right side of the measurement unit which are now accessible

through the mast adapter of the support unit. See Figure 78 on page
118.

Connecting Signal Lines to Receiver Interface Unit

1. Open the interface unit box. See Figure 63 on page 107.
2. Wire the cables through the cable feed-throughs at the bottom of
the box.
3. Connect the cables that are shown circled in Figure 84 on page 126.
4. The receiver interface unit must be connected to the
two-wire RS-485 module bus (terminals 8 and 10) which connects
all intelligent modules within the LT31 system with each other. In
addition, a two-wire synchronization line (terminals 6 and 7)
connects the modulation clock between transmitter and receiver.

NOTE The cable shield must be properly grounded in the cable gland of the
interface unit enclosure, see Figure 35 on page 77

5. Tighten the cable feed-through carefully, wind the rest of the cable
length to a ring and secure it with standard cable tabs. For details,
see section Communication Cable on page 76.

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1104-107

Figure 84 Receiver Unit LTR111 Wiring

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Mounting and Connecting Optional

TERMBOX-1200 (Junction Box)
TERMBOX-1200 enables a separate connection of the underground
cables and incorporates enhanced overvoltage protection. The
TERMBOX is to be mounted below the interface unit enclosure and
enables short cable ways between foundation and cable feed-throughs.

WARNING The AC power must be switched off until all power line wiring is
finalized.

1. Open the TERMBOX-1200 cover plate.

2. Take the screws and the grounding cable out of the box.
3. Connect a short grounding cable (4 mm2) to grounding clamp at the
left bottom side of the box. See Figure 82 on page 121.
4. Screw the box onto the receiver mast below the interface unit. Use
screws from the delivery and hex socket driver size 5.
5. Connect the grounding cable carefully to the grounding bolt at the
base plate below the box. The wrench size is 17. See Figure 82 on
page 121.
6. Use a short 3-conductor power cable (3 x 1.5 mm2) with free wire
ends. Connect this power cable to the corresponding terminals
inside the TERMBOX. Use a fitting cable feed-through.

CAUTION Double-check that the AC power is switched off.

No voltage must be detected at the end of the underground power cable.

7. Connect the respective data terminals of the interface unit with the
recommended communication cable type, see section Cable
Selection on page 73. Prepare a cable of sufficient length to
interconnect the interface unit with the respective terminals inside
the TERMBOX.
8. Route the underground cable through a fitting cable feed-through
and connect the power lines carefully.
9. Proceed the same way with the underground communication cable
for the connection to the host computer system.
10. Tighten the cable feed-through carefully and close the cover plate.

The receiver interface unit is equipped with the master CPU LTC212.
This CPU is responsible for communication with all intelligent units
within the LT31 system and for the communication with the host

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computer system. For details on different interface types and settings, see
section Communication Options on page 78.

Connecting Communication and Power Cables

Connecting Communication Cable to Receiver Interface Unit

The receiver interface unit is equipped with the master CPU LTC212.
This CPU is responsible for communication with all intelligent units
within the LT31 system and for the communication with the host
computer system.

1. Connect the respective data terminals of the interface unit with the
recommended communication cable type, see section Cable
Selection on page 73. The communication cable should be either
connected into the Vaisala TERMBOX (see above) or routed
straight through the opening in the base plate of the mast and
further into a local junction box. Prepare a cable of sufficient length
to interconnect the interface unit with the local junction box.
2. Route the cable through a fitting cable feed-through at the bottom
of the interface unit.
3. For details, see section Communication Options on page 78.

Connecting Power Cable to Receiver Interface Unit

The power cable should be either connected into the Vaisala TERMBOX
(see above) or routed straight through the opening in the base plate of the
mast and further into a local power distribution box. Use a 3-conductor
power cable (3 x 1.5 mm2) of sufficient length with free wire ends. Make
sure to connect the wires in the correct way, especially the ones to the
protective ground (usually yellow/green). See Figure 84 on page 126 for
wiring instructions.

Switching on LT31 System

After the wiring work is finalized, the common power for the receiver
and transmitter location can be switched on and the power switches
inside the interface unit enclosures of the transmitter and receiver can be
switched on as well.

In case the battery backup LTBB111 is installed, the battery switches can
now be switched on as well on both locations.

The LEDs on the power supply of the interface unit must illuminate. See
Figure 85 on page 129 and Figure 86 on page 129.

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0401-146

Figure 85 Power Supply Switches

0808-005

Figure 86 Power Supply LEDs

The green LED on the master CPU LTC212 inside the interface unit of
the receiver must flash. See Figure 87 on page 130.

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0806-038

Figure 87 Master CPU LTC212 LEDs

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The LEDs behind the right window of the measurement units of LT31
transmitter and receiver must flash. See Figure 88 below.

0401-123

Figure 88 Measurement Unit LEDs

At transmitter location, the white transmitter light must be observable

through the left window.

WARNING LASER class 1 product; do not stare directly into the bright light beam.

Double-checking Transmitter Alignment

1. Place the ladder behind the receiver mast.
2. The main bright transmitter light beam must be observable in the
area of the receiver head.
3. Should the transmitter light not be visible, the transmitter has to be
realigned.
4. Check if the transmitter head has to be turned or tilted and in which
direction.
5. Go back to the transmitter location and correct the alignment
accordingly.
6. Do not forget to tighten all the fastening screws carefully.

If all parts seem to be operational, you can continue from section Startup
Procedure on page 132.

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Startup Procedure

LT Maintenance Interface

Connection to Maintenance Terminal

Any computer equipped with terminal emulation software or a VT100-
compatible terminal with an RS-232 serial interface can be used as a
maintenance terminal for LT31. The maintenance cable QMZ101
(included in the delivery) provides a 9-pin D connector for the computer
and a connector for LT31. The external maintenance connector of LT31
is located at the right side of the receiver interface unit (see Figure 89
below). The connector is protected with a cap. Replace the protective cap
after disconnecting the maintenance cable. LT maintenance interface is
shown in Figure 90 on page 133.

0401-124

Figure 89 External Maintenance Line Connector

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0905-005

Figure 90 LT Maintenance Interface

The maintenance cable for Vaisala MILOS 500 can also be used, but in
this case the door of the receiver interface unit LTI211 must be opened
and the original maintenance line connector in the LTC212 board
disconnected (see Figure 91 on page 134). Note that the data line for RS-
485 connection is located in the same connector causing it also to be
disconnected when the maintenance line connector is removed.

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Reset
Status
RXD
TXD

Reset Button

1
2
3
4
5 +12V Out
6
7
8 GND
9 VB+
Sensor 10 VB-
line 11 RS-485+
12 RS-485-
13 CTS
Data 14 RTS
line PC
15 RXD
RS-232
16 TXD
17 GND D-9 pin

Data 18 RS-485+
line 19 RS-485- 2 RXD
RTS 7
Maint. 20 RXD TXD 3 TXD
CTS 8
line 21 TXD RXD
RS-232 22 GND GND 5 GND
23 MOD1 RS-232
Module Communication
24 MOD2
line
(modem) 25 MOD3
(modem) 26 MOD4
27 MOD5

0401-120

Figure 91 Maintenance Line connector on LTC212

Settings for Maintenance Terminal

Select the VT100 emulation mode. Set the transmission rate of the
terminal to 9600 bps and the data frame to contain 8 data bits, 1 stop bit,
and no parity.

Entering and Exiting Command Mode

Before any commands can be given to LT31, the maintenance or data
line of LT31 has to be assigned to the operator. Otherwise, it is assigned
to automatic messages or polled communication.

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OPEN Command
Users enter the command mode with the OPEN command.

Type the following:

open

If an ID is configured, use command:

open *

If no ID is configured, the LT31 sensor answers as follows:

0> LT31 line opened for operator commands

If no input is given within 10 minutes (default), LT31 closes the line

automatically and gives the following answer:
LT31 line closed

CLOSE Command
The line can be released to automatic data messages with the CLOSE
command.

If no ID is configured, LT31 answers as follows:

LT31 line closed

For details, see Chapter 5, Operation, on page 145.

Initial Settings
Vaisala Transmissometer LT31 is typically interfaced to a host computer
or a data logger in an automatic weather observation system. After the
physical connection has been made, the details of the communication can
be configured into the LT31 software. Suitable communication settings
depend on the implementation of the entire system.

By default the sensor transmits a new ASCII data message through the
data port in RS-232 mode every 10 seconds. The user can change the
interval and message type. The sensor can also be used in a polled mode,
in which a data message is only sent when the host computer requests
one with a special command. The default settings are listed in Table 9 on
page 136.

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Table 9 Default Settings

Setting Default
Transmission rate 9600 bps
Communication parameters 8N1
Mode RS-232
Polled or automatic mode, Automatic mode, message 2, interval
message type 10 seconds
Message port Data
Baseline 30 m
Sensor ID No ID set

In multipoint communication where several sensors share the same

communication line (RS-485/Modem), LT31 must be used in the polled
mode and the individual sensors must have distinct identifiers (ID).

You can check/set the basic configuration parameters of the LT31 in an

easy way. Select the maintenance menu by first typing the OPEN
command as explained above and then the MENU command. See Figure
92 below.

0803-001

Figure 92 Main Menu

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Pressing A on the keyboard gives access to the configuration parameters

of LT31. Figure 93 below shows the default configuration.

0401-127

Figure 93 Configuration Menu

To change settings, press the related letter in brackets on the keyboard.

The menu will then offer either an item list or a free-form input for the
desired changes. For detailed explanations and instructions see Chapter 5,
Operation on page 145.

Note the following remarks:

- The Identifier parameter is * only if one LT31 is connected on the

data line and no ID is needed.
- The Baseline parameter must be set according to the real installation
situation. The default is 30.
- In terms of the system, change LM sensor to the On position, if
Background Luminance Sensor LM21 is installed.
- In terms of the system, PWD Sensor must be switched on for normal
operation including auto-calibration. PWD Sensor should only and
exclusively be switched off, if auto-calibration is not allowed.
- Message type 1 is the standard LT31 message. If the PW data option
was ordered, message type 2 is configured by default. For details on
other messages and descriptions, see Chapter 5, Operation on page
145.
- The standard Interval of a message is 10 seconds. If connected to a
MIDASIV system, 15 seconds must be configured.
- For messages, the standard Port is data for communication with a host
system. To check the data message, the port can be temporarily
changed to Maintenance. If the optional DMX501 modem is installed,
the port has to be configured to module.

NOTE Normal communication will be interrupted during this intervention!

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- For the data port, the Speed and Format must correspond with the
peripheral equipment.
- For the data port, Mode has to be selected according to the used
interface connections on the LTC212 (RS232 or RS485).
- Configure the desired modem Mode in the module section if the
optional DMX501 modem is installed.

NOTE To exit the MENU pages, press ESC.

Checking Operation
Pressing B on the keyboard on the MAIN MENU page gives access to
the LT31 System Status page (see Figure 94 below).

0401-131

Figure 94 System Status

TRANSMITTER must indicate OK in all positions.

RECEIVER must indicate OK in all positions.

PWD Sensor must indicate OK.

LM Sensor must indicate OK, when installed (optional).

SPECIALS are to be indicated as OK after the final alignment and the

calibration were successfully carried out. For details on fine-alignment
and calibration, see sections Fine Alignment on page 231 and Section
Calibration on page 232.

NOTE To exit the MENU pages, press ESC.

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NOTE Use the CLOSE command after the configuration and operation check
is finalized.

Final Alignment
During the assembly as explained in Chapter 4, Installation on page 67,
the LT31 receiver and transmitter have already been roughly aligned in
relation to each other. To optimize this alignment, an acoustically
supported, menu-guided, automatic fine-alignment is a standard LT31
procedure.

CAUTION Final alignment must be carried out at this point of the LT31
installation.

For details on the final alignment, see section Alignment on page 225.

NOTE Always select Raw and Fine Alignment mode for the final alignment
during installation.

Calibration
As the achievable measurement signal depends on the individual
tolerances of the opto-electronic systems and the alignment result itself,
the measurement signal has to be correlated to the prevailing visibility to
bring the Transmissometer into a final operational condition. This
process is carried out in the frame of the LT31 Calibration. The LT31
Calibration Procedure is menu-guided.

CAUTION Calibration must be carried out at this point of the LT31 installation. A
sufficient fine-alignment quality is required in any case.

To do the calibration, follow the instructions in section Calibration on

page 232.

NOTE Always select the Base Calibration mode for the initial calibration
during installation.

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Finalizing Installation
After all the installation steps have been proceeded and the fine-
alignment as well as the calibration have been carried out successfully,
the LT31 system can be finally prepared for operational use. When the
fastening screws for support units and measurement units are carefully
fixed, the measurement head covers can be installed to LT31 receiver and
transmitter.

Mounting Measurement Head Cover

When the measurement head cover is attached to the support unit, a
special fast locking mechanism is utilized to secure the hood in its
intended position using six lock screws. These screws are equipped with
an ejector spring in order to achieve easy removal of the measurement
head cover. The placement of the screws is indicated in Figure 95 below.

0904-025

Figure 95 Lock-Screws at Measurement Head Cover

The counterpart of the lock screws are part of the support unit and the
lock screws can be fastened by a quarter turn using a suitable
screwdriver. One of the receptacles inside the measurement head cover is
shown in Figure 96 below.

0905-016

Figure 96 Receptacle Inside the Measurement Head Cover

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To mount the measurement head cover, follow these instructions:

1. Place the ladder to the left and right sides of the instrument for best
access to the three lock screws at each side of the cover.
2. Lower the measurement head cover carefully on top of the support
unit whereas the six lock screws have to slide into the associated
gaps in the support unit.
3. The six lock screws have to be fastened one by one as follows:
a. Use a screwdriver of suitable size and align the lock screw so
that the screw slot is in an upright position which ensures that
the screw stud fits into the associated receptacle of the
support unit (see Figure 97 a below).

NOTE The figure shows the basic mechanism only; no ejector spring or sheet
metal items are shown.

b. Insert the lock screw into the associated receptacle while

keeping the screw slot upright (see Figure 97 b below).

CAUTION Be sure to insert the lock screw into the associated receptacle only with
the correct orientation. Otherwise the receptacle mechanism might be
damaged!

c. When the lock screw is completely inserted and the sheet

metal of the measurement head cover is pressed against the
support unit, the screw is to be fastened by a quarter
clockwise turn (see Figure 97 c below).

0906-096

Figure 97 Using Lock Screws at Measurement Head Cover

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To remove the measurement head cover, follow these instructions:

1. Loosen the fastening screws carefully by a quarter

counterclockwise turn. Due to the included ejector the lock screws
will easily slide out from the associated receptacle.
2. The measurement head cover has to be lifted up before it can be
removed from the support unit.

Closing Interface Units

When all wiring activities are finalized and the cable feed-throughs are
carefully tightened, the interface units can be closed.

The interface unit enclosure is equipped with two locks. The unit locks
are shown in Figure 98 below. A fitting key is part of the delivery.

0401-104

Figure 98 Interface Unit Locks

Mounting Interface Unit Covers

After the interface unit has been carefully closed, the interface unit
covers can be installed.

The lower interface unit cover (see Figure 99 on page 143) is to be

fastened with the four mushroom head screws which are part of the
delivery.

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0401-107

Figure 99 Lower Interface Unit Cover

The upper interface unit cover can be mounted without any tools.

Hook-in the upper interface unit cover by lowering the cover vertically so
that the screws in the cover slide into the corresponding notches of the
mast construction. See Figure 100 below.

0906-091

Figure 100 Hooking-in Upper Interface Unit Cover

Then press the upper cover part carefully against the mast construction.
The whole cover is now in the final locked position. Tighten the screws
on both sides of the lower part of the unit cover. See Figure 101 on page
144.

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0906-092

Figure 101 Upper Interface Unit Cover

Disassembly for Transportation

If the LT31 system or a single LT31 transmitter or receiver has to be
transported to a different location, the instrument has to be disassembled
to the same degree as during delivery and shipment.

NOTE Only in a disassembled state, the LT31 system can be protected from
damages due to transportation shocks.

1. Remove upper and lower interface unit covers.

2. Remove measurement head cover.
3. Remove measurement unit and store it carefully in a separate box.
4. Remove weather protection hood.
5. Store interface unit covers, measurement head cover, weather
protection hood and separately packed measurement unit together
with the mast in the original transportation box.
6. Remove PWD Forward Scatter Sensor and store it carefully in the
provided box.
7. Remove optional LM21 with support arm and store it carefully in a
separate box.
8. Remove optional LT31OBS and store it carefully in a separate box.

For an illustration of the disassembled state, see Figure 46 on page 93

and Figure 65 on page 109.

NOTE Use the original boxes from the delivery during the transportation of the
equipment.

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CHAPTER 5
OPERATION

This chapter contains information that is needed to operate this product.

Vaisala Transmissometer LT31 is a fully automatic instrument that does

not need any regular operation by the user. The measurement parameters
set at the factory do not usually need to be changed.

LT31 visibility messages are either send automatically or polled by the

host computer.

The LT31 system parameters can be set with the SET command or by
menu mode which is entered by the MENU command in the command
line mode. Other commands can be used to display the system status and
other maintenance data. All commands can be given either via the
maintenance line or data line. Commands given via the maintenance line
do not interfere with the message sending of the data line visibility.

LT31 Commands
NOTE All command parameters must be separated from each other by a space
character and every command must end with pressing ENTER.

Entering and Exiting Command Mode

Before any commands can be given to LT31, the maintenance or data
line of LT31 has to be assigned to the operator. Otherwise, it is assigned
to automatic messages or polled communication.

OPEN Command
You enter the command mode with the OPEN command.

If no device identifier (ID) is defined, type the following:

OPEN

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If an ID is defined, for example as A, type the following:

OPEN A

If an ID is defined but forgotten, type the following:

OPEN *

NOTE The ID can be alphanumeric.

An ID is not needed if the device type LT is included in the OPEN

command. This device type can be used when there is only one LT31 on
the line. This command is similar to the OPEN * command with the
exception that it only opens the LT31 command mode and not the other
possible sensors on the line. The following is an example of the
command.
OPEN LT

If there are other sensors on the line with the same ID, the command
mode of LT31can be opened by including the device type LT with the
ID. This command is similar to the OPEN ID command with the
exception that it only opens the LT31 command mode and not the other
sensors on the line with the same ID. If an ID is defined, for example as
1, type the following:
OPEN LT 1

The LT31 sensor with the ID 1 answers as follows:

LT31-1 LINE OPENED FOR OPERATOR COMMANDS
0>

If no input is given within 10 minutes (default), LT31 closes the line

automatically and gives the following answer:
LT31-1 LINE CLOSED

The user can select the time-out time.

Also, if LT31 receives an OPEN command targeted to another device

while in the command mode, the line for the first opened device will be
closed (the CLOSE command executed).

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CLOSE Command
The line can be released to automatic data messages with the CLOSE
command.

Type the following:

0> CLOSE

The LT31 sensor with the ID 1 answers as follows:

LT31-1 LINE CLOSED

Available Commands
With the HELP command the operator can get information about the
available commands. By typing the HELP string (where string is any
valid command), the operator can get information about a specific
command.

The hierarchy and description of the commands are described in Table 10

on page 148. There is a user level command set and also a second,
advanced, in-depth maintenance and service level command set. To
access the advanced level, use the LEVEL 1 command. The command
prompt shows the current command set level. For the user level it is 0>
and for the advanced level it is 1>.

Command Completion
The operator only needs to remember the first few letters of the
command and the system can automatically identify the command and its
parameters. This can be done if the letters the user enters can
unambiguously determine the command or parameter.

If the command cannot be unambiguously identified, the system output is

as follows:

- If the command cannot be completed to any known command the

system answers the following:

COMMAND NOT FOUND

- If the input entered by the user cannot be unambiguously identified as
a command, the system outputs a list of matching commands. The
new system prompt includes the user's previous input allowing the
user to complete the command using the list of matching commands.
- If the command entered by the user is identified but the parameters are
incomplete, unambiguous, or otherwise not possible, the system

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outputs a help text. The new system prompt includes the user's
previous input up to the first parameter requiring clarification. This
allows the user to complete the command with the correct parameter.
The help text contains explanations for only those parameters that
match the operator's input.

Table 10 User Level Commands

Command Description
CLOSE Releases the port for message transmission.
DUMP Prints out a snapshot of the current instrument system
situation and configuration.
ECHO
ECHO OFF Disables input character echo. Set off when operating with
the RS-485 connection.
ECHO ON Enables input character echo (default)
EVENTS Gives the events logged by the system. It gives for each
event listed the following information:
- the time of occurrence backward from now by
years, months, days, hours, minutes, and
seconds
- the time of occurrence in operating seconds
- the type ( S - single, B bad, or G - good) and
the event text is given
EVENTS page The list is given by separate pages. The page length is
configured by SET TERMINAL_LINES.
EVENTS string The event list is reduced to event which event text contains
string.
EVENTS string page The reduced event list is given by separate pages.
HELP Lists available command.
HELPcommand Shows information about a specific command.
LEVEL 1 Changes the operation level to the advanced level.

or The password can be given as a parameter. If it is not given

as a parameter, the user is asked for it.

LEVEL 1 [password] The default password for the advanced level is LT31 (case
sensitive).
MEAS_SYNC Resets the measurement sequence. The following data
message is measured starting from the moment all the
average values are erased. The next message is transmitted
in seconds (interval +2).
MENU Starts the menu-driven operation. For a proper display, the
terminal application must be set to VT100 emulation. For
proper operation, the terminal height in lines used by the
terminal application must be configured to LT31 with the
command SET TERMINAL_LINES number (minimum 24).
Also, any 'line wrap'- and 'translate CR to CR/LF' -option
must be switched off for the terminal application in use.
MESSAGE Displays the default message without transmission frames.
MESSAGE ALL LT31 displays all messages.
MESSAGE message_number LT31 displays the selected message with transmission
frames.
NAME Displays the device type, device name given by the user,
and the ID.
PARAMETERS Prints configuration parameters
SET
SET DATA_PORT BAUD number yes/no Sets data port serial line transmission rate to 300 ... 19200.
Verification yes/no asked only if the command is given
through DATA PORT (default 9600).

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Command Description
SET DATA_PORT MODE RS-232 Uses the RS-232 serial line, where the hardware flow control
HW_FLOW_CNTR yes/no RTS and CTS lines are used. Verification yes/no asked only
if the command is given through DATA PORT.
SET DATA_PORT MODE RS-232 Uses the RS-232 serial line where no flow control is used.
NO_FLOW_CNTR yes/no Verification yes/no asked only if the command is given via
DATA PORT (default).
SET DATA_PORT MODE RS-485 Uses the RS-485 serial line. Verification yes/no asked only if
yes/no the command is given through DATA PORT.
SET DATA_PORT PARITY 7E1 yes/no RS-232 and RS-485 serial lines use the following
communication parameters: 7 data bits, even parity, 1 stop
bit. Verification yes/no asked only if the command is given
through DATA PORT (default).
SET DATA_PORT PARITY 8N1 yes/no RS-232 and RS-485 serial lines use the following
communication parameters: 8 data bits, no parity, 1 stop bit.
Verification yes/no asked only if the command is given
through DATA PORT.
SET MAINTENANCE_PORT BAUD Sets the Maintenance port serial line transmission rate to
number 300 ... 9600. Communication parameters: 8N1 (fixed).
SET MESSAGE INTERVAL number Sets the interval in seconds for message sending. Zero
ignores the automatic sending.
SET MESSAGE PORT DATA Message is directed to the Data port (default)
SET MESSAGE PORT MAINTENANCE Message is directed to the Maintenance port
SET MESSAGE PORT MODULE Message is directed to the optional communication module,
for example modem module
SET MESSAGE TYPE FD12MSG2 Selects the FD12 message 2 emulation mode. The data
message, message frames, and polling format are the same
as they are in Vaisala FD12 Visibility Meter.
SET MESSAGE TYPE FD12MSG7 Selects the FD12P message 7 emulation mode. The data
message, message frames, and polling format are the same
as they are in Vaisala FD12P Weather Sensor.
SET MESSAGE TYPE message_number Selects the transmitted message type
SET MESSAGE TYPE MITRAS Selects the MITRAS emulation mode. The data message,
message frames, and polling format are the same as they
are in Vaisala MITRAS Transmissometer.
SET MESSAGE TYPE SKOPO2 Selects the SKOPOGRAPH II Flamingo emulation mode.
The data message, message frames, and polling format are
the same as they are in Impulsphysik SKOPOGRAPH II
Flamingo.
SET MODULE MODEM V21 300 bps modem mode, ANSWER mode and no carrier on
ANSWER_NC standby. The carrier is on only during message sending.
Verification yes/no asked only if the command is given
through MODULE PORT.
SET MODULE MODEM V21 ANSWER 300 bps modem mode, ANSWER mode. Verification yes/no
asked only if the command is given through MODULE
PORT.
SET MODULE MODEM V21 ORIGINATE 300 bps modem mode, ORIGINATE mode. Verification
yes/no asked only if the command is given through
MODULE PORT.
SET MODULE MODEM V22 ANSWER 1200 bps modem mode, ANSWER mode. Verification
yes/no asked only if the command is given through
MODULE PORT.

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Command Description
SET MODULE MODEM V22 ORIGINATE 1200 bps modem mode, ORIGINATE mode. Verification
yes/no asked only if the command is given through
MODULE PORT.
SET MODULE MODEM V22BIS 2400 bps modem mode, ANSWER mode. Verification
ANSWER yes/no asked only if the command is given through
MODULE PORT.
SET MODULE MODEM V22BIS 2400 bps modem mode, ORIGINATE mode. Verification
ORIGINATE yes/no asked only if the command is given through
MODULE PORT.
SET MODULE MODEM V23 BACK_FDX 75 bps modem mode, Back Channel, Full Duplex.
Verification yes/no asked only if the command is given
through MODULE PORT.
SET MODULE MODEM V23 BACK_HDX 75 bps modem mode, Back Channel, Half Duplex.
Verification yes/no asked only if the command is given
through MODULE PORT.
SET MODULE MODEM V23 MAIN_FDX 1200 bps modem mode, Main Channel, Full Duplex.
Verification yes/no asked only if the command is given
through MODULE PORT.
SET MODULE MODEM V23 MAIN_HDX 1200 bps modem mode, Main Channel, Half Duplex.
Verification yes/no asked only if the command is given
through MODULE PORT.
SET NAME name Sets name, an alphanumerical string that can be used
during installation to identify for example unit location. The
string must not include spaces.
SET PORT_TIMEOUT number Sets the command mode time-out to 0 ... 30, in minutes.
Zero disables the time-out and the default is 10 minutes.
SET TERMINAL_LINES number Adjusts printouts used for terminal screen. Minimum 5 lines
(default 24).
SET UNIT_ID id Sets unit identifier: An alphanumerical character for
message and polling identification. A '-' character removes
the ID. By default no ID is set and space character is used in
the message headings.
STATUS Prints the status message.
STATUS CHECK Prints a short status check message.
STATUS QUALITY Prints information about window contamination, alignment
quality, and last time of autocalibration.
SYSTEM Gives system information: type, ID, software version,
hardware modules, serial numbers.
VERSION Gives the software version.
WORKINGHOURS Gives the elapsed time of powered operation in hours and
seconds. The time is given also in elapsed years, months,
days, hours, minutes and seconds up to now.

To access the advanced level, type LEVEL 1 password (default LEVEL

1 password: LT31). The password is case sensitive.

Table 11 Advanced Level Commands

Command Description
CALIBRATE
CALIBRATE FORCED_AUTOCAL Performs autocalibration. Most conditions for normal
autocalibration are neglected, except proper operation of
PWD and transmissometer hardware.
CALIBRATE WINDOW_CLEAN LM Sets the clean references for contamination control (and
backscatter control) in the background luminance sensor
LM21.
CALIBRATE VISIBILITY observer_ MOR Starts base calibration in command line mode.
When observer_MOR is given, it is used for visibility
calibration.

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Command Description
CALIBRATE TEMPERATURE EXTERNAL Makes single point calibration with the external
value temperature sensor connected to the interface unit, the
value should be between -99 and 99.
EVENTS evt A forced event is inserted into the event list.
LEVEL 0 Changes the operation level to the user level.
PASSWORD string Change password of advanced level.
RESET yes/no Hardware reset by watchdog circuitry, verification yes/no
asked.
SET
SET AC_STATUS_SUMMARY OFF Indication of autocalibration in the status word (refer to
Table 17 on page 166) is given for the time 5 seconds
prior to the message reporting (default).
SET AC_STATUS_SUMMARY ON Indication of autocalibration in the status word (refer to
Table 17 on page 166) is given for the time since the
last message reporting.
SET BASELINE number Configures the LT31 system base length in meters.
SET BL_RANGE minimum_BL maximum_BL Under and overrange values for BL readings are set. The
settings will be taken into account for SKOPO2
emulation only.
SET BLOW_ON_WMO number Whenever PWD codes a WMO code greater than or
equal to number, the blowers will be switched on for at
least five minutes.
SET LM ON Enables background luminance sensor LM21 data
polling in the LT31 system.
SET LM OFF Disables background luminance sensor LM21 data
polling and photo switch reading in the LT31 system
(default).
SET CONTAMINATION_COMPENSATION Disables the compensation function of LM21.
LM OFF

SET CONTAMINATION_COMPENSATION Enables measurement value compensation caused by

LM ON window contamination in background luminance sensor
LM21 (default).
SET DEFAULTS yes/no Restores the default factory settings, verification yes/no
asked.
SET DEW _HEATER LM OFF Disables dew heaters operation and monitoring of LM21.
Set OFF when operated in environments where dew
heaters of LM21 are not needed (ambient temperature
does not drop below 12 C).
SET DEW _HEATER LM ON Enables the monitoring and automatic operation of the
dew heaters of LM21 (default). The dew heaters start to
warm up optical surfaces when ambient temperature
drops below a preset level.
SET HEADER string The Two-Character-Header can be configured (default:
LT)
SET HEAT_ON_BATTERY YES The heaters of the measurement units will be switched
on, when working on battery. (Only available when the
battery backup option is available).
SET HEAT_ON_BATTERY NO The heaters of the measurement units will be switched
off when working on battery. (Only available when the
battery backup option is available.)
SET HOOD_HEATERS LM OFF Disables LM21 hood heater operation and monitoring.
Set OFF when operated in environments where hood
heaters are not needed and the sensor is only operated
with DC power.
SET HOOD_HEATERS LM ON Enables the monitoring and automatic operation of the
LM21 hood heater (default). The hood heaters are turned
on by the unit when the hood temperature drops below a
preset level.

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Command Description
SET INTERNAL_FILTER number Sets filter value of the internal filter. When an internal
filter is applied (for example for installations with baseline
length of less than 30 m), it is necessary to set the
internal filter value accordingly. Otherwise the displayed
SIGNAL values in Alignment Menu and Calibration Menu
will be incorrect.
Example: Configure an internal filter with 56 %
transmission respectively an attenuation factor of 0.56
with the command SET INTERNAL_FILTER 0.56
SET MESSAGE SECOND Selects the message type to be transmitted as the
TYPEmessage_number Second Message.
SET MESSAGE SECOND TYPE MITRAS Selects the MITRAS Single Base emulation mode for the
Second Message. The data message and message
frames are the same as they are in Vaisala MITRAS
Transmissometer.
SET MESSAGE SECOND TYPE Selects the FD12 message 2 emulation mode for the
FD12MSG2 Second Message. The data message and message
frames are the same as they are in Vaisala FD12
Visibility Meter.
SET MESSAGE SECOND TYPE Selects the FD12 message 7 emulation mode for the
FD12MSG7 Second Message. The data message and message
frames are the same as they are in Vaisala FD12P
Weather Sensor.
SET MESSAGE SECOND TYPE SKOPO2 Selects the SKOPOGRAPH II Flamingo emulation mode
for the Second Message. The data message and
message frames are the same as they are in
Impulsphysik SKOPOGRAPH II Flamingo.
SET MESSAGE SECOND PORT DATA The Second Message is directed to the Data port.
SET MESSAGE SECOND PORT The Second Message is directed to the Maintenance
MAINTENANCE port.
SET MESSAGE SECOND PORT MODULE The Second Message is directed to the optional
communication module, for example, modem module.
SET MESSAGE SECOND PORT NONE No Second Message is given to any port (default).
SET METAR OFF Disables METAR messaging (default).
SET METAR ON Enables METAR messaging. without framing
SET METAR 8 Enables METAR messaging. with framing
SET PWD OFF Disables PWD data polling in the LT31 system.
SET PWD ON Enables PWD data polling in the LT31 system.
SET PWD_MOR_ONERROR [ON|OFF] The current status is reported by set pwd_mor_on_error.
The response is PWD MOR is (NOT) used on error.
To disable the use of PWD MOR in error situations use
the command set pwd_mor_on_error off. The response
would be PWD MOR is NOT used on error.
To enable the use of PWD MOR in error situations
(default) use the command set pwd_mor_on_error on.
The response is PWD MOR is used on error.
SET PWD_PITCH number During auto-calibration, derived PWD MOR data will be
corrected by a factor with the value of number. To take
PWD data as it is, number has to be set to 1.000
(default).
SET MESSAGE FRAME OFF Message frames not included in data messages.
SET MESSAGE FRAME ON Message frames included in data message transmission
(default).
SET MOR_RANGE minimum_MOR Under and overrange values for MOR readings are set.
maximum_MOR Values are to be given in meters. The absolute minimum
value is 10. The absolute maximum value is 10 000. The
minimum value is set according to the baselength. A
baselength up to 35 m will allow underrange values
down to 1/3 of the baselength. A baselength above 35 m
will allow underrange values down to 1/2 of the
baselength.

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Command Description
SET PW_OPTION string A valid keyword for string will enable the PW option
(Message 2). An invalid keyword will disable the PW
option. To verify the status of the PW option, give the
SYSTEM command.
SET +12VOUT OFF Disables +12 V output on LTC212 board (default).
SET +12VOUT ON Enables +12 V output on LTC212 board.
SIMULATE
SIMULATE MANUAL_MESSAGE string User definable data message for system testing
purposes.
SIMULATE OFF Disables simulation message sending and returns to
normal operation mode.
SIMULATE SEQUENCE Data for MOR, overall status, background luminance
status will be taken from simulation sequences defined
on the Simulation Menu.
SIMULATE TEST_MESSAGE Sets the fixed, predefined data message as the
transmitted message type for system testing purposes.
See section Fixed Test Messages on page 187.

Terminal Line Adjustment

The printout of the command response can be adjusted to the screen size
of the used terminal. This feature stops the printout every time the
terminal screen is full. This is especially useful with the small screens of
palmtops. Without this feature the user will only see the bottom lines of
the long command responses such as the STATUS message.

The terminal size adjustment is done using the following command:

SET TERMINAL_LINES number

The standard PC terminal program has 24 visible lines. This is also the
default value for the parameter. The minimum number of lines is 5. This
parameter has no effect on the message sending when the commands are
given in the command polling format.

Command Polling
All commands can also be executed without opening the command line.
This feature can be used in the host system, for example, for making
automatic status message inquiries of active warnings or alarms.

The general command polling format (with 1 as the unit ID) is the
following:

!LT1"&DO_COMMAND-#CSUM$-*

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where
! = Start of heading
" = Start of text
&DO_ = ID for command polling (the '_' indicates space)
COMMAND = Command name
- = Carriage return
# = End of text
CSUM = Checksum
$ = End of transmission
* = Line feed

The response for the command is framed similarly, see below:

!LT1"LT31-RESPONSE#CSUM$-*

where
! = Start of heading
LT1
" = Start of text
LT31- = Response of LT31
RESPONSE
# = End of text
CSUM = Checksum
$ = End of transmission
- = Carriage return
* = Line feed

For instance, the following command

!!LT1"&DO_STATUS-#3D64$-*

prints the status messages in frames (equal to message 3), whereas the
command below

!LT1"&DO_MEAS_SYNC-#7357$-*

resets the measurement sequence.

The following command

!LT1"&DO_NAME-#A689$-*

displays the device type, device name given by the user, and the ID.

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CRC16 Checksum
The CRC16 checksum can be calculated using the following algorithm
written in the programming language C:
/* 16 bit type */
typedef unsigned short Word16;

/* Calculate CRC-16 value as used in LT31 protocol */

Word16 crc16(const unsigned char *buf, int len)

{
Word16
crc;
int
i,
j;

crc=0xffff;

for (i=0;i<len;++i)
{
crc^=buf[i]<<8;

for (j=0;j<8;++j)
{
Word16
xmask=(crc&0x8000)?0x1021:0;

crc<<=1;
crc^=xmask;
}
}

return crc^0xffff;
}

The calculation of the checksum starts after the ! (Start of Heading,

ASCII 1) character and ends after the # (End of text, ASCII 3) character.

Checksums should be used for proper operation. However, giving XXXX

(four times X [ASCII 88DEC/58HEX]) will skip checksum testing in the
LT31 system.

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Menu-driven Operation
Operational actions which are necessary for installation, such as,
configuration, alignment, calibration, maintenance, and troubleshooting,
can be undertaken in the so called menu-driven mode. You can enter this
mode by typing the MENU command at the user or advanced level.

NOTE The menu-driven operation is designed for VT100 terminal with 80

columns. For proper operation, the terminal height in lines used by the
terminal application must be configured to LT31 with the command
SET TERMINAL_LINES number (minimum 24). In addition, any 'line
wrap'- and 'translate CR or CR/LF'-option must be switched off for the
terminal application in use.

The Main Menu is shown in Figure 102 below.

0803-001

Figure 102 Main Menu

The following characters refer to Figure 102 above.

A = Enter Configuration menu
B = Enter System Status menu
C = Enter Alignment menu
D = Enter Calibration menu
E = Enter Linearity Test menu
F = Enter Simulation menu
G = Enter PWD Sensor menu
H = Enter Service menu
I = Enter Events menu

Entering submenus is done by using one-character commands. The valid

one-character commands are listed in each menu by a character enclosed

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in round brackets ((A), (B), ...). Leaving a submenu, aborting actions, or

leaving the menu-driven operation itself is done by pressing ESC once.

Submenus

Configuration
The Configuration Menu is shown in Figure 103 below.

0401-127

Figure 103 Configuration Menu

The following characters refer to Figure 103 above.

A = Configure header (analog to SET HEADER)
B = Configure identifier (analog to SET UNIT_ID)
C = Configure baseline length (analog to SET BASELINE)
D = Configure PWD sensor (analog to SET PWD)
E = Configure LM sensor (analog to SET LM)
F = Configure message type (analog to SET MESSAGE TYPE)
G = Configure message interval (analog to SET MESSAGE
INTERVAL)
H = Configure message port (analog to SET MESSAGE PORT)
I = Configure data port transmission rate (analog to SET
DATA_PORT BAUD)
J = Configure data port mode (analog to SET DATA_PORT
MODE)
K = Configure data port format (analog to SET DATA_PORT
PARITY)
L = Configure modem (analog to SET MODULE MODEM)

Selecting any of the above items will open either an item list or free-form
input.

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Item List
In Figure 104 below, the configuration is completed by choosing an
offered speed from the list by typing the appropriate character, in this
case, E for 9600 bps.

0401-128

Figure 104 Configuration Menu: Data Port Speed

Free-form Input
In Figure 105 below the configuration is completed by giving a valid
value for the message interval. For example, 15 for 15 s automatic
message interval or 0 for disabling automatic message interval (polled
mode).

0401-129

Figure 105 Configuration Message Interval

System Status
The System Status menu (see Figure 106 on page 159) gives the
possibility to have a quick overview into the entire LT31 system. All

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available status messages for the transmitter, receiver, PWD sensor, LM

sensor, and system related objects are given.

0401-131

Figure 106 Status Menu

In Figure 106 above all subsystems indicate status OK. MOR is reporting
an INDICATION (2) which has the meaning of MOR overrange (refer to
Table 15 on page 165).

Alignment
The Alignment menu offers a two step alignment. Raw and fine
alignment can be started or fine alignment separately. For details, see
section Alignment on page 225.

Calibration
The Calibration menu offers two different calibration procedures: offset
and visibility calibration or visibility calibration only. For details, see
section Calibration on page 232.

Linearity Test
The Linearity Test menu supports the user in performing a linearity test.
For details, see section Linearity Test on page 245.

Simulation
The Simulation menu offers the definition of simulation sequences. For
details, see section Simulation Sequences on page 189.

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PWD Sensor
The PWD Sensor menu offers a subset of PWD sensor commands. The
commands can be executed through the LT31 maintenance terminal
connection. For details, see section PWD Sensor on page 253.

Service
The Service menu offers an in depth view to the transmitter and receiver
units. Raw data values are shown and a set of direct impact commands
are available. If the MENU command has been given at the user level,
the possibilities are restricted. Giving the MENU command at the
advanced level, no restrictions are made. For details, see section Service
Menu on page 250.

Events
In the event log you are asked whether you want displayed a full list with
all entries or a reduced list, whose entries contain a given text.

In Figure 107 below, a reduced list is given with entries with the word
ALIGN chosen.

0401-133

Figure 107 Event Log

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As shown in Figure 108 below, each line contains one event log entry.
Each event holds a time stamp, qualifier, and the event text.

0401-134

Figure 108 Event Log: Reduced List

Table 12 below explains some of the log contents.

Table 12 Event Log Entry

Log Content Explanation
-0y0m0d04:16:33 From the current moment, the event
happened
0 years,
0 months,
0 days,
4 hours,
16 minutes and
33 seconds ago
31031436 The event occurred after 31 031 436 seconds
of powered operation.
S An event can be
Single,
Bad, or
Good.
A single event will be logged each time the
event occurs.
If an event describes a good/bad event,
successive good/bad situations will be logged
by the first good/bad situation only. By this,
only a change from good to bad and vice
versa will be logged.
A9:Alignment... The event text describes the event

Message Formats
In the command mode (entered with the OPEN command), the LT31
messages can be displayed using the MESSAGE command. In the
independent mode (entered with the command CLOSE), LT31 can be

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polled or it can send predefined messages automatically at selected

intervals. There are eight message formats to select from. As a default,
LT31 adds frame strings to all polled and LT31-specific, automatic
messages. The polling request can ask for the automatic (default) or some
other message (with a message number).

When used as a part of an existing RVR system, LT31 can emulate the
messages of Vaisala MITRAS Transmissometer, FD12 Visibility Meter,
and Impulsphysik SKOPOGRAPH II Flamingo. To find out which
message types can be emulated, see sections Message 5 and 6 - FD12
Emulation on page 177, Message 7 - MITRAS Emulation on page 188,
and Message 8 - Flamingo Emulation on page 180. LT31 also accepts
MITRAS, FD12, and Flamingo type of polling by the RVR computer
when the emulation is on.

MESSAGE Command
The MESSAGE command is used for displaying messages. The
command format is the following:

MESSAGE Message_number

If the message number is not specified, LT31 displays a default message.

The default message is the message that has been selected for automatic
or polled mode with the SET MESSAGE TYPE command.

NOTE When displaying a message with the MESSAGE command, the

message frames are not displayed.

All message frames end with -* which represent the carriage return and
line feed characters. These characters are also used as line separators in
multi-line messages.

The contents of the messages are described in the next sections.

Message 1 - LT31 Standard Message

Message 1 is of fixed length where both meteorological optical range
(MOR) and background luminance values (in case optional LM21 is
installed) are shown. The message format is as follows:

!LT1"VIS 10000.0 AL 00000000000000000000 BL 01000 AL 0#CSUM$-*

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Start Content Description

at Byte
No.
1 ! Start of the heading (ASCII 1).
2 LT LT31 sensor identifier.
4 1 Unit identifier, if the ID is not defined it is
replaced by a space character.
5 " Start of the text (ASCII 2).
6 VIS 10000.0 MOR value, unit m, 1-min-average.

18 AL 00000000000000000000 Alarm status of the LT31 system (see Table 13

on page 164).
42 BL 01000 Background luminance identifier and BL value,
unit, cd m-2.
51 AL 0 Alarm status of the background luminance
sensor LM21.
55 # End of the text (ASCII 3).
56 CSUM Checksum CRC16.
60 $ End of the transmission (ASCII 4).
61 -* CR + LF (ASCII 13 + ASCII 10).

The total amount of characters in the message is 62. The transmission

times are: 2.1 s at 300 bps (10-bit char), 0.26 s at 2400 bps, 0.07 s at
9600 bps.

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Status Codes

LT31 System
For decoding the status, see Table 13 below.

Table 13 Status Codes

Byte Number Description
21 LT31 overall status
22 LT31 MOR status
23 LT31 power supply
24 LT31 auto-calibration
25 LT31 window contamination
26 LT31 alignment
27 LT31 reserved for future extension
28 LT31 reserved for future extension
29 LT31 transmitter status
30 LT31 transmitter -> main transmitter module
31 LT31 transmitter -> window transmitter module
32 LT31 transmitter -> measurement CPU
33 LT31 transmitter -> misc.
34 LT31 receiver status
35 LT31 receiver -> master CPU status
36 LT31 receiver -> main receiver module
37 LT31 receiver -> window transmitter module
38 LT31 receiver -> measurement CPU
39 LT31 receiver -> misc.
40 PWD status

For decoding the overall status, see Table 14 below.

Table 14 LT31 Overall Status (Byte Number 21)

Code Explanation Comment
0 OK
I Indication of AC off, operating on battery.
abnormal Auto-alignment in process.
situation
W Warning Examples:
Window contamination reached
warning level.
Transmitter LED aging.
Backup battery voltage low.
A Alarm Examples:
Communication error in LT31 system.
Internal monitoring alarm signal
exceeded limit.
E Error Examples:
Transmitter, receiver, PWD (if
configured) or LM21 (if configured) is
missing.
S Service User interface has been opened.

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The indication of the over-range as well as the under-range is always

correlated to the configured baseline length and therefore, to the specific
measurement range. If the configured baseline length is 30 m, the under-
range is indicated for MOR values below 10 m and the over-range
indication is set for MOR readings above 10 000 m.

The indication of code 4 represents a special case. This code indicates

that the reported MOR is based on the PWD visibility measurement and
occurs when the transmissometer measurement shows a vital error but
the PWD status is still OK and PWD visibility data are available. See
Table 15 below.

NOTE Alternatively to the above described default behavior, it is possible to

disable the use of PWD MOR in error situations. The intended
respective behavior can be configured using the
PWD_MOR_ON_ERROR [ON|OFF] command as follows:

To disable the use of PWD MOR in error situations use the command set
pwd_mor_on_error off. The response would be PWD MOR is NOT
used on error.

To enable the use of PWD MOR in error situations (default) use the
command set pwd_mor_on_error on. The response is PWD MOR is
used on error.

Table 15 LT31 MOR Status (Byte Number 22)

Code Explanation Comment
0 OK
1 Underrange MOR below measurement range limit.
2 Overrange MOR above measurement range limit.
4 Based on MOR value is based on PWD
PWD measurement. (only if configured
accordingly)
8 Not valid System error.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 2 and code 4 occur at the
same time, it will lead to code 6.

For decoding the power supply status, see Table 16 on page 166.

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Table 16 LT31 Power Supply (Byte Number 23)

Code Explanation Comment
0 OK
1 Battery mode No AC power.
LT31 is working on a backup battery.
2 Low backup No AC power.
battery LT31 is working on a backup battery.
voltage
LT31 will shut
down soon

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

For decoding the auto-calibration status, see Table 17 below.

Table 17 LT31 Auto-calibration (Byte Number 24)

Code Explanation Comment
0 OK
1 Auto-calibration Auto-calibration situation detected.
situation Auto-calibration in process.
4 Auto-calibration PWD not configured.
not available
8 Auto-calibration Example:
switched off PWD is missing.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

For decoding the window contamination status, see Table 18 below.

Table 18 LT31 Window Contamination (Byte Number 25)

Code Explanation Comment
0 OK
1 Contamination Cleaning required soon.
warning level 1
2 Contamination Immediate cleaning required.
warning level 2
4 Contamination Contamination too high.
level too high
8 Contamination
measurement
error

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NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

For decoding the alignment status, see Table 19 below.

Table 19 LT31 Alignment (Byte Number 26)

Code Explanation Comment
0 OK
1 Automatic
alignment in
process
2 Reduced
alignment
quality level 1
3 Reduced New fine alignment should be carried
alignment out.
quality level 2
4 Reduced Very poor alignment quality, re-
alignment alignment is necessary.
quality level 3
8 Error Alignment procedure failed or
alignment hardware error.

Table 20 LT31 Future Extension 1 (Byte Number 27)

Code Explanation Comment
0 OK

Table 21 LT31 Future Extension 2 (Byte Number 28)

Code Explanation Comment
0 OK

The byte shown in Table 22 on page 168 indicates the overall status of
the LTT111 (LT31 transmitter unit).

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Table 22 LT31 Transmitter Status (Byte Number 29)

Code Explanation Comment
0 OK
W Warning Examples:
Window receiver saturated.
Main/window transmitter light source
aging warning.
Window contamination above limit.
A Alarm Example:
Internal monitoring alarm exceeded
limit.
E Error Hardware error.
C Communication Device not responding.
error

The byte shown in Table 23 below indicates the status of the main
transmitter module LTL112 which is located in the transmitter
measurement unit LTM112 of the LTT111 (LT31 transmitter).

Table 23 LT31 Transmitter: Main Transmitter Module (Byte

Number 30)
Code Explanation Comment
0 OK, normal
operation
2 Light source Transmitter derating signal
aging warning reached warning limit.
4 Hardware Internal monitoring alarm signal
warning exceeded limit.
8 Hardware error Examples:
Intensity signal out of limits.
EEPROM and/or temperature
sensor failure.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 24 on page 169 indicates the status of the
window transmitter module LTL212 which is located in the transmitter
measurement unit LTM112 of the LTT111 (LT31 transmitter).

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Table 24 LT31 Transmitter: Window Transmitter Module (Byte

Number 31)
Code Explanation Comment
0 OK, normal
operation
2 Light source Transmitter derating signal
aging warning reached warning limit.
4 Hardware Internal monitoring alarm signal
warning exceeded limit.
8 Hardware error Examples:
Intensity signal out of limits.
EEPROM and/or temperature
sensor failure.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 25 below indicates the status of the

measurement CPU LTC112 which is located in the transmitter
measurement unit LTM112 of the LTT111 (LT31 transmitter).

Table 25 LT31 Transmitter: Measurement CPU (Byte Number

32)
Code Explanation Comment
0 OK, normal
operation
2 Window receiver Direct or indirect solar radiation.
saturated
4 Hardware Internal monitoring alarm signal
warning exceeded limit
8 Hardware error Examples:
EEPROM and/or temperature
sensor failure.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 26 on page 170 indicates the status of the optics
alignment mechanism and the window heater unit located in the
transmitter measurement unit LTM112 of the LTT111 (LT31
transmitter).

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Table 26 LT31 Transmitter: Miscellaneous (Byte Number 33)

Code Explanation Comment
0 OK, normal
operation
4 Window heater Temperature difference between
error window and enclosure temperature
below limit when heater is on.
8 Alignment No horizontal and/or vertical
mechanism position signal.
No movement.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 27 below indicates the overall status of the
LTR111 (LT31 receiver unit).

Table 27 LT31 Receiver Status (Byte Number 34)

Code Explanation Comment
0 OK
W Warning Examples:
Window receiver is saturated.
Window transmitter light source
aging warning.
Window contamination above limit.
A Alarm Examples:
Main receiver saturated.
Internal monitoring alarm signal
exceeded limit.
E Error Hardware error.
C Communication Device not responding on module
error bus request.

The byte shown in Table 28 below indicates the status of the LT31
master CPU LTC212 which is located in the receiver interface unit
LTI211.

Table 28 LT31 Receiver: Master CPU Status (Byte Number 35)

Code Explanation Comment
0 OK
4 Hardware warning
8 Error Memory error

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NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 29 below indicates the status of the LT31 main
receiver module LTD112 which is located inside the receiver
measurement unit LTM212.

Table 29 LT31 Receiver: Main Receiver Module (Byte Number

36)
Code Explanation Comment
0 OK, normal
operation
1 Self-test in
process
2 Receiver Direct or indirect solar radiation.
saturated
4 Hardware Internal monitoring alarm signal
warning exceeded limit
8 Hardware error Examples:
Self-test failed.
EEPROM and/or temperature
sensor failure.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 30 below indicates the status of the LT31
window transmitter module LTL212 which is located inside the receiver
measurement unit LTM212.

Table 30 LT31 Receiver: Window Transmitter Module (Byte

Number 37)
Code Explanation Comment
0 OK, normal
operation
2 Light source Transmitter derating signal
aging warning reached warning limit.
4 Hardware Internal monitoring alarm signal
warning exceeded limit.
8 Hardware error Examples:
Intensity signal out of limits.
EEPROM and/or temperature
sensor failure.

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NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 31 below indicates the status of the LT31
measurement CPU LTC112 which is located inside the receiver
measurement unit LTM212.

Table 31 LT31 Receiver: Measurement CPU (Byte Number 38)

Code Explanation Comment
0 OK, normal
operation
1 Synchronization Main receiver synchronization
error signal not available.
2 Window receiver Direct or indirect solar radiation.
saturated
4 Hardware Internal monitoring alarm signal
warning exceeded limit
8 Hardware error Examples:
EEPROM and/or temperature
sensor failure.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

The byte shown in Table 32 below indicates the status of the optics
alignment mechanism and the window heater unit located inside the
receiver measurement unit LTM212.

Table 32 LT31 Receiver: Miscellaneous (Byte Number 39)

Code Explanation Comment
0 OK, normal
operation
4 Window heater Temperature difference between
error window and enclosure temperature
below limit when heater is on.
8 Alignment No horizontal and/or vertical position
mechanism signal.
No movement.

NOTE The listed status codes do not cover all possible cases. Combinations are
possible if applicable. If, for example, code 1 and code 2 occur at the
same time, it will lead to code 3.

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The byte shown in Table 33 below indicates the status of the PWD
forward scatter sensor which is mounted to the transmitter support unit
LTS111 of the LTT111 (LT31 transmitter).

Table 33 LT31 PWD Status (Byte Number 40)

Code Explanation Comment
0 OK
I Indication of Cleaning required.
abnormal
situation
W Warning Hardware warning.
A Alarm Immediate cleaning required.
E Error Hardware error.
C Communication Device not responding.
error
X Not configured LT31 is not configured to handle
PWD (auto-calibration is off).

The byte shown in Table 34 below indicates the status of the LM21
Background Luminance Sensor which is optionally connected to the
receiver interface unit LTI211.

Table 34 Background Luminance Sensor LM21 (optional)

Alarm Status Information (Byte Number 54)
Code Explanation Comment
0 OK
I Indication of Hood or dew heater problem.
abnormal
situation
A Alarm Sensor does not react to changes
in luminance.
LM21 has detected the field
calibrator LMA21.
Backscatter has increased.
E Error Memory error.
C Communication Device not responding.
error
X Not configured LT31 is not configured to handle
LM21.

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Message 2 - LT31 Message Including PW

Data (optional)
Message 2 is of fixed length where, in addition to Message 1, present
weather (PW) values are shown.

NOTE Present weather values are available only when a valid PW-Option-
Keyword has been entered (refer to the SET PW_OPTION command
string shown in Table 11 on page 150). If no valid keyword has been
entered, the present weather values are replaced by a slash / (ASCII 47).

The message 2 format is as follows:

!LT1"VIS 10000.0 AL 00000000000000000000 BL 01000 AL 0 WMO

61 WMO15M 61 WMO60M 61RN1M 0.33 RNSUM 12.16 SNSUM 999
TMP 23.4 AL 0#CSUM$-*

Start at Content Description

Byte No.
1 ! Start of the heading (ASCII 1)
2 LT LT31 sensor identifier
4 1 Unit identifier, if the ID is not defined it is
replaced by a space character
5 " Start of the text (ASCII 2)
6 VIS 10000.0 MOR value, unit m, 1-min-average
18 AL 00000000000000000000 Alarm status of the LT31 system (see Table
13 on page 164)
42 BL 01000 Background luminance identifier and BL
value, unit, cd m-2
51 AL 0 Alarm status of the background luminance
sensor (see Table 34 on page 173)
56 WMO 61 Instant WMO code
63 WMO15M 61 15-minute WMO code
73 WMO60M 61 60-minute WMO code
83 RN1M 0.33 Rain intensity 1-minute average, unit
mm h-1
95 RNSUM 12.16 Rain sum, unit mm
108 SNSUM 999 Snow sum, unit mm
119 TMP 23.4 Temperature, C
129 AL 0 Alarm/status for PWD (see Table 33 on
page 173)
133 # End of the text (ASCII 3)
134 CSUM Checksum CRC16
138 $ End of the transmission (ASCII 4)
139 -* CR + LF (ASCII 13 + ASCII 10)

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The total amount of characters in the message is 140. The transmission

times are: 4.7 s at 300 bps (10-bit char), 0.58 s at 9600 bps.

For visibility alarm/status code explanations, refer to Table 13 on page

164.

For background luminance alarm/status code explanations, refer to Table

34 on page 173.

For PWD alarm/status code explanations, refer to Table 33 on page 173.

Message 3 - Status Message

The status message contains the results of the built-in tests. These
numerical internal monitoring measurement values can be requested with
the STATUS command. The short status check message which contains
a short, clear language report of the system status can be requested with
the STATUS CHECK command. The following is an example of a
status message:

!LT1" LT31 SYSTEM: OK

Status: 02000000000000000000
Interface unit:
Temperatures (unit C):
CPU: 32.7 external://///
Humidity:////
Voltages:
+12V: 11.8 +12Vout: 0.5 PVin: 20.8 V5I: ON
Transmitter unit: OK
Status
EEPROM:00 TEMP:00 ALIGN:00 MISC:00
HEATER:H BLOW:0
Temperatures (unit C):
Heater:+48.2 Enclosure:+42.9
WinRec:+43.7 WinTrm:+43.6 MainTrm:+41.1
Receiver unit: OK
Status
EEPROM:00 TEMP:00 ALIGN:00 MISC:00
HEATER:H BLOW:0 REC:S
Temperatures (unit C):
Heater:+51.0 Enclosure:+41.4
WinRec:+41.9 WinTrm:+43.2 MainRec:+37.9
PWD Sensor: OK
PWD STATUS
VAISALA PWD21 V 1.01 2001-11-01 SN:W42106 ID STRING: 0

SIGNAL 0.53 OFFSET 160.77 DRIFT 7.52

REC. BACKSCATTER 993 CHANGE 58
TR. BACKSCATTER -1.2 CHANGE -0.0
LEDI 2.6 AMBL -1.1
VBB 15.2 P12 11.5 M12 -11.3
TDRD 7 DRD 800 DRY 815.1
TS 5.5 TB 10
HARDWARE :

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OK

LM Sensor: OK
Background luminance sensor: OK
window cont: 3
contamination compensation: ON
luminance: 11914 uc: 11557 backscatter: 0
temperature: CPU: 29.4 hood: 25.7
heaters: hood: ON/0 dew: ON/0
voltage: +5Va: 4.9 +5Vd: 5.0 +12V: 11.9
dew heater current: 0
free memory: 499276 V5iso: ON raw:00002F / 469
E E C L
XCSUM T R F

where
! =Start of the heading
LT =LT31 sensor identifier
1 =Unit identifier, if the ID is not defined it is replaced by a
space character
" = Start of the text
MESSAGE BODY
# = End of the text
CSUM = Checksum CRC16
$ = End of the transmission
-* = CR + LF

The amount of characters in the message is approximately 975. The

transmission times for 975 characters are: 33 s at 300 bps (10-bit char),
4 s at 2400 bps, 1 s at 9600 bps.

Message 4 - Vaisala System Standard

Message
Message 4 is defined according to the Vaisala system standard format.
All units in the message are SI units. The message format is as follows:

!LT1"VIS(02000(AL(0)))BL(00100(AL(0)))#CSUM$-*

where

! = Start of the heading

LT = FS11 sensor identifier
1 = Unit identifier, if the ID is not defined it
is replaced by a space character
" = Start of the text
VIS(02000(AL(0))) = Visibility identifier and MOR value (unit
meter, 1 min average) and alarm status
related to visibility value

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BL(00100(AL(0))) = Background luminance identifier and BL

value (unit cd m-2) and alarm status
related to BL value
# = End of the text
CSUM = Checksum CRC16
$ = End of the transmission
-* = CR + LF

For a list of alarm status code explanations, refer to Table 13 on page

164.

The total amount of characters in the message is 46. The transmission

times are: 1.5 s at 300 bps (10-bit char), 0.19 s at 2400 bps, 0.05 s at
9600 bps.

Message 5 and 6 - FD12 Emulation

For compatibility in the system level, LT31 also supports the message
format, frames, and polling format of Vaisala Visibility Meter FD12. The
FD12 polling format is accepted only when the FD12 message emulation
is selected.

The FD12 polling format is as follows:

%FD id message_number-

The Message_number parameter refers to the FD message number.

Message 5 - FD12 Message 2

The emulation mode for FD12 message 2 is selected with the SET
MESSAGE TYPE FD12MSG2 command.

The message format of the emulated Vaisala FD12 message number 2 is

presented below:

!FD A"00 1810 1353 //// // /////#-*

----------------------------------------
¦¦ ¦ ¦ ¦ ¦ ¦_____reserved for options
¦¦ ¦ ¦ ¦ ¦________reserved for options
¦¦ ¦ ¦ ¦_________reserved
¦¦ ¦ ¦_________visibility 10 min average in meters
¦¦ ¦________visibility 1 min average in meters
¦¦____hardware status 0 = OK
¦ 1 = alarm (data OFF, see STATUS message)
¦ 2 = warning (see STATUS message)
¦_______data status 0 = normal, FD12 alarm limits not supported

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The total amount of characters in the message is 40. The transmission

times are: 1.3 s at 300 bps (10-bit char), 0.17 s at 2400 bps, 0.04 s at
9600 bps.

Message 6 - FD12 Message 7

The FD12 message 7 emulation mode is selected with the
SET MESSAGE TYPE FD12MSG7 command.

NOTE Present weather values are available only when a valid PW-Option-
Keyword has been entered (refer to the SET PW_OPTION command
string shown in Table 11 on page 150). If no valid keyword has been
entered, the present weather values are replaced by a slash / (ASCII 47).

The message format of the emulated Vaisala FD12 message number 7 is

presented below:

Message with framing:

!FD A"00 10000 10000 L 52 61 61 0.33 12.16 0 13.2 01000-*

-*
-*
#-*

Message contents:
00 10000 10000 L 52 61 61 0.33 12.16 0
13.2 01000
-----BL cd/m2
----- TS temperature
---- cumulative snow sum
------ cumulative water sum
------ precipitation water intensity, mm/h
-- one hour present weather code, 0 . . . 99
-- 15 minute present weather code, 0 . . . 99
-- instant present weather code, 0 . . . 99
--- instant present weather, NWS codes
----- visibility ten minute average
----- visibility one minute average
- 1=hardware error, 2 = hardware warning
- data status 0 = normal, (FD12 alarm limits not supported)

, Empty row
, Empty row

The total amount of characters in the message is 74. The transmission

times are: 2.5 s at 300 bps (10-bit char), 0.31 s at 2400 bps, 0.08 s at
9600 bps.

Message 7 consists of three lines. The measured background luminance

value will be displayed in cd/m2 only if LM21 is attached to LT31.

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Message 7 - MITRAS Emulation

For compatibility on the system level, LT31 also supports the message
format, frames, and polling format of Vaisala MITRAS
Transmissometer. The MITRAS polling format is accepted only when
the MITRAS message emulation is selected with the SET MESSAGE
TYPE MITRAS command.

The MITRAS polling format is the following:

P<Space><ID>-*.

The emulated Vaisala MITRAS Transmissometer message is presented

below:

"ID 1 V 4550 B ///// S010101 -*#

------------------------------------
¦ ¦ ¦ ¦ ¦ ¦ ¦¦ ¦ ¦___receiver 2 status (fixed to 01)
¦ ¦ ¦ ¦ ¦ ¦ ¦¦ ¦___receiver 1 status
¦ ¦ ¦ ¦ ¦ ¦ ¦¦___transmitter status
¦ ¦ ¦ ¦ ¦ ¦ ¦____S, status heading
¦ ¦ ¦ ¦ ¦ background luminance value
¦ ¦ ¦ ¦ ¦___B,background luminance heading
¦ ¦ ¦ ¦_____ visibility m, 1 min average
¦ ¦ ¦________V, visibility heading
¦ ¦________unit ID (one character only)
¦_________ID, start indicator

The total amount of characters in the message is 35. The transmission

times are: 1.2 s at 300 bps (10-bit char), 0.15 s at 2400 bps, 0.04 s at
9600 bps.

The binary status is in hexadecimal notation. The status bits emulate the
MITRAS status as follows.

The transmitter status correspondence between MITRAS and LT31 is the

following:

BIT MITRAS LT31

BIT.0=1 MEAS MODE ON
II BIT.1=2 CONT/OTHER OFF
BIT.2=4 OPTICAL SURFACE Window contamination
BIT.3=8 POWER SUPPLY Power supply failure
BIT.4=1 HEATING OFF
I BIT.5=2 FLASH LAMP LED failure
BIT.6=4 BL METER BL sensor status (ON/OFF)
BIT.7=8 MEASUREM. LOOP OFF
SIGNAL

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The receiver status correspondence between MITRAS and LT31 is the

following:

BIT MITRAS LT31

BIT.0=1 MEAS MODE ON
IV BIT.1=2 CONT/OTHER Any other LT31 alarm
BIT.2=4 OPTICAL SURFACE OFF
BIT.3=8 POWER SUPPLY OFF
BIT.4=1 HEATING OFF
III BIT.5=2 CALIBRATION Receiver saturated
BIT.6=4 TEST OFF
BIT.7=8 CONSISTENCY OFF

For example, status 4101 means that

I 4 = BL sensor ON
II 1 = ON
IV 1 = ON

In other words, Background Luminance Sensor LM21 is set as the

background luminance sensor.

Message 8 - Flamingo Emulation

For compatibility in the system level, LT31 also supports the message
format, frames, and polling format of Impulsphysik transmissometer
SKOPOGRAPH II “Flamingo”. The FLAMINGO polling format is
accepted only when the FLAMINGO message emulation is selected. The
FLAMINGO emulation mode is selected with the SET MESSAGE
TYPE SKOPO2 command.

The FLAMINGO polling format is as follows:

"H0C!S0P_ _ _ _ _ _ _ _ _ _0B$

NOTE Read the ' _ ' character as a space.

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The emulated Impulsphysik transmissometer SKOPOGRAPH II

“Flamingo” message is presented below:
"S0TA" 935 2000 -87440 5300 > R0100 10 5 50 00 00 00 A0-*$
--------------------------------------------------------------------------------
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦ ¦ ¦ ¦ ¦_Checksum, 2’s cmplmt.
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦ ¦ ¦ ¦ for all char’s
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦ ¦ ¦ ¦ ¦_Status, not used
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦ ¦ ¦ ¦_Baselength, m
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦ ¦ ¦_Contrast threshold, %
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦ ¦_Update time, s
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦¦_Simulation Value, %
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦¦_Test generator (1-on, 0-off)
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦¦_Test filter (1-on, 0-off)
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦¦_Transmitter (1-on, 0-off)
¦ ¦¦ ¦ ¦ ¦ ¦¦¦¦_1st telegram after reset
¦ ¦¦ ¦ ¦ ¦ ¦¦¦_Remote/Local
¦ ¦¦ ¦ ¦ ¦ ¦¦_BGL out of range (<, >)
¦ ¦¦ ¦ ¦ ¦ ¦_MOR out of range (<, >)
¦ ¦¦ ¦ ¦ ¦_Background luminance, cd m-2
¦ ¦¦ ¦ ¦_ln(Et) * 104
¦ ¦¦ ¦_Visibility, m
¦ ¦¦_Transmission, ‰
¦ ¦_STILBUS II indication
¦_Header

The total amount of characters in the message is 80. The transmission

times are: 2.7 s at 300 bps (10-bit char), 0.33 s at 2400 bps, 0.08 s at
9600 bps.

Message 9 - Service Raw Values

Message 9 offers a set of raw values. The contents of message 9 are
subject to change.

Message Sending Modes

Automatic Mode
In the automatic mode the LT31 system sends a predefined message at
selected intervals. The automatic and default polled message can be
defined with the SET MESSAGE TYPE command. For available
messages, see section Message Formats on page 161. The format of the
SET MESSAGE TYPE command is presented below:

SET MESSAGE TYPE Message_number

The following command selects message 2 as the default automatic

message:

SET MESSAGE TYPE 2

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The automatic message interval is selected with the command SET

MESSAGE INTERVAL. The message interval is given in seconds. An
interval of zero which ignores the automatic sending, is used when the
messages are polled. The command format is as follows:

SET MESSAGE INTERVAL Message_interval

The following command selects the message to be sent once a minute:

SET MESSAGE INTERVAL 60

The following command cancels the automatic sending:

SET MESSAGE INTERVAL 0

Polled Mode
In the polled mode the LT31 system sends a data message when the host
computer transmits a polling command. The automatic message sending
mode is disabled by setting the message interval to zero with the
following command:

SET MESSAGE INTERVAL 0

The polling command format is as follows:

%LT<id><message_number>-

where

% = The ASCII character number 5 (CTRL + E)

id = The identifier selected in configuration; if replaced
by blank space all LT31 systems on the line will
respond
message = Optional message identifier (two characters); if the
number message number is omitted, the sensor answers with
the default message selected with the SET
MESSAGE TYPE command
- = CR (ASCII 13)

Examples of polling commands:

%LT- = If only one LT unit is on the line (no ID is

necessary)
%LTA03- = The LT31 unit A (ID = A) is polled for message
number 03 (status message). This is used when
several devices are on the same line.

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The LT31 system does not echo the polling character string.

When there are several devices on the same modem line, the polled unit
turns the modem (DMX501 option) carrier on after it has acknowledged
the request. Turning on the carrier will cause extra characters to be added
in front of the ! character. The LT31 system waits for about 100 ms after
the carrier has been turned on before it sends the message. When LT31
has sent the message, it turns the carrier off causing another set of extra
characters that will also have to be ignored by the host.

When the FD12, MITRAS, or SKOPO2 emulation is on (message types

FD12MSG2, FD12MSG7, MITRAS or SKOPO2 are selected), LT31
does not respond to the above polling command but to polling commands
sent in FD12, MITRAS or SKOPO2 formats, respectively.

METAR Messaging
In order to support host systems in generating comprehensive Weather
Reports, LT31 is able to provide additional information on the prevailing
weather situation. When METAR Messaging is enabled, PWD22
Message 7 (FD12P Message 7) is transmitted immediately after any
original LT31 Message. For a specification of PWD Message 7, refer to
PWD22 User's Guide (see section Related Manuals on page 16.

The following command enables METAR Messaging without framing.

This command is available in Advanced Level only.

SET METAR ON

The following command enables METAR Messaging with framing. This

command is available in Advanced Level only.

SET METAR 8

The following command disables METAR Messaging. This command is

available in Advanced Level only.

SET METAR OFF

METAR Messaging is available only when LT31 PW Option is enabled.

By default, METAR Messaging is disabled.

The original LT31 message and the additional 'METAR' message cannot
be sent to different ports, nor can they use different communication
settings. The message interval, the message port and the port settings are
the same as for the original LT31 messaging.

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Original LT31 messages from 1 to 9 are unchanged. The visibility data of

PWD22 message 7 (one minute and ten minutes averages) is invalidated
by slashes.

The unit identifier given in the PWD22 message 7 is the same as set for
LT31 unit identifier. When the unit identifier for LT31 is removed, the
unit identifier given in the PWD22 message 7 is 0 (zero, ASCII-0x30).

There is no crosscheck for whether the message interval and port settings
are able to offer a bandwidth large enough to transmit all the data (for
example, 300 bps, 15 s, LT31 message 9, METAR-Messaging enabled
does not work).

System Configuration
The SET command is used to set or update system communication and
user interface related parameters.

The CALIBRATE command is used to set or update visibility, window

contamination and temperature measurement calibrations. This command
is available only at the advanced level. For calibration instructions, see
section Calibration on page 232.

The current system parameters can be displayed by using the

PARAMETERS command. Type the following:
0> PARAMETERS

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The output is the following:

LT31 parameter values:
identifier: -
name:
header: LT
base length: 30
command terminal:
lines: 24
timeout: 10
message:
type: 2
interval: 10
port: data
frame: on
simulation: off
data port:
speed: 9600
mode: rs-485
data format: 8n1
2nd port: none
2nd type: 0
maintenance port:
speed: 9600
system:
modem: off
LM: on
PWD: on
+12V out: off
PW option: enabled
PWD Pitch: 1
Internal Filter 1
Blow on WMO: 0
Heat on Battery:yes
MOR range: 10 to 10000 meter
BL range: 1 to 100000 cd/sqm

The system parameter values are saved in the non-volatile Flash memory.

Default Factory Settings

The default parameter settings set at the factory are listed in Table 35
below. All the listed parameters can be set to their default values with the
SET DEFAULTS YES command in the advanced level.

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Table 35 Default Factory Parameter Settings

Parameter Default Setting
Message type 1
Message interval 10
Message port Data
Unit_id -
Name -
Header LT
Data_port baud 9600
Data_port mode RS-232
Data_port parity 8N1
Module modem Off
Maintenance_port baud 9600
Port_timeout 10
Terminal_lines 24
Contamination_compensation bl_sensor On
LM Off
PWD On
Hood_heaters bl_sensor On
Dew_heaters bl_sensor On
Message_frame On
+12VOUT Off
Blow on WMO 0
PWD pitch 1.000
Internal Filter 1.00
Heat on Battery Yes
Base Length 30 meters
MOR Range 10 to 10 000 meters
BL Range 1 to 100 000 cd/sqm

External Sensors (Optional)

Background Luminance Sensor LM21

The SET LM ON configuration command is used to add the LM21
sensor output into the LT31 system and the data messages. With this
command, the interface unit CPU starts polling the LM21 sensor via the
internal RS-485 sensor interface.

RVR System Test Commands

LT31 can be set to a simulation mode which includes transmitting fixed,
user defined or sequenced visibility and status information. This function
is intended for system testing purposes. The normal visibility detection
mode takes effect after a reset. The simulation mode can also be stopped
with the SIMULATE OFF command in the advanced level.

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The user can either choose a fixed test message, define an entire message
content, or define data sequences. The fixed simulation message is
selected with the SIMULATE TEST_MESSAGE command in the
advanced level.

There is a predefined fixed test message for each message type. If a fixed
test message is selected, one of the messages in the following section is
transmitted depending on the selected message type. The data values in
the messages are as shown in the following section. The frames of the
messages are not fixed but depend on the current configuration.

When changing the predefined, fixed test message type, first turn the
simulation off by giving the SIMULATE OFF command in the
advanced level and then change the message type (with the SET
MESSAGE TYPE number command). Turn the simulation on again by
giving the SIMULATE TEST_MESSAGE command.

When changing the user defined data sequences, first turn the simulation
off by giving the SIMULATE OFF command on the advanced level and
then change the message type (with the SET MESSAGE TYPE number
command). Turn the sequence simulation on again by giving the
SIMULATE SEQUENCE command.

Message 3 (Status message) has no simulation message. Message 3

(Status message), Message 5 (FD12 MSG2), and Message 6 (FD12
MSG7) are not supported by simulation sequences.

Fixed Test Messages

Message 1
!LT1"VIS 1850.0 AL 00000000000000000000 BL 00622 AL
0#F363$-*

Message 2
!LT1"VIS 1850.0 AL 00000000000000000000 BL 00671 AL 0 WMO
00 WMO15M 00 WMO60M 00 RN1M 0.00 RNSUM 0.00 SNSUM 0
TMP 23.3 AL 0#24CE$-*

Message 4
!LT1"VIS(01850(AL(0)))BL(00671(AL(0)))#9063$-*

Message 5 - FD12 Message 2 Emulation

!FD 1"00 1850 2000 //// // /////#-*

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Message 6 - FD12 Message 7 Emulation

!FD 1"00 1850 2000 /// // // // ////// ////// //// 23.3
01100-*
-*
-*
#-*

Message 7 - MITRAS Emulation

"ID 1 V 1850 B 595 S4101 -*#

Message 8 - SKOPOGRAPH II Flamingo

"S0TA" 948 1850 -108189 587 R0100 10 5
33 00 00 00 -*$

Manual Simulation Message

The user can define the entire content of the message body which is the
string between the SX (start of the text) character and the EX (end of the
text) character. The message string must be given after the SIMULATE
MANUAL_MESSAGE command in the following way:

SIMULATE MANUAL_MESSAGE string

The LT31 sensor does not make any validity checks to the message body
content which means that the message is transmitted as it is written.

Special characters can be added to the message by using the following

notations:
\c Message counter (incrementally increased by a sent message)
\n New line
\r CR
\t TAB
\s Space
\[ [
\] ]
\xXX Character corresponding to hexadecimal XX

An example of a manual simulation message is presented below:

The command

SIMULATE MANUAL_MESSAGE this\sis\stestmessage

sends the following message:

!LT"this is testmessage#CSUM$-*.

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In the automatic message sending mode, the frames are not displayed
with the MESSAGE command.

Simulation Sequences
The user can define up to ten sequences of visibility, status, and
background luminance data.

Each sequence has its own duration. A sequence duration of 0 seconds

switches the sequence off. For a proper operation of Simulation
Sequences, at least one sequence must be enabled by a duration greater
than 0 seconds.

The sequences are defined in the Simulation menu. To enter the

Simulation menu, it is necessary to open the maintenance terminal (see
section Entering and Exiting Command Mode on page 145) and then
enter the menu-driven operation (see section Menu-driven Operation on
page 156).

0401-110

Figure 109 Simulation Menu

By default all sequences are disabled (0 s), visibility is set to 5000 m,

status contains of all zeroes and background luminance is set to
20 000 cd/m2. Default settings can be set by (M) RESET SEQUENCES.

Sequences changing is invoked by pressing the appropriate key (A) to

(J). In the Info Area, the user can accept or modify each set of data. If
the current data is kept, ENTER has to be pressed.

Entering 0 (zero) for a visibility or background luminance value will lead

to an indication of an appropriate value (slashes will be transmitted
instead).

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The following is an example of modifying sequence A:

Sequence (A) Duration In Seconds: [0]

Typing 300<CR> will change the duration to 300 s.

Sequence (A) VIS In m: [5000.0]

Typing 8000<CR> will change the visibility data to 8000 m.

Sequence (A) AL: [00000000000000000000]

Typing E<CR> will change the first character of the overall status to E.
Further characters remain unchanged.
Sequence (A) BL In cd*m^-2: [20000]

Typing 1000<CR> will change background luminance data to 1000 cd/m2.

Sequence (A) AL: [0]

Typing <CR> will keep the background luminance status data unchanged.

The summary of sequence data is displayed in Figure 110 below.

0401-111

Figure 110 Summary of Sequence Data

Simulation can either be switched on or off by pressing (K) SWITCH

ON or (L) SWITCH OFF or by giving SIMULATE SEQUENCE or
SIMULATE OFF on the command line on the advanced level.

Once the simulation sequences are enabled, the data of each sequence is
valid for a given time. If the time of a valid sequence runs out, the next
valid sequence (duration greater than zero) is chosen. If the last valid
sequence runs out, the first valid sequence is chosen. Refer to the
example given in the paragraph on the next page.

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Example:

Assuming that only sequence (A) (300 s) and sequence (D) (600 s) are
valid sequences, the data of sequence (A) will be reported for 300 s (five
minutes). After that the of sequence (D) will be reported for 600 s (ten
minutes).

After a total time of 15 minutes the reported data will be taken from
sequence (A) again. After a total time of 25 minutes the reported data
will be taken from sequence (D) and so on.

If simulation sequences are enabled for all messages, the value for MOR,
status, and background luminance are replaced by sequence data.

The LT31 sensor does not make any validity checks to the sequence data
contents.

As all messages do not have the capability to give all the defined data,
some message formats use only parts of the defined sequence data.

Message 1
Full data replacement.

Message 2
Full data replacement.

Message 4
Full data replacement with the following exception: visibility status is
replaced by the first character of the overall status. Other characters of
the overall status are omitted.

Message 7 (MITRAS)
Visibility and background luminance data are replaced. The status is
replaced by the first six characters of the overall status. Other characters
of the overall status are omitted.

Message 8 (SKOPO 2)
Visibility and background luminance data are replaced. The
transmittance is calculated based on visibility data and configured
baseline length.

The value for ln(εt) is calculated based on the background luminance

data.

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Flags for MOR and background luminance overrange are replaced by the
first two characters of the overall status. Other characters of the overall
status are omitted.

As Skopograph II Flamingo does not indicate invalid values, minimum

values are given when the invalid values have been set.

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CHAPTER 6
MAINTENANCE

This chapter provides information that is needed in basic maintenance of

the product.

Periodic Maintenance
Due to the unique concept of the LT31 transmissometer incorporating
automatic calibration, automatic alignment, and permanent window
contamination measurement, the required periodic maintenance for this
transmissometer system is significantly reduced.

The periodic maintenance interval has also been significantly extended.

The built-in self check processes will generate related status messages, if
any maintenance and/or repair activity is required besides the periodic
maintenance (for example, window cleaning or alignment).

Periodic maintenance includes inspection and cleaning of all

transmissometer system components, optical fine alignment, calibration,
and checking the blower system. Also the optional heated weather
protection hoods are checked in this frame.

NOTE Periodic maintenance once a year during the springtime is mandatory.

However, periodic maintenance twice a year in the spring and autumn is
recommended.

Depending on the environmental conditions, cleaning windows of the

instrument might be necessary more often, LT31 requests for window
cleaning with a window contamination warning. For details, see Chapter
5, Operation, starting from page 145.

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Cleaning Instructions
The major maintenance aspect is in cleaning of the LT31 transmitter and
receiver windows, the PWD Sensor lenses, PWD RAINCAP® Rain
Detector, and all the equipment housings.

NOTE Always calibrate LT31 after cleaning. Do not clean the windows if the
weather conditions do not allow calibration to be performed immediately
afterwards, that is, if it rains or if visibility is less than 10 kilometers.

Cleaning LT31 Windows and Weather

Protection Hoods
The LT31 windows need to be cleaned occasionally, either due to a
window contamination warning (see section Status Messages on page
261) or during the periodic maintenance. In this frame, the inner surface
of the weather protection hoods should be cleaned as well.

To access the LT31 windows, no parts have to be removed.

1. Wipe the windows with a soft, lint-free cloth and normal glass
cleaner or isopropyl alcohol. Be careful not to scratch the glass
surfaces. The windows should dry up rapidly. There should be no
wiping stripes or remaining dirt on the surfaces. See Figure 111
below.

0401-063

Figure 111 LT31 Windows

2. Check from the outside that the inner window surfaces are free of
any water condensation.
3. Check that the weather protection hood is free of ice or snow
deposits.
4. Clean the inner weather protection hood surface in the same way as
the window surfaces.

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5. Clean the LT31 receiver and transmitter windows.

The LT31 system runs a permanent window contamination measurement.

NOTE After a window contamination warning, only the windows need to be

cleaned.

NOTE After window cleaning, run a visibility calibration for the

transmissometer; see section Visibility Calibration on page 238.

Cleaning PWD Forward Scatter Sensor

The structure of the PWD forward scatter sensor is explained in Figure
112 below.

1 3 2

6 5 6
0401-121

Figure 112 PWD Hardware Structure

The following numbers refer to Figure 112 above:

1 = Transmitter
2 = Controller/Receiver
3 = Vaisala RAINCAP® Rain Sensor
4 = Pt-100 temperature sensor tube
5 = Mounting clamp
6 = Hood heaters (optional)

Cleaning Lenses and Hoods

The PWD Forward Scatter Sensor lenses need to be cleaned occasionally,
either due to a contamination warning or during the periodic
maintenance. In this frame, the inner surface of the weather protection
hoods should be cleaned as well.

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To clean the PWD Forward Scatter Sensor lenses, no parts have to be

removed.

1. Wipe the lenses with a soft, lint-free cloth and normal glass cleaner
or isopropyl alcohol. Be careful not to scratch the lens surfaces.
The lenses should dry up rapidly. There should be no wiping stripes
or remaining dirt on the surfaces. See Figure 113 below.

0401-164

Figure 113 PWD Sensor Lenses

2. Check that the inner window surfaces are free of any water
condensation.
3. Check that the weather protection hood is free of ice or snow
deposits.
4. Wipe the dust from the inner and outer surfaces of the hoods.

NOTE Do not use any sharp tools to remove ice or snow from the hoods,
especially from their heated surfaces. This can easily break the heating
foils. Do not try to insert any tools under the foils because the foil glue
may then fail to hold the foil on the hood surface. It is essential that the
foils are firmly attached to the hood surface, otherwise the heat transfer
from the foil piece to the hood is impaired resulting in a burned foil.

5. After the windows have dried up, the cleaning activity should be
finalized with the CLEAN command (A) for the PWD Forward
Scatter Sensor. Select PWD Sensor (G) from the maintenance
section on the Main menu page (see section CLEAN Command on
page 241) to access the CLEAN command.

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Cleaning RAINCAP®
Vaisala RAINCAP® Rain Sensor should be cleaned every six months or
more often depending on the conditions.

NOTE The measuring principle does not allow for proper ESD protection of the
RAINCAP® electronics, so you must follow the instructions given
below:

- Ground your hand first by touching grounded metallic parts of the

installation to remove excessive static charges from your body.
- Clean the RAINCAP® rain detector carefully with a soft, lint-free
cloth moistened with mild detergent. Be careful not to scratch the
surface.
- Check that the detector is free of ice and snow deposits

Cleaning Window of Optional LM21

The LM21 window needs to be cleaned occasionally either due to a
contamination warning or during the periodic maintenance. In this frame,
the inner surface of the weather protection hood should be cleaned as
well.

To access the LM21 window, no parts have to be removed.

1. Wipe the window with a soft, lint-free cloth and normal glass
cleaner or isopropyl alcohol. Be careful not to scratch the window
surface. The window should dry up rapidly. There should be no
wiping stripes or remaining dirt on the surfaces.
2. Check that the weather protection hood is free of ice or snow
deposits.
3. Wipe the dust from the inner and outer surface of the hood.

NOTE Do not use any sharp tools to remove ice or snow from the hood,
especially from its heated surface. This can easily break the heating foil.
Do not try to insert any tools under the foil because the foil glue may
then fail to hold the foil on the hood surface. It is essential that the foil is
firmly attached to the hood surface, otherwise the heat transfer from the
foil piece to the hood is impaired resulting in a burned foil.

4. After the windows have dried up, the cleaning activity should be
finalized with the CALIBRATE WINDOW_CLEAN LM
command.
- Enter the LT31 command mode (OPEN command)

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- Change to the advanced level (LEVEL 1 command)

- Enter the CALIBRATE WINDOW_CLEAN LM command
- Leave the LT31 command mode (CLOSE command)

For details, refer to LM21 User's Guide. For details on the LT31
command mode, see Chapter 5, Operation, starting from page 145.

NOTE There must not be any objects in front of the sensor when the
CALIBRATE WINDOW_CLEAN LM command is given.

NOTE The window must be dry before giving the CALIBRATE

WINDOW_CLEAN LM command.

Inspection and Cleaning Winter Dirt

The periodic maintenance includes an inspection and cleaning of all the
system component housings of the transmissometer.

1. Use a soft cloth and warm water with a household cleaning agent
and clean especially the following:
- Measurement head covers of the transmitter and receiver
- Masts and interface unit covers
- PWD Sensor housing parts
- Housing of the optional Background Luminance Sensor LM21
- Optional Obstruction Light LT31OBS
2. During the cleaning, check all the housing surfaces for mechanical
damages.

Blower System
The LT31 transmitter and receiver are equipped with a blower system.
The blowers are controlled by the master CPU software. By default, the
blowers are switched on continuously. An air duct on top of each
measurement unit lead the blower air stream towards the window area.

In the frame of the periodic maintenance, the operation of the blowers

should be checked. The cleanliness of the particle filters should also be
checked, and the filters should be changed if necessary. See instructions
in the next pages.

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WARNING Touching an operational blower fan can cause injury. Keep your fingers
and tools away from the air inlet. Power down the whole LT31, or switch
off the blowers in the service menu (see section Checking Operation
below) before replacing the particle filters and keep the menu open
during the operation. After replacement, switch on the blowers again.

Checking Operation
To check the operation, do the following:

1. Access the Main menu of the LT31 user interface.

2. Press H on the keyboard to access the Service page.
3. Then press A (BLOW) to easily switch on and off the blowers of
the receiver and transmitter.

Now a significant blower air stream must be noticeable in front of the

window panes of the measurement units of the LT31 transmitter and
receiver. During operation, the blowers must not generate any abnormal
noise. If abnormal noise is generated, the blowers need to be replaced.
See section Replacing Blower Unit LTB111 on page 220.

Replacing Particle Filter

To replace the particle filters which are mounted directly in front of the
air stream inlet of the blower system, the measurement head covers have
to be removed first.

1. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
2. Loosen the securing screw to access the particle filter. Tilt the
mounting frame for the particle filter slightly away from the
blower. Particle filter holder is shown in Figure 114 on page 200.

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0401-165

Figure 114 Particle Filter Holder

3. The filter material can now be removed. See Figure 115 below.

0401-166

Figure 115 Particle Filter

4. After checking and replacing the filter material, the securing screw
of the particle filter holder has to be tightened and the measurement
head cover has to be mounted and locked carefully.

Optional Heated Weather Protection

Hoods
The LT31 transmitter and receiver can be optionally equipped with
heated weather protection hoods. These heated hoods are automatically

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switched on during low temperatures in the environment and they protect

the hoods from snow and ice build-up in harsh environments. In the
frame of the periodic maintenance, the operation of the hood heaters
should be checked.

Checking Operation
To check the operation, do the following:

1. Access the Main menu of the LT31 user interface.

2. Press H on the keyboard to access the Service page.
3. Then press B (HEAT) to be able to easily switch on and off the
optional hood heaters of the receiver and transmitter.

When the hood heaters are switched on, a significant temperature

increase must be noticeable on the inner surfaces of the weather
protection hoods of the LT31 transmitter and receiver.

Repair Instructions

Replacing Fuses
The FSP102 Power Supply Board contains two glass tube fuses in the
AC line: in both the phase and return lines. The secondary side of the
transformer also holds two glass tube fuses: one in the low voltage AC
line of the heater power and the other in the DC voltage line. The fuses
are located inside the black, horizontally mounted fuse holders on the
FSP102 board. The fuse holders are visible through a transparent plastic
shield covering the live circuits; see Figure 116 on page 202.

To replace the fuses, proceed as follows:

WARNING Make sure to switch off the AC switch before replacing the fuses.

1. Switch off the AC power switch.

2. Remove the plastic protector shield from FSP102.
3. Remove the fuse from its fuse holder. In case of fuse F1 and F2 the
fuse holder cap has to be removed before accessing the actual fuse.
4. Replace the fuse with a spare one according to the specification
given by the sticker on the plastic protector shield of the FSP102.
5. Attach the plastic protector shield back to the FSP102 board.

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6. Turn the AC power switch back on and verify that both LEDs are
lit on the FSP102 board to indicate that both output power supply
lines are OK.

0808-007

Figure 116 Replacing Fuses

The following numbers refer to Figure 116 above:

1 = Heater and DC line fuses

2 = AC line fuses

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Replacing Power Supply FSP102

The power supply unit FSP102 is located inside the interface units of the
LT31 transmitter and receiver.

1. Remove the upper interface unit enclosure and open the interface
unit with the provided key.

WARNING Make sure to switch off the AC switch before replacing the power
supply.

2. All connectors have to be disconnected and the four spacer bolts

which carry the plastic protector shield, have to be opened to
remove the complete FSP102 module.
3. After replacing FSP102, reconnect all the connectors carefully and
re-install the plastic protector shield properly.
4. Turn on the AC power switch (and also the battery backup switch,
if installed).
5. Close the interface unit and mount the upper interface unit cover
after replacement is finalized.

Replacing Master CPU LTC212

The master CPU LTC212 (see Figure 117 on page 204) is located inside
the interface unit of the LT31 receiver.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off before you proceed with the next steps. In case
of the optional LTBB111, switch the battery backup switch off as well.
See Figure 40 on page 83.

2. Disconnect all the connectors from the LTC212 board.

3. Open the four screws holding the cover plate and remove it.
4. Open the two screws holding the LTC212 board in place and
withdraw the board carefully.

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0401-167

Figure 117 LTC212

CAUTION Handle the board carefully. Do not touch the components on the board to
avoid ESD damage to them. Ground yourself before attempting to touch
the board.

CAUTION Do not let the board get wet.

5. Push the replacement LTC212 board back to its place and attach
the two screws securing it into the correct position.
6. Attach the cover plate by tightening the four screws holding it.
7. Insert the connectors onto the LTC212 board and note their correct
order (refer to the numbers on the cover plate).
8. Turn on the AC power switch (and also the battery backup switch,
if installed).
9. Close the interface unit and mount the upper interface unit cover
after replacing is finalized.

Replacing Optional Modem DMX501

Optional modem DMX501 is shown in Figure 118 below.

0401-106

Figure 118 DMX501

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If the optional modem DMX501 needs to be replaced, the instructions for

replacing the master CPU LTC212 should be followed. If the LTC212 is
removed from the interface unit, the replacement modem can simply be
plugged on the master CPU as shown in Figure 119 below.

NOTE Pay attention to the correct position of the connectors and the orientation
of the module.

0401-158

Figure 119 LTC212 with Optional Modem DMX501

Pay attention to the handling notes given above for the master CPU
LTC212.

Carry out the re-assembling like described for LTC212 above.

Replacing Receiver Measurement Unit

LTM212
To replace the complete receiver measurement unit, the AC power has to
be switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off, before you proceed with the next steps. In
case of the optional LTBB111, switch the battery backup switch off as
well. See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex-socket screws,
size 4 can be reached. See Figure 78 on page 118.

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4. The replacement unit must be mounted in the same way as

described in section Mounting Receiver Measurement Unit on page
124.
5. Make sure that the enclosure is parallel with the support unit.
6. Secure the fastening screws of the measurement unit carefully.
7. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After replacing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration on
page 232.

8. Re-install the measurement head cover and lock it carefully.

9. Close the interface unit and mount the upper interface unit cover
after the replacement, fine alignment, and calibration is finalized.

Replacing Transmitter Measurement Unit

LTM112
To replace the complete transmitter measurement unit, the AC power has
to be switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off, before you proceed with the next steps. In case
of the optional LTBB111, switch the battery backup switch off as well.
See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex-socket screws,
size 4, can be reached. See Figure 78 on page 118.
4. The replacement unit must be mounted in the same way as
described in section Mounting Transmitter Measurement Unit on
page 114.
5. Make sure that the enclosure is parallel with the support unit.
6. Tighten the fastening screws of the measurement unit carefully.

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7. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After replacing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration on page
232.

8. Re-install the measurement head cover and lock it carefully.

9. Close the interface unit and mount the upper interface unit cover
after the replacement, fine alignment, and calibration is finalized.

Replacing Receiver Optics Unit LTO212

NOTE To replace the complete receiver optics unit, the receiver measurement
unit has to be opened. For this reason, it is highly recommended to
dismount the complete measurement unit and carry out the replacement
on a clean workbench in a dry room.

To remove the complete receiver measurement unit, the AC power has to

be switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off before you proceed with the next steps. In case
of the optional LTBB111, switch the battery backup switch off as well.
See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex-socket screws,
size 4 can be reached. See Figure 78 on page 118.

The following steps should be carried out in a closed room. The

measurement unit cover has to be opened carefully.

1. Loosen the four fastening screws of the cover plate, hex-socket,

size 5, and remove the cover with air duct. See Figure 72 on page
115.

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2. To remove the optics unit, two fastening screws (hex-socket size

3), and one fastening bolt (socket wrench, size 7) need to be
loosened. See Figure 120 below.

1104-108

Figure 120 Receiver Measurement Unit Opened

3. Disconnect the three plugs at the rear end of the LTC112 board.
4. Check if it is necessary to clean the measurement unit windows
from the inside. Wipe the window panes with a soft, lint-free cloth
and normal glass cleaner or isopropyl alcohol. Be careful not to
scratch the window surfaces.
5. After replacing the optics unit, pay attention to the correct position.
The optics unit should be mounted in parallel to the enclosure.
6. Reconnect the three plugs at the rear end of the LTC112 board.
7. Tighten the two fastening screws and the bolt carefully.
8. The complete optics unit consists of fully adjusted and tested
modules. For that reason, no adjustments are necessary.
9. Close the measurement unit carefully. Pay attention to the position
of the cover seal.
10. The repaired measurement unit must be mounted in the same way
as described in section Mounting Receiver Measurement Unit on
page 124.
11. Make sure that the enclosure is parallel with the support unit.
12. Secure the fastening screws of the measurement unit carefully.
13. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After repairing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration.

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14. Re-install the measurement head cover and lock it carefully.

15. Close the interface unit and mount the upper interface unit cover
after fine alignment and calibration is finalized.

Replacing Transmitter Optics Unit

LTO112
NOTE To replace the complete receiver optics unit, the receiver measurement
unit has to be opened. For this reason, it is highly recommended to
dismount the complete measurement unit and carry out the replacement
work on a clean workbench in a dry room.

To remove the complete transmitter measurement unit, the AC power has

to be switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off, before you proceed with the next steps. In
case of the optional LTBB111, switch the battery backup switch off as
well. See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex- socket screws,
size 4, can be reached. See Figure 78 on page 118.

The following steps should be carried out in a closed room. The

measurement unit cover has to be opened carefully.

1. Loosen the four fastening screws of the cover plate, hex-socket,

size 5 and remove the cover with air duct. See Figure 72 on page
115.
2. To remove the optics unit, two fastening screws (hex-socket size 3)
and one fastening bolt (socket wrench, size 7) have to be loosened.
See Figure 121 on page 210.

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1104-109

Figure 121 Transmitter Measurement Unit Opened

3. Disconnect the three plugs at the rear end of the LTC112 board.
4. Check if it is necessary to clean the measurement unit windows
from the inside. Wipe the window panes with a soft, lint-free cloth
and normal glass cleaner or isopropyl alcohol. Be careful not to
scratch the window surfaces.
5. After replacing the optics unit, pay attention to the correct position.
The optics unit should be mounted in parallel to the enclosure.
6. Reconnect the three plugs at the rear end of the LTC112 board.
7. Tighten the two fastening screws and the bolt carefully.
8. The complete optics unit consists of fully adjusted and tested
modules. For that reason, no adjustments are necessary.
9. Finally close the measurement unit carefully. Pay attention to the
position of the cover seal.
10. The repaired measurement unit must be mounted in the same way
as described in section Mounting Receiver Measurement Unit on
page 124.
11. Make sure that the enclosure is in parallel with the support unit.
12. Secure the fastening screws of the measurement unit carefully.
13. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After repairing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration on
page 232.

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14. Re-install the measurement head cover and lock it carefully.

15. Close the interface unit and mount the upper interface unit cover
after fine alignment and calibration is finalized.

Replacing Measurement CPU/Window

Receiver LTC112
The measurement units of the LT31 transmitter and receiver are equipped
with identical measurement CPUs LTC112. In case of a faulty window
receiver, the measurement CPU has to be replaced as well. The procedure
is identical for the receiver and transmitter measurement unit.

NOTE To replace the measurement CPU LTC112, the measurement unit has to
be opened. For this reason, it is highly recommended to dismount the
complete measurement unit and carry out the replacement work on a
clean workbench in a dry room.

To remove the complete measurement unit, the AC power has to be

switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off before you proceed with the next steps. In case
of the optional LTBB111, switch the battery backup switch off as well.
See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex-socket screws,
size 4, can be reached. See Figure 78 on page 118.

The following steps should be carried out in a closed room. The

measurement unit cover has to be opened carefully.

1. Loosen the four fastening screws of the cover plate, hex-socket size
5, and remove the cover with air duct. See Figure 72 on page 115.
2. To replace the LTC112, the optics unit has to be removed first.
Two fastening screws (hex-socket size 3) and one fastening bolt
(socket wrench, size 7) have to be loosened. See Figure 121 on

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page 210 as example. Disconnect the three plugs at the rear of the
LTC112 board.
3. Check if it is necessary to clean the measurement unit windows
from the inside. Wipe the window panes with a soft, lint-free cloth
and normal glass-cleaner or isopropyl alcohol. Be careful not to
scratch the window surfaces.
4. Open five hex-socket screws, size 2.5 (see Figure 122 below), from
the measurement CPU board and remove it from the optics unit.
Remove all connectors.

1104-110

Figure 122 Fastening Screws of LTC112

CAUTION Handle the board carefully. Do not touch the components on the board to
avoid ESD damage to them. Ground yourself before attempting to touch
the board.

CAUTION Do not let the board get wet.

5. Replace the LTC112 and tighten all five fastening screws carefully.
Attach all connectors.
6. The spare LTC112 board is fully adjusted and tested, no additional
adjustments are necessary.
7. After replacing the LTC112 board, pay attention to the correct
position of the optics unit inside the measurement unit enclosure. It
should be mounted in parallel to the enclosure walls.
8. Reconnect the three plugs at the rear end of the LTC112 board.
9. Tighten the two fastening screws and the bolt carefully.
10. Close the measurement unit carefully. Pay attention to the position
of the cover seal.

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11. The repaired measurement unit must be mounted in the same way
as described in sections Mounting Receiver Measurement Unit on
page 124 and on page 114.
12. Make sure that the enclosure is parallel with the support unit.
13. Secure the fastening screws of the measurement unit carefully.
14. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After repairing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration on
page 232.

15. Re-install the measurement head cover and lock it carefully.

16. Close the interface unit and mount the upper interface unit cover
after fine alignment and calibration is finalized.

Replacing Window Transmitter Module

LTL212
The measurement units of the LT31 transmitter and receiver are equipped
with identical window transmitter modules LTL212. In case of a faulty
window transmitter, the optics unit has to be removed from the
measurement unit to access the window transmitter. The procedure is
identical for the receiver and transmitter measurement unit.

NOTE To replace the window transmitter module LTL212, the measurement

unit has to be opened. For this reason, it is highly recommended to
dismount the complete measurement unit and carry out the replacement
work on a clean workbench in a dry room.

To remove the complete measurement unit, first the AC power has to be

switched off and then the measurement head cover has to be removed.

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1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off, before you proceed with the next steps. In
case of the optional LTBB111, switch the battery backup off as well.
See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex-socket screws,
size 4, can be reached. See Figure 78 on page 118.

The following steps should be carried out in a closed room. The

measurement unit cover has to be opened carefully.

1. Loosen the four fastening screws of the cover plate, hex-socket,

size 5, and remove the cover with air duct. See Figure 72 on page
115.
2. To replace the LTL212, the optics unit has to be removed first.
Two fastening screws (hex-socket size 3) and one fastening bolt
(socket wrench, size 7) have to be loosened. For an example, see
Figure 121 on page 210. Disconnect the three plugs at the rear end
of the LTC112 board.
3. Check if it is necessary to clean the measurement unit windows
from the inside. Wipe the window panes with a soft, lint-free cloth
and normal glass cleaner or isopropyl alcohol. Be careful not to
scratch the window surfaces.
4. Open two hex-socket screws, size 2, from the window transmitter
module and remove it from the optics unit. Use an edge cutter to
cut the cable tie carefully. See Figure 123 below.

1104-111

Figure 123 Fastening Screws of LTL212

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CAUTION Handle the module carefully. Do not touch the components on the board
to avoid ESD damage to them. Ground yourself before attempting to
touch the board.

CAUTION Do not let the module get wet.

5. Replace the LTL212 and tighten the fastening screws carefully.

Secure the cables with a cable tie.
6. The spare LTL212 module is fully adjusted and tested, no
additional adjustments are necessary.
7. After replacing the LTL212 module, pay attention to the correct
position of the optics unit inside the measurement unit enclosure. It
should be mounted in parallel to the enclosure walls.
8. Reconnect the three plugs at the rear end of the LTC112 board.
9. Tighten the two fastening screws and the bolt carefully.
10. Close the measurement unit carefully. Pay attention to the position
of the cover seal.
11. The repaired measurement unit must be mounted in the same way
as described in sections Mounting Receiver Measurement Unit on
page 124 and Mounting Transmitter Measurement Unit on page
114.
12. Make sure that the enclosure is parallel to the support unit.
13. Secure the fastening screws of the measurement unit carefully.
14. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After repairing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration on
page 232.

15. Re-install the measurement head cover and lock it carefully.

16. Close the interface unit and mount the upper interface unit cover
after fine alignment and calibration is finalized.

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Replacing Main Transmitter Module

LTL112
The LTM112 transmitter measurement unit of the LT31 is equipped with
the main transmitter module LTL112. In case of a faulty main transmitter
module, the measurement unit has to be opened for replacing purposes.

NOTE To replace the main transmitter module LTL112, the measurement unit
has to be opened. For this reason it is highly recommended to dismount
the complete measurement unit and carry out the replacement work on a
clean workbench in a dry room.

NOTE If the weather conditions allow, it is possible to replace the main

transmitter module on site. In that case, the measurement unit can
remain installed on the mast.

To access the transmitter measurement unit, the AC power has to be

switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off, before you proceed with the next steps. In
case of the optional LTBB111, switch the battery backup switch off as
well. See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. The measurement unit is fixed with two fastening screws. Through
the mast adapter of the support unit, these two hex-socket screws,
size 4 can be reached. See Figure 78 on page 118.

The following steps should be carried out in a closed room. The

measurement unit cover has to be opened carefully.

1. Loosen the four fastening screws of the cover plate, hex-socket,

size 5, and remove the cover with the air duct.
2. Loosen one hex-socket set screw, size 1.5 (see Figure 124 on page
217), and remove the main transmitter module from the optics unit.
Use an edge cutter to cut the cable tie carefully.

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1104-112

Figure 124 Securing Screw of LTL112

CAUTION Handle the module carefully. Do not touch the components on the board
to avoid ESD damage to them. Ground yourself before attempting to
touch the board.

CAUTIO Do not touch the front window of the LTL112 module. Ensure that this
window is free of damages, fingerprints and dirt!

CAUTION Do not let the module get wet.

3. Replace the LTL112, reconnect the cabling, tighten the securing

screw carefully, and secure the cables with a cable tie. Pay
attention not to damage the optical tube of the main transmitter
module while tightening the screw!
4. The spare LTL112 module is fully adjusted and tested, no
additional adjustments are necessary.
5. After replacing the LTL112 module, close the measurement unit
carefully. Pay attention to the position of the cover seal.
6. The repaired measurement unit must be mounted in the same way
as described in section Mounting Transmitter Measurement Unit on
page 114.
7. Make sure that the enclosure is parallel with the support unit.
8. Secure the fastening screws of the measurement unit carefully.
9. Turn on the AC power switch (and also the battery backup switch,
if installed).

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CAUTION After repair of the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignment on page 231 and Calibration on
page 232.

10. Re-install the measurement head cover and lock it carefully.

11. Close the interface unit and mount the upper interface unit cover
after finalizing fine alignment and calibration.

Replacing Main Receiver Module LTD112

The LTM212 receiver measurement unit of the LT31 is equipped with
the main receiver module LTD112. In case of a faulty main receiver
module, the measurement unit has to be opened for replacement
purposes.

NOTE To replace the main receiver module LTD112, the measurement unit has
to be opened. For this reason it is highly recommended to dismount the
complete measurement unit and carry out the replacement work on a
clean workbench in a dry room.

NOTE If the weather conditions allow, it is possible to replace the main

receiver module on site. In that case, the measurement unit remains
installed on the mast.

To access the receiver measurement unit, the AC power has to be

switched off first and then the measurement head cover has to be
removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

NOTE Switch the AC switch off, before you proceed with the next steps. In
case of the optional LTBB111, switch the battery backup switch off as
well. See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.

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3. The measurement unit is fixed with two fastening screws. Through

the mast adapter of the support unit, these two hex-socket screws,
size 4, can be reached. See Figure 78 on page 118.

The following steps should be carried out in a closed room. The

measurement unit cover has to be opened carefully.

1. Loosen the four fastening screws of the cover plate, hex-socket,

size 5, and remove the cover with the air duct. See Figure 72 on
page 115.
2. Loosen one hex-socket set screw, size 1.5 (see Figure 125 below),
and remove the main receiver module from the optics unit. Use an
edge cutter to cut the cable tie carefully.

1104-113

Figure 125 Securing Screws of LTD112

CAUTION Handle the module carefully. Do not touch the components on the board
to avoid ESD damage to them. Ground yourself before attempting to
touch the board.

CAUTION Do not let the module get wet.

3. Replace the LTD112, reconnect the cabling, tighten the securing

screw carefully, and secure the cables with a cable tie. Pay
attention not to damage the optical tube of the main receiver
module while tightening the screw!
4. The spare LTD112 module is fully adjusted and tested, no
additional adjustments are necessary.

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5. After replacing the LTD112 module, close the measurement unit

carefully. Pay attention to the position of the cover seal.
6. The repaired measurement unit must be mounted in the same way
as described in section Mounting Receiver Measurement Unit on
page 124.
7. Make sure that the enclosure is parallel with the support unit.
8. Secure the fastening screws of the measurement unit carefully.
9. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION After repairing the measurement unit, fine alignment and calibration
must be carried out to establish normal operational conditions. For
details, see sections Fine Alignmenton page 231 and Calibrationon page
232.

10. Re-install the measurement head cover and lock it carefully.

11. Close the interface unit and mount the upper interface unit cover
after finalizing fine alignment and calibration.

Replacing Blower Unit LTB111

After approximately five years of operation, the blower units of LT31
need to be replaced. The blower units are identical for the LT31
transmitter and receiver.

To replace a blower unit, the AC power has to be switched off first and
then the measurement head cover has to be removed.

1. Remove the upper interface unit cover and open the interface unit
with the provided key.

CAUTION Make sure to switch off the AC switch before replacing the blower unit.

NOTE In case of the optional LTBB111, switch the battery backup switch off
as well. See Figure 40 on page 83.

2. Loosen the three lock screws at each side of the measurement head
cover and carefully remove the cover from the measurement head.
See Figure 95 on page 140.
3. To access the fastening screws of the blower unit, the measurement
unit has to be removed first.

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4. The measurement unit is fixed with two fastening screws. Through

the mast adapter of the support unit, these two hex-socket screws,
size 4, can be reached. See Figure 78 on page 118.
5. Now the four hex-socket screws, size 3, are accessible, which fix
the blower onto the support unit. See Figure 126 below.

0408-018

Figure 126 Fastening Screws of Blower Unit

6. Open the junction box and disconnect the blower lines from the
terminals.
7. Install the new blower unit (including blower capacitor and particle
filter). Take care of the cable routing through the feed-through hole
in the support unit, and the cable feed-through of the junction box.
8. Tighten the four blower fastening screws carefully.
9. Reconnect the blower lines and the blower capacitor to the
terminals and close the cable feed-through and the junction box
tightly.
10. The measurement unit must now be mounted in the same way as
described in sections Mounting Receiver Measurement Unit on
page 124 and Mounting Transmitter Measurement Unit on page
114.
11. Make sure that the enclosure is parallel with the support unit.
12. Secure the fastening screws of the measurement unit carefully.
13. Turn on the AC power switch (and also the battery backup switch,
if installed).
14. Carry out an operation check for the blower system as described in
section Checking Operation on page 201.

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CAUTION After any movement of the measurement unit, fine alignment and
calibration must be carried out to establish normal operational
conditions. For details, see sections Fine Alignment on page 231 and
Calibration on page 232.
15. Re-install the measurement head cover and lock it carefully.
16. Close the interface unit and mount the upper interface unit cover
after exchange, fine alignment and calibration is finalized.

Replacing Battery in Case of Optional

LTBB111
In case of the optional battery backup LTBB111, a battery loader module
QBR101 and a 2 Ah battery are located inside the interface units of the
LT31 receiver and transmitter. After approximately 5 years of operation,
the battery should be replaced to ensure a safe backup operation in case
of an AC power break-down.

1. Remove the upper interface unit enclosure and open the interface
unit with the provided key.

NOTE Switch the AC switch off and also the battery backup switch off, before
you proceed with the next steps. See Figure 40 on page 83.

2. Open the four screws holding the cover plate for the battery and
remove it. See Figure 127 on page 223.

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0810-074

Figure 127 Backup Battery

3. Disconnect the two power connectors from the battery.

4. The battery can now be withdrawn carefully.

CAUTION Do not short circuit the battery.

NOTE A valve-regulated battery is used for the battery backup. According to

the ISO 14001 standard, the battery must be recycled.

5. Push the replacement battery back to its place and reconnect the
power connectors. Watch the right polarity!
6. Attach the cover plate by tightening the four screws holding it.
7. Turn on the AC power switch and the battery backup switch.
8. Close the interface unit and mount the upper interface unit cover
after exchange work is finalized.

Replacing PWD Sensor

The PWD sensor is mounted to the support arm of the LT31 transmitter
unit.

To replace the PWD sensor, the AC power has to be switched off first.

1. Remove the upper interface unit enclosure and open the interface
unit of the LT31 transmitter unit with the provided key.

NOTE Switch the AC switch to off before proceeding with the next steps. In
case of the optional LTBB111, switch the battery backup switch to off
as well. See Figure 40 on page 83.

2. Disconnect the PWD cable plug.

3. Loosen the PWD attachment screw and remove the faulty PWD
sensor.
4. Install the PWD spare unit using the attachment screws and
washers.
5. Connect the cable plug to PWD. Make sure that the plug is
carefully and completely screwed on the PWD receptacle. See
Figure 80 on page 119.

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6. Turn on the AC power switch (and also the battery backup switch,
if installed).

CAUTION When the PWD forward scatter sensor is replaced, the spare unit must
be prepared for LT31 communication using the Spare Unit Init
function on the PWD sensor menu. For details, see section Spare Unit
Init on page 244.

7. Close the interface unit and mount the upper interface unit cover.

Replacing optional LM21 Sensor

The LM21 background luminance sensor is optionally mounted to the
LM21 support arm at the LT31 receiver unit.

To replace the LM21 sensor, the AC power has to be switched off first.

1. Remove the upper interface unit enclosure and open the interface
unit of the LT31 receiver unit with the provided key.

NOTE Switch the AC switch to off before proceeding with the next steps. In
case of the optional LTBB111, switch the battery backup switch to off
as well. See Figure 40 on page 83.

2. Disconnect the LM21 cable plug.

3. Loosen the LM21 attachment screws and remove the faulty LM21
sensor.
4. Install the LM21 spare unit using the attachment screws and
washers.
5. Connect the cable plug to LM21. Make sure that the plug is
carefully and completely screwed on the LM21 receptacle.
6. Turn on the AC power switch (and also the battery backup switch,
if installed).
7. Close the interface unit and mount the upper interface unit cover.

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Alignment
Any mechanical change to the LT31 system demands an alignment. The
mechanical changes can be the following:

- Initial set-up of the LT31 system.

- Full replacement of the transmitter or receiver measurement unit.
- Replacement of any PCB of the transmitter or receiver measurement
unit
- Indication of Reduced Alignment Quality in the overall status (see
section Status Codes on page 164).

Depending on the mechanical impact, a raw and fine alignment or a fine

alignment only must be carried out.

The alignment procedure is heavily supported by the Alignment menu.

To enter the Alignment menu, it is necessary to open the maintenance
terminal (see section Entering and Exiting Command Mode on page 134)
and enter the menu-driven operation (see section Menu-driven Operation
on page 156).

0401-112

Figure 128 Alignment Menu

As Figure 128 above shows, the Alignment menu consist of three areas:

- On the left, the available actions are displayed. During the alignment
process the progress is indicated.
- On the right, necessary information is given for the transmitter and
receiver measurement unit.
The overall Status and the Axes Position Data of the transmitter and
receiver measurement unit are displayed.
For the receiver measurement unit, the Synchronization Signal status
and the Main Receiver Signal Strength are displayed additionally.
The Synchronization Signal Status indicates the quality of the

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synchronization signal which is transmitted by the transmitter

measurement unit and received by the receiver measurement unit.
The Main Receiver Signal Strength is the relation between the main
receiver signal value and the expected maximum receiver signal value.
The expected maximum receiver signal value is a calculated value
which is derived from the configured baseline length. A reading of a
100 % or even above is not an indication of a receiver signal
saturation.
- The lower part is used for user interaction.

Any action is started by simply pressing the appropriate key:

A Start Raw and Fine Alignment

B Start Fine Alignment only

All status information has to be read as OK for proper operation during

the alignment.

During the raw alignment, all axes of the transmitter or receiver

measurement unit are set to the mid position.

When all the mid positions have been reached, an acoustical signal is
generated at the transmitter and receiver location. Information about the
finalisation of raw alignment by the user has to be given to the LT31
system.

After that the fine alignment will be started by the LT31 system. During
the fine alignment, all axes are scanned to find the axis’ position for the
highest main receiver signal available.

To run the fine alignment, you must have at least some transmitter light
at the LT31 receiver.

It is recommended to perform the calibration procedure after the fine

alignment.

Raw and Fine Alignment

To start the raw and fine alignment, press A after entering the Alignment
menu.

CAUTION The transport protection in the transmitter and receiver measurement

unit must be removed before moving any optics.

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NOTE It is important for the alignment procedure, that no significant

atmospheric visibility variations occur during the procedure. For this
reason, it is recommended to do the procedure in weather conditions that
are stable without precipitation and with visibility above 10 km.

To make sure that the transport protection in the transmitter and receiver
measurement unit have been removed, the user has to give a
confirmation. See Figure 129 below and the example below:
HAS THE TRANSPORT PROTECTION IN THE T/R OPTIC UNITS BEEN
REMOVED? (yes/no) :

To confirm, type yes<CR>.

To abort the action, type no<CR>.

As the current position of all axes will be lost, the user has to confirm the
movement to the mid position for all axes. See the example below:
Move Optics To Mid Position. Are You Sure? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

Now all axes are moved to the mid position, one by one. Indication of the
process is given on the left of the menu.

0401-113

Figure 129 Moving the Axes to Mid-position

To get an impression of the main receiver signal value without reading

the display, an acoustic signal is generated at the transmitter and receiver
location.

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- The main receiver signal is less than 20 % of the expected maximum

receiver signal: The acoustical signal is modulated with a short signal
and a long break in the beat of one second.
- The main receiver signal is between 20 % and 80 % of the expected
maximum receiver signal: The acoustical signal is modulated with a
mid signal and a mid break in the beat of one second.
- The main receiver signal is greater than 80 % of the expected
maximum receiver signal: The acoustical signal is permanent and
uninterrupted.

It is desirable to reach a main receiver signal greater than 40 % of the

expected maximum receiver signal by raw alignment.

If the main receiver signal is lower than 40 %, the horizontal and vertical
position of the transmitter and receiver have to be double-checked.

To double-check the receiver location, do the following:

1. Place the ladder behind the receiver.

2. Remove the measurement head cover and check if the transmitter
head image is exactly visible through the sight hole and if the
measurement unit is turned in line with the support unit (refer to
Figure 53 on page 98). If this is not the case, the receiver alignment
has to be improved.

During the following steps, check if the acoustic signal changes to a

longer or permanent signal:

1. If the transmitter image in the sight hole is shifted left or right,

loosen the two fastening screws of the support unit (lower two hex-
socket screws, size 4, refer to Figure 52 on page 97) and then turn
the support unit slightly until the orientation is perfect.
2. Tighten the support unit fastening screws.
3. Loosen the two fastening screws of the receiver measurement unit
(upper two hex-socket screws, size 4, refer to Figure 74 on page
116) and turn it in line with the support unit.
4. Tighten the measurement unit fastening screws.
5. If the transmitter image in the sight hole is shifted above or below,
loosen the foundation nuts slightly and turn the front nuts to move
the base plate according to your observation (refer to Figure 66 on
page 111).
6. Tighten the foundation nuts carefully after you have finished.
7. Re-install the measurement head cover.
8. If the receiver alignment check did not come to a sufficient result,
the raw alignment of the transmitter has to be checked as well.

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To double-check the transmitter location, do the following:

1. Place the ladder behind the receiver and check if the transmitter
light beam (diameter approx. 1.5 m in 30 m distance) must be
further adjusted.
2. Check the edges of the light beam.

If the light is visible even in a distance of approx. 0.5 m below, above,

left, and right from the receiver head, the transmitter is sufficiently
aligned for the following steps.

1. Place the ladder behind the transmitter. The acoustical signal must
be audible.
2. Remove the measurement head cover and check if the receiver
head image is exactly visible through the sight and if the
measurement unit is turned in line with the support unit.

During the following steps, check if the acoustic signal changes to longer
or even permanent signal.

1. If the receiver image in the sight hole is shifted left or right, loosen
the two fastening screws of the support unit (lower two hex-socket
screws, size 4) and turn the support unit slightly until the
orientation is perfect.
2. Tighten the support unit securing screws.
3. Loosen the two securing screws of the transmitter measurement
unit (upper two hex-socket screws, size 4) and turn it carefully in
line with the support unit. Check if you can find a position where
the acoustical signal turns to a longer or even a permanent signal.
4. After optimizing, tighten the measurement unit securing screws.
5. If the receiver image in the sight hole is shifted above or below and
the acoustical signal indicates still a too low signal strength, loosen
the foundation nuts slightly and turn the front nuts to move the base
plate according to your observation.
6. Tighten the foundation nuts carefully after you have finished.
7. Re-install the measurement head cover.

NOTE In case that the main receiver signal does not exceed 40 % even after
double-checking the raw alignment, proceed with the completion of the
raw alignment followed by the fine alignment as described below.

The completion of the raw alignment by the user has to be confirmed (see
Figure 130 on page 230):
Perform Raw Alignment! Done? (yes/no):

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To confirm, type yes<CR>.

To abort the action, type no<CR>.

From now on, the acoustic signal is switched off. To continue, a

confirmation to start fine alignment has to be given.
Start Fine Alignment? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

Now all the axes are scanned to find the position of axis for the highest
main receiver signal available, one by one. Indication of the process is
given on the left of the menu. On the right, the current positions and the
main receiver signal are changing according to the ongoing process. In
the lower part, found positions for the highest main receiver signal are
displayed after each scan.

0401-114

Figure 130 Scanning Axes during Raw and Fine Alignment

The user is requested to start calibration procedure immediately.

NOTE A Signal indication of at least 60 % is recommended to be achieved after

the fine alignment procedure is finalized. Otherwise please doublecheck
the raw alignment.

NOTE It is highly recommended to perform calibration procedure after fine

alignment.

Proceed with Calibration? (yes/no):

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To confirm, type yes<CR>.

To abort the action, type no<CR>.

Fine Alignment
To start the fine alignment, press B in the Alignment menu.

CAUTION The transport protection in the transmitter and receiver measurement

unit have to be removed before moving any optics.

NOTE It is important for the alignment procedure, that no significant

atmospheric visibility variations occur during the procedure. For this
reason, it is recommended to do the procedure in weather conditions that
are stable without precipitation and with visibility above 10 km.

To make sure that the transport protection in the transmitter and receiver
measurement unit have been removed, the user has to give a confirmation
(see Figure 131 on page 232):
HAS THE TRANSPORT PROTECTION IN THE T/R OPTIC UNITS BEEN
REMOVED? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

Now all axes are scanned to find the position of axis for the highest main
receiver signal available, one by one. Indication of the process is given
on the left of the menu. On the right, the current positions and the main
receiver signal are changing according to the ongoing process. In the
lower part, found positions for the highest main receiver signal are
displayed after each scan.

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0401-115

Figure 131 Scanning Axes during Fine Alignment

The user is requested to start the calibration procedure immediately.

NOTE A Signal indication of at least 60 % is recommended to be achieved after

the fine alignment procedure is finalized. Otherwise please doublecheck
the raw alignment.

NOTE It is highly recommended to perform the calibration procedure after the

fine alignment.

Proceed with Calibration? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

Calibration
The LT31 subsystems, such as transmitter/receiver measurement units
and PWD sensor, are calibrated at the factory. As the achievable
measurement signal depends on the individual tolerances of the
opto-electronic systems and the alignment result itself, the measurement
signal has to be correlated to the prevailing visibility to bring the
transmissometer into a final operational condition.

Due to the LT31 system topology, an overall calibration is necessary

during initial set-up of the LT31 system. Furthermore, calibration is
necessary whenever any alignment procedure has been performed.

Depending on the situation, an offset and visibility calibration or a

visibility calibration only must be carried out.

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During the calibration process, no significant visibility variations should

occur. Stable weather conditions without precipitation and a visibility
above 10 km are recommended.

The offset and visibility calibration are supported by the Calibration

menu. The visibility calibration can alternatively be performed by a
command line command.

Calibration Menu
To enter the Calibration menu, it is necessary to open the maintenance
terminal (see section Entering and Exiting Command Mode on page 145)
and enter the menu-driven operation (see section Menu-driven Operation
on page 156).

0401-136

Figure 132 Calibration Menu

As shown in Figure 132 above, the Calibration menu consist of three

areas:

- On the left side, the available actions are displayed. Progress is

indicated during the calibration process.
- On the right side, necessary information is given for the
transmitter/receiver measurement unit and the PWD sensor. The
transmitter/receiver measurement unit and PWD sensor overall status
are displayed.
For the receiver measurement unit, the Synchronization Signal status
and the Main Receiver Signal Strength are displayed additionally.
The Synchronization Signal status indicates the quality of the
synchronization signal which is transmitted by the transmitter
measurement unit and received by the receiver measurement unit.
The Main Receiver Signal Strength is the relation between the Main
Receiver Signal value and the expected Maximum Receiver Signal

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value. The expected Maximum Receiver Signal value is a calculated

value which is derived from the configured baseline length. A reading
of a 100 % or even above is not an indication of a Main Receiver
Signal saturation. The absolute value of the displayed Main Receiver
Signal has no direct relation to the prevailing transmittance since it is
a non-calibrated reading.
Furthermore the currently active calibration values Offset and
CALfact are displayed.
The lower part is used for user interaction.
Any action is started by simply pressing the appropriate key:

A Start Offset and Visibility Calibration

B Start Visibility Calibration only

All status information have to be read as OK for proper operation during

the calibration.

During offset calibration, the optical path is blocked and the system uses
the remaining Main Receiver signal as the offset value.

During the menu-guided procedure, the zero signal level (offset) without
any receiver signal (receiver optical system blocked) and the signal level
in accordance to the prevailing visibility are measured. LT31 calculates
automatically the appropriate calibration parameters from these two
measurement values in combination with the information about the
prevailing visibility. The visibility information can be created either from
the integrated PWD Sensor or from an observer estimation. After
finalising the procedure, the calibration parameters are automatically
stored.

Before any visibility calibration, all the optical surfaces of the

transmitter/receiver measurement unit and PWD sensor have to be
cleaned.

NOTE Always select the offset and visibility calibration mode for the initial
calibration during installation or in the frame of periodic maintenance.

Offset and Visibility Calibration

To start the offset and visibility calibration, press A in the Calibration
menu.

Have the optical blocker available, to block the optical path at the
receiver measurement head. Figure 133 on page 235 shows the optical
blocker mounted.

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To make sure that the operation is stable, the LT31 system should be
powered up for at least 20 minutes. The user has to give a confirmation.
System Powered For At Least 20 Min? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

For offset calibration, the optical path has to be blocked. This will have
to be done at the receiver location. The user has to confirm the blockage.

0401-137

Figure 133 Optical Blocker Mounted on Receiver Support Unit

Block Receiver! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

Now the Main Receiver Signal is evaluated and taken as an offset value.
An indication of progress is given on the left of the menu (see Figure 134
on page 236).

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0401-138

Figure 134 Offset Calibration

When the offset signal has been taken, the optical path has to be
unblocked. The user has to confirm the clearance of the optical path.
Unblock Receiver! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

NOTE For a proper visibility calibration, all the optical surfaces of the
transmitter/receiver measurement unit and PWD sensor have to be clean.

Clean Windows! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

Now the Main Receiver Signal is evaluated and taken as signal for a
visibility value. An indication of process is given on the left of the menu
(see Figure 135 on page 237).

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0401-139

Figure 135 Visibility Calibration

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The taken Main Receiver Signal has to be linked to a MOR reading (see
Figure 136 below).

0401-140

Figure 136 Choosing MOR Reading

Type A for applying the PWD sensor visibility or type B to give an

observer estimation for MOR. When using PWD sensor visibility has
been selected no extra input is needed. If the user wants to give an
observer estimation, the system will prompt for an extra input:
Give Observer’s Visibility Estimation In Meter:

Type the value and terminate the input by pressing <CR>.

The new calibration data is displayed on the right side. The new value
have to be accepted by the user.
Accept Calibration? (yes/no):

To accept the new values, type yes<CR>.

To discard the new values, type no<CR>.

An indication about the storage of the data is given. When the calibration
is completed, Done! is displayed in the lower part of the menu.

Visibility Calibration
To start the visibility calibration, press B in the Calibration menu.

NOTE For a proper visibility calibration, all the optical surfaces of the
transmitter/receiver measurement unit and PWD sensor must be clean.

Clean Windows! Done? (yes/no):

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To confirm, type yes<CR>.

To abort the action, type no<CR>.

Now the Main Receiver Signal is evaluated and taken as a signal for a
visibility value. An indication of progress is given on the left of the menu
(see Figure 137 below).

0401-141

Figure 137 Visibility Calibration

The taken Main Receiver Signal has to be linked to an MOR reading (see
Figure 138 below).

0401-142

Figure 138 Choosing MOR Reading

Type A for applying the PWD sensor visibility or type B to give an

observer estimation for MOR. When using PWD sensor, visibility has
been selected and no extra input is needed. If the user wants to give an
observer estimation, the system is prompting for an extra input:
Give Observer’s Visibility Estimation In Meter:

Type the value and terminate the input by pressing <CR>.

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The new calibration data is displayed on the right side. The new value
have to be accepted by the user.
Accept Calibration? (yes/no):

To accept the new values, type yes<CR>.

To discard the new values, type no<CR>.

An indication about the storage of the data is given. When the calibration
is completed, Done! is displayed in the lower part of the menu.

Command Line Calibration

Visibility Calibration
When using the command line for visibility calibration, it can only be
performed on the advanced user level.

NOTE For a proper visibility calibration, all the optical surfaces of the
transmitter/receiver measurement unit and PWD sensor have to be clean.

To perform visibility calibration by taking the PWD sensor MOR

reading, just enter the following at the command prompt:
1> calibrate visibility<CR>

The answer will be as follows:

PWD MOR taken:14458.3
BaseCalfact:1.24266e-06
CALIBRATION SUCCEEDED

To perform visibility calibration by taking an observer’s MOR

estimation, enter the reading in meters as an additional parameter.
1> calibrate visibility 15000<CR>

The answer will be as follows:

Given MOR taken:15000
BaseCalfact:1.24314e-06
CALIBRATION SUCCEEDED

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PWD Sensor
Due to the system topology, the PWD sensor is connected to the module
bus and hidden from user access. For your convenience, a subset of PWD
maintenance commands is available through the LT31 system
maintenance port, namely the PWD Sensor menu. The menu is shown in
Figure 139 below.

0401-116

Figure 139 PWD Sensor Menu

CLEAN Command
After cleaning the optical surfaces of the PWD sensor, the cleanliness of
the optical surfaces has to be announced to the PWD sensor.

The CLEAN command does not need any parameter.

Select execution of the CLEAN command by pressing A. The user has to

confirm the cleaning of the optical surfaces.
Clean PWD lenses! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

For details, refer to PWD22 User's Guide.

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The PWD sensor response is displayed (see Figure 140 below).

0401-118

Figure 140 PWD Sensor Menu, CLEAN Command

ZERO Command
To calibrate the PWD sensor offset signal, the optical path of the receiver
has to be blocked. When the optical path of the receiver is blocked, the
ZERO command can be executed.

The ZERO command does not need any parameter.

Select execution of the ZERO command by pressing B. The user has to

confirm the applying of the blocker.
Apply Signal Blocker! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

The PWD sensor response is displayed.

For details, refer to PWD22 User's Guide.

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CAL Command
To calibrate the PWD sensor visibility signal, the opaque plates have to
be applied. When the opaque plates have been applied, the CAL
command can be executed.

Select execution of the CAL command by pressing C. The user has to

confirm the applying of the opaque plates.
Apply Opaque Glass Plates! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

The CAL command needs the Calibrator Signal Value which is printed
on the opaque plates, as a parameter.

Give Calibrator_Signal_Value:

Enter the Calibrator Signal Value and terminate the input by <CR>.

The PWD sensor response is displayed.

For details, refer to PWD22 User's Guide.

CHEC Command
PWD sensor calibration can be checked when the opaque plates have
been applied. When the opaque plates have been applied, the CHEC
command can be executed.

The CHEC command does not need any parameter.

Select execution of the CHEC command by pressing D. The user has to

confirm the applying of the opaque plates.
Apply Opaque Glass Plates! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR>.

The PWD sensor response is displayed.

For details, refer to PWD22 User's Guide.

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Other Commands
NOTE Refer to PWD User's Guide before maintaining the PWD sensor through
the PWD Sensor menu.

Any other PWD sensor command can be forwarded to the PWD Sensor.

Select execution of the other commands by pressing E. The user has to

confirm the undertaking of necessary preparations.
Any Necessary Preparations Done? (yes/no):

To conform, type yes<CR>.

To abort the action, type no<CR>.

Now the command and, if applicable, further parameters can be given.

Enter the command string:

Enter the command and parameters and terminate the input by <CR>.

The PWD sensor response is displayed.

Spare Unit Init

CAUTION When the PWD forward scatter sensor is replaced, the spare unit must be
prepared for LT31 communication using the Spare Unit Init function
on the PWD sensor menu.

Select execution of the Spare Unit Init function by pressing F.

Now the spare PWD forward scatter sensor is initialized for LT31
application automatically.

The initialization result is displayed.

Full Command Set Access

In order to access the full command set of the PWD according to the
PWD User’s Guide, the RS-232 interface of a computer with terminal
emulation has to be connected to the PWD maintenance interface located
at the transmitter interface unit of the LT31 using the QMZ101

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maintenance cable (which is delivered together with the LT31). PWD

maintenance interface is shown in Figure 141 below.

Apart from getting access to the full PWD command set this interface
provides a convenient means for updating the PWD software.

0905-004

Figure 141 PWD Maintenance Interface

Linearity Test
For test purposes, it might be necessary or helpful to ‘create’ visibility
situations in order to evaluate or verify the measurement performance of
the LT31 system. ‘Creation’ of visibility, namely reduced transmittance,
can be done by applying optical filters to the optical path of the
transmissometer.

The LT31 system provides the Linearity Test menu, thus there is no
need for manual calculations.

In combination with the LT31 Optical Filter Set (LTOF111), the linearity
test is an easy to handle procedure. LTOF111 consists of a filter holder
and several calibrated optical filters. Before applying any optical filter to
the optical path, the filter holder has to be mounted to the Support Unit
Receiver (LTS211). Figure 142 on page 246 shows the filter holder
mounted.

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0401-119

Figure 142 Filter Holder Mounted to the Receiver Support Unit

The filter holder is able to hold up to three optical filters. By combining

optical filters, a wide variety of filter values can be applied to the optical
path.

0401-120

Figure 143 Linearity Test Menu

For MOR readings and measured transmittance, a reference value, a

calculated value, a measured value and deviation values are given (see
Figure 143 above). In addition, two items are displayed to judge the

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stability of the current measurement and any change in prevailing

visibility conditions.

- Reference: Entering the Linearity Test menu stores current MOR

reading and transmittance. The reference value is valid as long as it
remains in the Linearity Test menu or until another prevailing
visibility/transmittance is chosen to be the new reference.
- Calculated: When giving a filter value, the target values are
calculated based on the reference values and the filter value. When no
filter value is available, reference and calculated values are the same.
- Measured: Currently measured values.
- Deviation: Deviation between measured and calculated values. The
deviation is given both, as absolute (abs.) and relative (rel.) values.

- 2-minutes-Variation-versus-current-Measurement: The 2-minutes-

Variation-versus-current-Measurement is the minimum (down) and
maximum (up) visibility compared to the current, 'Measured' data.

The 2-minutes-Variation-versus-current-Measurement is given only

when the latest action (entering the menu page, applying the filter,
removing of filter) is longer than two minutes ago. Otherwise it is
replaced by dashes ('---------'). The values are given as absolute
differences.
- Reference-Quality-Indication (Ref. Quality): The Reference-
Quality-Indication is a three step quality indication of the prevailing
transmittance compared to the formerly taken reference transmittance.
The Reference-Quality-Indication is given only when, by procedure,
no filter is applied and latest action (entering the menu page,
removing of filter) is longer than two minutes ago. Otherwise it is
replaced by dashes ('--------'). The Reference-Quality-Indicator is
reported as
- 'very steady, +/- 0.05% abs.' when
TMAX 2 min / Tref  1.0005  TMIN 2 min / Tref  0.9995 or
- 'steady, +/- 0.50% abs.' when
TMAX 2 min / Tref  1.005  TMIN 2 min / Tref  0.995 or
- otherwise as 'unsteady, >+/- 0.5% abs.'
- APPLIED FILTER VALUE: In addition, the filter value taken into
account for calculated values and deviation, is displayed.

The deviation values are used to evaluate or verify the measurement

performance of the LT31 system.

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User's Guide ______________________________________________________________________

NOTE A proper linearity test demands stable and homogenous atmospheric

situations. Whenever the APPLIED FILTER VALUE is displayed as
NONE, the 2-minutes-Variation-versus-current-Measurement deviation
and the Reference-Quality-Indication may be used to judge the change
of the prevailing atmospheric situation.

For standard linearity tests it is recommend that the Reference-Quality-

Indication be read as 'steady' or 'very steady'.

If the prevailing visibility has changed so much that the once taken
reference is not longer a proper one, the current visibility can be set as
the new reference by pressing R.

Once the filter holder is mounted to the receiver support unit and an
optical filter has been chosen to be applied to the optical path, the user
has to give the filter value to the system.
Enter Filter Value (e.g. 0.5)! (ESC to exit):

NOTE The filter values are given as decimal values. A so called 50 % filter has
the filter value of 0.5.

NOTE If a combination of optical filters will be applied, the overall filter value
is the product of all individual filter values. To calculate the overall filter
value for combinations of optical filters, the individual filter values have
to be multiplied.

Example: Three optical filters will be applied. The individual filter

values are 0.72, 0.71, and 0.51. The overall filter value is
0.72 x 0.71 x 0.51 = 0.264384.

The filter value of any optical filter is mentioned on the calibration label
on the optical filter.

After typing the filter value (e.g. 0.7239<CR>), the user is asked to apply
the filter.
Apply Filter! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR> .

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Calculated values and deviation are updated continuously and the

deviation can be read (see Figure 144 below).

0401-125

Figure 144 Linearity Test Menu, Filter Applied

The applied filter value is given. The calculated values are displayed as
269.8 m for MOR and 71.6732 % for transmittance. The deviation of the
current measurement compared to the calculation can be read by the
means of the values for Deviation (abs.) and (rel.) for MOR and
transmittance.

NOTE Take into account that all measured values are 1-minute-averages. After
applying an optical filter, the system needs its response time to handle
the new situation.

NOTE The 2-minutes-Variation-versus-current-Measurement is given only

when the latest action (entering the menu page, applying the filter,
removing the filter) occurred longer than two minutes ago. Otherwise it
is replaced by dashes ('---------').

The Reference-Quality-Indication is given only when, by procedure, no

filter is applied and the latest action (entering the menu page, removing
the filter) occurred longer than two minutes ago. Otherwise it is replaced
by dashes ('--------'). See Figure 145 on page 250.

If any other optical filter will be applied or the linearity test is finished,
the user has to remove the currently applied filter first.
Remove Filter After Measurement! Done? (yes/no):

To confirm, type yes<CR>.

To abort the action, type no<CR> .

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User's Guide ______________________________________________________________________

The linearity test result is sufficient when the MOR deviation is less than
±10 %. The tolerable transmittance deviation varies for different MOR
simulations.

0904-024

Figure 145 Linearity Test Menu, Reference Update

Service Menu
Due to the system topology, the transmitter/receiver measurement units
are connected to the module bus and are thus hidden for user access. For
the users' convenience, a subset of commands for the measurement unit
maintenance are available by the LT31 system maintenance port, namely
the Service menu. Furthermore, an overview of measured raw values are
displayed. On the user command level, the subset of commands and the
display of values is restricted. Full access is given on the advanced
command level only.

Figure 146 and Figure 147 on page 251 show the Service menu on user
command level and advanced command level, respectively.

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0401-130

Figure 146 Service Menu on User Command Level

1104-114

Figure 147 Service Menu on Advanced Command Level

Displayed Values
All displayed values are updated every five seconds.

Every sub unit (transmitter/receiver measurement unit, PWD sensor, LM

sensor) is described by its overall status. Further details are given for
each sub unit.

Transmitter/Receiver Measurement Unit

Blow
ON Blower is switched on.
OFF Blower is switched off.

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User's Guide ______________________________________________________________________

Heat
ON Heater is switched on.
OFF Heater is switched off.

Beep
??? Status of acoustical signal is unknown.
ON Acoustical signal is generated. Pattern of acoustical signal is
dependent of Main Receiver Signal Value. See section
Alignment on page 225.
OFF Acoustical signal is not generated.

Status
The transmitter and the receiver part of the overall status is displayed.
For message decoding, see section Status Codes on page 164.

Freq (Advanced Command Level only)

Transmitter Generated Main Transmitter Modulation Frequency.
Receiver Detected Synchronization Signal Frequency.
This may differ from the generated Main Transmitter
Modulation Frequency but must be in the same range.

Frequency values are updated only when any frequency command has
been invoked.

mRdc (Receiver only) (Advanced Command Level only)

Main Receiver DC value (0-1023)

mTint (Transmitter only) (Advanced Command Level only)

Main Transmitter Intensity value (0-1023)

mTdera (Transmitter only) (Advanced Command Level only)

Main Transmitter derating value (0-1023)

wRdc (Advanced Command Level only)

Window Receiver DC value (0-1023)

wTint (Advanced Command Level only)

Window Transmitter Intensity value (0-1023)

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wTdera (Advanced Command Level only)

Window Transmitter Derating value (0-1023)

ver (Advanced Command Level only)

Vertical Position value (0-1023)

hor (Advanced Command Level only)

Horizontal Position value (0-1023)

mRac (Receiver only) (Advanced Command Level only)

Main Receiver AC value (0-1048575)
Receiver Response Mode is given as FAST or SLOW.

wRac (Advanced Command Level only)

Window Receiver AC value (0-1048575)

PWD Sensor

Visi
One-minute average visibility provided by PWD Sensor

WMO1min
Instant present weather code provided by PWD Sensor

LM Sensor

BGL
Background luminance provided by LM Sensor

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User's Guide ______________________________________________________________________

Commands
Nearly every command is selected, specified and invoked by one-key
commands. Once the user has selected a command (A-G, A-C for the
user command level), the command has to be specified in further detail.
Available choices for a specification are given in the lower area of the
Service menu when a command has been selected.

Blower
The blower can be switched on or off for the transmitter or receiver unit.

Select the command by pressing A.

The user has to select whether to handle the transmitter or receiver unit.
BLOWER (R)ECEIVER/(T)RANSMITTER

The transmitter unit blower is selected by pressing T.

The receiver unit blower is selected by pressing R.

The user has to select whether the blower shall be switched on or off.
BLOWER TRANSMITTER O(N)/OF(F) or
BLOWER RECEIVER O(N)/OF(F)

The blower is switched on by pressing N.

The blower is switched off by pressing F.

The result can be read at the displayed values at Blow.

Example: On the user command level, the receiver unit blower shall be
switched off. See Figure 148 on page 255.

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0401-143

Figure 148 Switching Off Receiver Unit/Blower

Heater
The heater can be switched on or off for the transmitter or receiver unit.

Select the command by pressing B.

The user has to select whether to handle the transmitter or receiver unit.
HEATER (R)ECEIVER/(T)RANSMITTER

The transmitter unit heater is selected by pressing T.

The receiver unit heater is selected by pressing R.

The user has to select whether the heater shall be switched on or off.
HEATER TRANSMITTER O(N)/OF(F) or
HEATER RECEIVER O(N)/OF(F)

The heater is switched on by pressing N.

The heater is switched off by pressing F.

The result can be read at the displayed values at Heat.

Beeper
The beeper can be switched on or off for the transmitter or receiver unit.

Select the command by pressing C.

The user has to select whether to handle the transmitter or receiver unit.
BEEPER (R)ECEIVER/(T)RANSMITTER

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User's Guide ______________________________________________________________________

The transmitter unit beeper is selected by pressing T.

The receiver unit beeper is selected by pressing R.

The user has to select whether the beeper shall be switched on or off.
BEEPER TRANSMITTER O(N)/OF(F) or
BEEPER RECEIVER O(N)/OF(F)

The beeper is switched on by pressing N.

The beeper is switched off by pressing F.

The result can be read at the displayed values at Beep.

Frequency (Advanced Command Level only)

CAUTION Changes to the Main Transmitter Modulation Frequency can result in

loss of performance to the LT31 system. Only trained or advised users
shall use this feature.

The Main Transmitter Modulation Frequency can be increased,

decreased, set to a fixed value, or written to the EEPROM of the main
transmitter module.

Select the command by pressing D.

The user has to select whether to increase, decrease, write the current
value to the EEPROM of the main transmitter module or to set a fixed
value.
FREQ (+)/(-)/(W)RITE/(V)ALUE

The Main Transmitter Modulation Frequency is increased by pressing +.

The Main Transmitter Modulation Frequency is decreased by pressing -.

The current Main Transmitter Modulation Frequency is written to the

EEPROM of the main transmitter module by pressing W.

The Main Transmitter Modulation Frequency is set to a fixed value by

pressing V.

When the Main Transmitter Modulation Frequency is set to a fixed value,

the user has to give the desired frequency.
MAIN FREQUENCY TO SET (GIVE NUMBER):

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Give the desired frequency value and terminate the input with <CR>.

Any change to the Main Transmitter Modulation Frequency will result in

the closest available value the transmitter measurement unit is able to
realise due to the restricted frequency generator resolution.

The result can be read at the displayed values at Freq.

Example: On the advance command level, the Main Transmitter

Modulation Frequency must be decreased (see Figure 149 below).

1104-115

Figure 149 Decreasing Main Transmitter Modulation Frequency

Example: On the advance command level, the Main Transmitter

Modulation Frequency must be set to 994 Hz (see Figure 150 below).

1104-116

Figure 150 Setting Main Transmitter Modulation Frequency to

994 Hz

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Movement (Advanced Command Level only)

CAUTION Changing positions of the optics units may result in loss of performance
to the LT31 system. Only trained or advised users shall use this feature

The horizontal and vertical position of the optics unit can be changed for
the transmitter or receiver unit.

Select the command by pressing E.

CAUTION The transport protection in the transmitter and receiver measurement

unit have to be removed before moving any optics.

Make sure that the transport protection in the transmitter and receiver
measurement unit have been removed. The user has to give a
confirmation.
HAS THE TRANSPORT PROTECTION IN THE T/R OPTIC UNITS BEEN
REMOVED? (yes/no) :

To confirm, type yes<CR> .

To abort the action, type no<CR>.

The user has to select whether to handle the transmitter or receiver unit.
MOVE (R)ECEIVER/(T)RANSMITTER

Moving a transmitter optics unit is selected by pressing T.

Moving a receiver optics unit is selected by pressing R.

The user has to select whether to move horizontal or vertical axis.

MOVE TRANSMITTER (H)ORIZONTAL/(V)ERTICAL or
MOVE RECEIVER (H)ORIZONTAL/(V)ERTICAL

Vertical movement is selected by pressing V.

Horizontal movement is selected by pressing H.

The user has to give the position to reach by its value.

GIVE VALUE (100...900):

Type the position to reach and terminate the input by <CR>.

The result can be read at the displayed values at ver resp. hor.

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Scanning (Advanced Command Level Only)

CAUTION Changing positions of the optics units may result in loss of performance
to the LT31 system. Only trained or advised users shall use this feature.

The horizontal and vertical axes of the optics unit can be scanned for
maximum Main Receiver Value for the transmitter or receiver unit.

Select the command by pressing F.

CAUTION The transport protection in the transmitter and receiver measurement

unit have to be removed before moving any optics.

Make sure that the transport protection in the transmitter and receiver
measurement unit have been removed. The user has to give a
confirmation.
HAS THE TRANSPORT PROTECTION IN THE T/R OPTIC UNITS BEEN
REMOVED? (yes/no) :

To confirm, type yes<CR>.

To abort the action, type no<CR> .

The user has to select whether to handle the transmitter or receiver unit.
SCAN (R)ECEIVER/(T)RANSMITTER

Scanning at transmitter optics unit is selected by pressing T.

Scanning at receiver optics unit is selected by pressing R.

The user has to select whether to scan horizontal or vertical axis.

SCAN TRANSMITTER (H)ORIZONTAL/(V)ERTICAL or
SCAN RECEIVER (H)ORIZONTAL/(V)ERTICAL

Vertical scanning is selected by pressing V.

Horizontal scanning is selected by pressing H.

Information about the scanning result can be read at the lower area of the
Service menu.

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User's Guide ______________________________________________________________________

Receiver Response Time (Advanced Command Level

Only)
The Main Receiver Response Time can be switched to slow or fast.

Select the command by pressing G.

The user has to select whether to switch the Main Receiver Response
Time to slow or fast.
RECEIVER (F)AST/(S)LOW

The Main Receiver Response Time switched to fast by pressing F.

The Main Receiver Response Time switched to slow by pressing S.

The result can be read at the displayed values at mRac.

Example:

On the advance command level the Main Receiver Response Time will
be switched to fast (see Figure 151 below).

1104-117

Figure 151 Setting Main Receiver Response Time to Fast

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Chapter 7 ___________________________________________________________ Troubleshooting

CHAPTER 7
TROUBLESHOOTING

This chapter describes common problems, their probable causes and

remedies, and contact information.

CAUTION The servicing of the equipment must be performed by qualified

personnel only.

CAUTION The AC power switch and the optional battery backup switch must
always be turned off before attempting any service actions described in
this chapter.

WARNING The equipment contains dangerous voltage of 230/115/100 VAC.

Status Messages
With each standard message (Message 1 and 2), a 20-character overall
status word is transmitted. The overall status word describes the current
status of the LT31 system in detail. Referring to Table 14 starting from
page 164 through Table 34 on page 173, host systems and users are able
to decode the status of the LT31 system.

For the convenience of the user, the current status can be read by the
STATUS CHECK command on any user level. The user must connect
to the maintenance port via the maintenance or data line (see section
Entering and Exiting Command Mode on page 145).

The system answers with an overview of the status of all active sub units
and a list of current events.
LT31 SYSTEM: OK
Transmitter unit: OK
Receiver unit: OK

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User's Guide ______________________________________________________________________

PWD Sensor: OK
LM Sensor: OK
Background luminance sensor: OK

Current Events:
A2:MOR; MOR overrange
A3:AutoCal: autocalsituation

An event is defined as a noticeable situation detected by the LT31 system

by sub-unit answers or internal algorithms. Each event or a combination
of events have characteristics information, warning, alarm, or error.

Table 36 below summarises all possible events to identify the current

LT31 system status easily.

Table 36 Events
Event Message Probable Cause Remedy

A1:MOR; MOR MOR value is below lower MOR Normal operation.

underrange measurement range limit.
A2:MOR; MOR MOR value is above upper MOR Normal operation.
overrange measurement range limit.
A3:AutoCal: Preconditions for auto-calibration Normal operation.
autocalsituation are valid. A new auto-calibration
factor is build.
A4:AutoCal; autocal Current auto-calibration factor is Reset LT31 system (refer to Table 11 on
error out of range. page 150) and perform an Offset and
Visibility Calibration (see section Offset and
Visibility Calibration on page 234).
A5:Window Measured window contamination Cleaning of optical surfaces within mid-term
Contamination has reached level 1 threshold. range (see section Cleaning Instructions on
Measurement; page 194). LT31 system is able to cope this
contamination level 1 situation.
A6:Window Measured window contamination Cleaning of optical surfaces within short-
Contamination has reached level 2 threshold. term range (see section Cleaning
Measurement; Instructions on page 194). LT31 system is
contamination level 2 able to cope this situation.

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A7:Window Measured window contamination Cleaning of optical surfaces is necessary

Contamination is high. (refer to see section Cleaning Instructions
Measurement; on page 194).
contamination too
high
A8:Window An error occurred during window Reset LT31 system (refer to Table 11 on
Contamination contamination measurement. page 150) and perform an Offset and
Measurement; Visibility Calibration (see section Offset and
contamination Visibility Calibration on page 234).
measurement error
A9:Alignment; The Alignment Menu has been -
automated alignment entered at maintenance port.
in progress
A10:Alignment; Alignment quality has been Fine Alignment might be carried out in mid-
reduced quality evaluated to be below level 1. term range (see section Alignment on page
level 1 225). LT31 system is able to cope this
situation.
A11:Alignment; Alignment quality has been Fine Alignment has to be carried out in
reduced quality evaluated to be below level 2. short-term range (see section Alignment on
level 2 page 225). LT31 system is able to cope this
situation.
A12:Alignment; Alignment quality has been Raw and Fine Alignment has to be carried
reduced quality evaluated to be below level 3. out (see section Alignment on page 225).
level 3
A13:Alignment; An error occurred during Check if Synchronization Signal is valid.
alignment procedure alignment quality evaluation.
Check if movement of all axes is possible
error
and position values are valid (see section
Service on page 250).
A14:Receiver Main Preconditions for main receiver –
Receiver LTD112; self-test are valid. Main receiver
self-test in progress self-test is in progress.
A15:Receiver Main Main receiver self-test failed. Replace Main Receiver Module (see
Receiver LTD112; section Replacing Main Receiver Module
self-test failed (no on page 218).
reaction on test LED)
A16:Receiver Main Main receiver self-test has Replacement of Main Receiver Module
Receiver LTD112; detected low sensitivity for main should be taken into account (see section
sensitivity is low receiver. Replacing Main Receiver Module on page
218). LT31 system is able to cope this
situation

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C2:Transmitter Window receiver signal is greater Check derating and intensity value for
Measurement CPU than overrange threshold while window transmitter module (wTdera, wTint)
LTC112; window window transmitter LED driver is (see section Service on page 250).
receiver AC detected as faulty.
Replace window transmitter module (see
overrange
section Replacing Window Transmitter
Module on page 213).
C3:Transmitter Window receiver signal is greater Replace transmitter measurement CPU
Measurement CPU than overrange threshold while (see section Replacing Measurement
LTC112; window window transmitter LED driver is CPU/Window Receiver on page 211).
receiver AC detected as OK.
overrange
C5:Receiver Window receiver signal is greater Check derating and intensity value for
Measurement CPU than overrange threshold while window transmitter module (wTdera, wTint)
LTC112; window window transmitter LED driver is (see section Service on page 250)
receiver AC detected as faulty.
Replace window transmitter module (see
overrange
section Replacing Window Transmitter
Module on page 213).
C6:Receiver Window receiver signal is greater Replace receiver measurement CPU (see
Measurement CPU than overrange threshold while section Replacing Measurement
LTC112; window window transmitter LED driver is CPU/Window Receiver on page 211).
receiver AC detected as OK.
overrange
L1:LM21; indication LM Sensor has detected a hood Refer to LM21 User's Guide.
of abnormal situation or dew heater problem.
L2:LM21; alarm -> LM Sensor has detected at least Refer to LM21 User's Guide.
BGL not valid one of the following situations:
- the sensor does not react to
changes in luminance
- field calibrator has been applied
- backscatter signal has increased
L3:LM21; error -> LM21 Sensor has detected a Refer to LM21 User's Guide.
BGL not valid memory error
L4:LM21; not LM21 sensor does not respond to Check whether the LM Sensor lines (power,
responding LT31 system polling request RS–485) are connected properly (see
section Mounting Optional LM21 on page
98).
LM sensor is not foreseen at all, but LM
Sensor is configured to be available (refer
to Table 11 on page 150).
L5:LM21; not LM Sensor is configured to be not –
installed available (refer to Table 11 on
page 150).

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P1:PWD; cleaning The PWD sensor has evaluated Cleaning of optical surfaces within mid term
required that cleaning is required. range. When cleaned, the CLEAN
command has to be executed at PWD
Sensor Menu (see section PWD Sensor on
page 241).
P2:PWD; immediate The PWD sensor has evaluated Cleaning of optical surfaces is necessary.
cleaning required that immediate cleaning is When cleaned the CLEAN command has to
required. be executed at PWD Sensor Menu (see
section PWD Sensor on page 241).
P3:PWD; hardware The PWD sensor has evaluated a Execute Other Command STA at the PWD
warning hardware warning. Sensor Menu (see section PWD Sensor on
page 241 and refer to PWD User's Guide).
P4:PWD; hardware The PWD sensor has evaluated a Execute Other Command STA on the PWD
error hardware error. Sensor Menu (see section PWD Sensor on
page 241 and refer to PWD User's Guide).
P5:PWD; MOR not The MOR reading of the PWD Execute Other Command STA on the PWD
valid Sensor is invalid and can not be Sensor Menu (see section PWD Sensor on
used for auto-calibration purposes page 241 and refer to PWD User's Guide).
P6:PWD; not PWD Sensor does not respond to Check whether the PWD Sensor lines
responding LT31 system polling request (power, RS–485) are connected properly.
PWD Sensor is not foreseen at all, but
PWD Sensor is configured to be available
(refer to Table 11 on page 150).
P7:PWD; not PWD Sensor is configured to be –
installed not available (refer to Table 11 on
page 150).
R1:Receiver Main A light source other than the Try to identify an additional light source of
Receiver LTD112; modulated transmitter signal is any nature that might be in sight of the Main
DC high detected at Main Receiver Module Receiver Module.
site. Either the signal is detected
R2:Receiver Main If an artificial light source has been
as high (R1) or the receiver is
Receiver LTD112; identified try to eliminate the influence.
saturated (R2).
saturated
R3:Receiver Main Main Receiver Signal is lower During start-up this event is generated as
Receiver LTD112; than the underrange level. the average building is not complete.
offset error
Check whether the optical light path
between transmitter measurement unit
(LTM112) and receiver measurement unit
(LTM212) is unblocked.
Perform an Offset and Visibility Calibration
(see section Offset and Visibility Calibration
on page 234).

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R4:Receiver Main Main Receiver Signal is higher –

Receiver LTD112; than expected for normal
AC high operation.

R5:Receiver Main Check Main Transmitter Module status

Receiver LTD112; (events T1 - T6). If any of the T1 - T6
AC overrange events are reported, handle them first. If no
such event is reported, replace the main
receiver module (see section Replacing
Main Receiver Module on page 218).
R6:Receiver Window
The intensity value of the Window Replacement of the Window Transmitter
Transmitter LTL212;
Transmitter LED is out of limits Module is necessary (see section
LED driver error
for normal operation or has been Replacing Window Transmitter Module on
R7:Receiver Window evaluated as to be off. page 213).
Transmitter LTL212;
LED driver error
R8:Receiver Window
Transmitter LTL212;
LED driver error
R9:Receiver Window The derating value of Window
Transmitter LTL212; Transmitter LED has been
LED driver error evaluated as to be off.
R10:Receiver The derating value of the Window Replacement of the Window Transmitter
Window Transmitter Transmitter LED has been Module in mid–term range (see section
LTL212; LED aged evaluated as to be high. Replacing Window Transmitter Module on
page 213).
R11:Receiver The derating value of the Window Replacement of the Window Transmitter
Window Transmitter Transmitter LED has been Module (see section Replacing Window
LTL212; LED error evaluated as to be too high. Transmitter Module on page 213).
R12:Receiver A light source other than the Try to identify an additional light source of
Measurement CPU modulated transmitter signal is any nature that might be in sight of the
LTC112; window detected at Window Receiver. window receiver. Try to eliminate the
receiver DC high Either the signal is detected as influence.
high (R12) or the receiver is
R13:Receiver saturated (R13).
Measurement CPU
LTC112; window
receiver saturated
R14:Receiver Window Receiver Signal is lower During start-up this event is generated as
Measurement CPU than the underrange level. the averaging is not complete.
LTC112; window
Check whether the optical light path at the
receiver AC off
V-shaped windows at receiver
measurement unit (LTM212) is blocked.
R15:Receiver –
Window Receiver Signal is higher
Measurement CPU
than expected for normal
LTC112; window
operation
receiver AC high

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R16:Receiver Check Window Transmitter Module status

Measurement CPU (events R6–R11).
LTC112; window
If any of the events R6–R11 are reported,
receiver AC
handle them first.
overrange
If no such event is reported, replace the
Receiver Measurement CPU (see section
Replacing Measurement CPU/Window
Receiver on page 211).
R17:Receiver Optics
Position values are out of range Check if movement of all axes is possible
Unit LTO212;
for normal operation and position values are valid (refer to
alignment
Service Menu)
mechanism vertical
underrange
Check for proper cabling at the receiver
R18:Receiver Optics optics unit
Unit LTO212;
alignment Check for firm fit of eccentric elements to
mechanism vertical gear motor axis and variable resistor axis
overrange
R19:Receiver Optics
Unit LTO212;
alignment
mechanism
horizontal
underrange
R20:Receiver Optics
Unit LTO212;
alignment
mechanism
horizontal overrange
R21:Receiver Replace Receiver Measurement CPU (see
The Receiver Measurement CPU
Measurement CPU section Replacing Measurement
is unable to read or write the
LTC112, general CPU/Window Receiver on page 211).
appropriate EEPROM device
EEPROM error

R22:Receiver Check for proper cabling between Receiver

Measurement CPU Measurement CPU and appropriate
LTC112, window module.
receiver EERPOM
error Replace appropriate module (see sections
starting from Repair Instructions on page
201).
R23:Receiver
Window Transmitter
LTL212; EEPROM
error

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R24:Receiver Main
Receiver LTD112;
EEPROM error
R25:Receiver
Replace Receiver Measurement CPU (see
Measurement CPU
section Replacing Measurement
LTC112; window
CPU/Window Receiver on page 211).
temperature sensor
error
R26:Receiver
Measurement CPU
LTC112; enclosure
temperature sensor
error
R27:Receiver
Check for proper cabling between Receiver
Measurement CPU
Measurement CPU and appropriate
LTC112; window
module.
receiver temperature
sensor error
Replace appropriate module (see sections
R28:Receiver starting from Repair Instructions on page
Window Transmitter 201).
LTL212; temperature
sensor error
R29:Receiver Main
Receiver LTD112;
temperature sensor
error

R30:Receiver Optics The Receiver Measurement CPU Check for proper cabling at the receiver
Unit LTO212; has detected out of range values optics unit.
alignment for the appropriate axis.
mechanism Check for firm fit of eccentric elements to
horizontal error gear motor axis and variable resistor axis.

R31:Receiver Optics
Unit LTO212;
alignment
mechanism vertical
error

R32:Receiver The Receiver Measurement CPU Check for proper cabling of

Measurement Unit is unable to detect a valid synchronization signal between
LTC112; synchronization signal frequency Transmitter Unit (LTT111) and Receiver
synchronization Unit (LTR111).
signal error

R33:Receiver The heater in the Receiver Check for proper cabling between Receiver
Measurement Unit Measurement CPU is unable to Measurement CPU and heating elements
LTM212; heater create the desired temperature
error difference between window and
enclosure temperature sensor.

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Chapter 7 ___________________________________________________________ Troubleshooting

R34:Receiver Unit Receiver Unit does not respond Check whether all the Receiver Unit lines
LTR111; not to LT31 system polling request are connected properly
responding
Check whether all connections concerning
the RS-485 Module Bus are connected
properly.
R35:Receiver Unit Any Receiver Unit Module has Perform an Offset and Visibility Calibration
LTR111; unknown been changed since last Offset (see section Offset and Visibility Calibration
and Visibility Calibration. on page 234).
T1:Transmitter Main The intensity value of the Main
Replacement of the Main Transmitter
Transmitter LTL112; Transmitter LED is out of limits for
Module is necessary (see section
LED driver error normal operation or has been
Replacing Main Transmitter Module on
evaluated as to be off.
T2:Transmitter Main page 216).
Transmitter LTL112;
LED driver error
T3:Transmitter Main
Transmitter LTL112;
LED driver error
T4:Transmitter Main
The derating value of Main
Transmitter LTL112;
Transmitter LED has been
LED driver error
evaluated as to be off.
T5:Transmitter Main The derating value of the Main Replacement of the Main Transmitter
Transmitter LTL112; Transmitter LED has been Module in mid–term range (see section
LED aged evaluated as to be high. Replacing Main Transmitter Module on
page 216).
T6:Transmitter Main The derating value of the Main Replacement of the Main Transmitter
Transmitter LTL112; Transmitter LED has been Module (see section Replacing Main
LED error evaluated as to be too high. Transmitter Module on page 216).
T7:Transmitter The intensity value of the Window
Replacement of the Window Transmitter
Window Transmitter Transmitter LED is out of limits for
Module is necessary (see section
LTL212; LED driver normal operation or has been
Replacing Window Transmitter Module on
error evaluated as to be off.
page 213).
T8:Transmitter
Window Transmitter
LTL212; LED driver
error
T9:Transmitter
Window Transmitter
LTL212; LED driver
error
T10:Transmitter
The derating value of Window
Window Transmitter
Transmitter LED has been
LTL212; LED driver
evaluated as to be off.
error

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T11:Transmitter The derating value of the Window Replacement of the Window Transmitter
Window Transmitter Transmitter LED has been Module in mid–term range (see section
LTL212; LED aged evaluated as to be high. Replacing Window Transmitter Module on
page 213).
T12:Transmitter The derating value of the Window Replacement of the Window Transmitter
Window Transmitter Transmitter LED has been Module (see section Replacing Window
LTL212; LED error evaluated as to be too high. Transmitter Module on page 213).
T13:Transmitter A light source other than the Try to identify an additional light source of
Measurement CPU modulated transmitter signal is any nature that might be in sight of the
LTC112; window detected at Window Receiver. Window Receiver. Try to eliminate the
receiver DC high Either the signal is detected as influence.
high (T12) or the receiver is
T14:Transmitter
saturated (T14).
Measurement CPU
LTC112; window
receiver saturated

T15:Transmitter Window Receiver Signal is lower During start-up this event is generated as
Measurement CPU than the underrange level. the averaging is not complete.
LTC112; window
receiver AC off Check whether the optical light path at the
V-shaped windows at Transmitter
Measurement Unit (LTM112) is blocked.

T16:Transmitter Window Receiver Signal is higher -

Measurement CPU than expected for normal
LTC112; window operation
receiver AC high

T17:Transmitter Check window transmitter module status

Measurement CPU (event T7 - T12). If any of the T7 - T12 are
LTC112; window reported, handle them first. If no such
receiver AC event is reported, replace the Transmitter
overrange Measurement CPU (see section Replacing
Measurement CPU/Window Receiver on
page 211).
T18:Transmitter Position values are out of range Check if movement of all axes is possible
Optics Unit LTO112; for normal operation. and position values are valid (see section
alignment Service on page 250).
mechanism vertical
Check for proper cabling at the Transmitter
underrange
Optics Unit.
T19:Transmitter
Check for firm fit of eccentric to gear motor
Optics Unit LTO112;
axis and variable resistor axis.
alignment
mechanism vertical
overrange

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Chapter 7 ___________________________________________________________ Troubleshooting

T20:Transmitter –
Optics Unit LTO112;
alignment
mechanism
horizontal
underrange
T21:Transmitter
Optics Unit LTO112;
alignment
mechanism
horizontal overrange
T22:Transmitter The Transmitter Measurement Replace Transmitter Measurement CPU
Measurement CPU CPU is unable to read or write the (see section Replacing Measurement
LTC112, general appropriate EEPROM device CPU/Window Receiver on page 211).
EEPROM error

T23:Transmitter Check for proper cabling between

Measurement CPU Transmitter Measurement CPU and
LTC112, window appropriate module.
receiver EERPOM
Replace appropriate module (see sections
error
starting from Repair Instructions on page
201).
T24:Transmitter
Window Transmitter
LTL212; EEPROM
error
T25:Transmitter Main
Transmitter LTL112;
EEPROM error
T26:Transmitter Replace Transmitter Measurement CPU
Measurement CPU (see section Replacing Measurement
LTC112; window CPU/Window Receiver on page 211).
temperature sensor
error
T27:Transmitter
Measurement CPU
LTC112; enclosure
temperature sensor
error
T28:Transmitter Check for proper cabling between
Measurement CPU Transmitter Measurement CPU and
LTC112; Window appropriate module.
Receiver temperature
Replace appropriate module ((see sections
sensor error
starting from Repair Instructions on page
T29:Transmitter
201).
Window Transmitter
LTL212; temperature
sensor error

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T30:Transmitter
Main Transmitter
LTL112;
temperature sensor
error
T31:Transmitter The Transmitter Measurement Check for proper cabling at the transmitter
Optics Unit LTO112; CPU has detected out of range optics unit.
alignment values for the appropriate axis.
Check for firm fit of eccentric elements to
mechanism
gear motor axis and variable resistor axis.
horizontal error
T32:Transmitter
Optics Unit LTO112;
alignment
mechanism vertical
error
T33:Transmitter The heater in the Transmitter Check for proper cabling between
Measurement Unit Measurement CPU is unable to Transmitter Measurement CPU and heating
LTM112; heater error create the desired temperature elements.
difference between window and
enclosure temperature sensor.
T34:Transmitter Unit Transmitter Unit does not respond Check whether all the Transmitter Unit lines
LTT111; not to LT31 system polling request are connected properly.
responding
Check whether all connections concerning
the RS-485 Module Bus are connected
properly.
T35:Transmitter Unit Any Transmitter Unit Module has Perform an Offset and Visibility Calibration
LTT111; unknown been changed since last Offset (see section Offset and Visibility Calibration
and Visibility Calibration. on page 234).
T36:Transmitter Main A light source other than the Try to identify an additional light source of
Transmitter LTL112; modulated transmitter signal is any nature that might be in sight of the Main
DC high detected by the Main Transmitter Transmitter. Try to eliminate the influence
monitor measurement. Either the
T37:Transmitter Main signal is detected as high (T36) or
Transmitter LTL112;
the receiver is saturated (T37).
saturated

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Chapter 7 ___________________________________________________________ Troubleshooting

Miscellaneous Problems
Table 37 Miscellaneous Problems
Problem Probable Cause Remedy
Data message is Dialogue not operating Check cable connections, make reset.
missing Wrong communication Check the communication parameters
parameters (default in RS-232/RS-485: 9600/8N1) and
the flow control parameters.
Check the cable connections.
To prevent collisions in half-duplex RS-485
bus, switch line feeds off from your terminal
software.
If a modem is used, verify that the same
modem standard is used in both modems
and one is in the answer mode and the other
in the originate mode.
Wrong polling command used In the MITRAS and FLAMINGO emulation
modes, the LT31 answers only to MITRAS
and FLAMINGO polls, respectively.
Wrong data port Open the terminal program and the
command mode through the data or
maintenance line and give the PARAMETER
command. Check that the message port is
DATA and that the data port mode is the
desired one.
Sensor is off or out of power Check that the power switch is on and the
LEDs on the FSP102 power board are lit and
the status LED on the interface unit is
blinking. If the AC switch is on but no LED is
lit, check the fuses on the FSP102 board
and verify the presence of a line voltage.
Dots instead of data Wrong font being used Select View and Font from the drop-down
message when menu, and click OK.
using Windows
HyperTerminal
Command mode Wrong serial port on the host Check the serial port your terminal software
does not open from computer is using.
the maintenance Maintenance line cabling is Try to open the command mode from the
connector faulty data port (connect your computer into the
data port using the connectors on the
LTC212 front panel) to make sure LTC212 is
OK.
Visibility value Active alarm (measurement Check which alarm is active and remove the
missing in the value not reliable and not cause for the alarm.
message shown)
Data messages Unit ID is set Use correct ID or the format OPEN * (or
arrive but command OPEN LT) that opens the command line
mode does not open regardless of the unit ID. Remove the unit ID
with the OPEN if not needed.
command. Wrong communication Check the communication parameters
parameters (default in RS-232/RS-485: 9600/8N1) and
the flow control parameters.
To prevent collisions in half-duplex RS-485
bus, switch line feeds off from your terminal
software.

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Problem Probable Cause Remedy

Line is broken or a wire is loose If you are using RS-232 or RS-485 data lines
verify that the TXD LED on the LTC212
board blinks when you send characters to
the line.
Check the cable connections.
Data messages Test message simulation has Give the command SIMULATE
arrive but the not been turned off TEST_MESSAGE OFF.
measurement values
are wrong and do
not change or
patterns of data sets
are given.
Reset LED inside An AC fuse or the DC output Check and replace the blown fuse on the
the LTI211 does not line fuse on FSP102 is blown FSP102 board.
light up when power
is turned on
Status LED inside LTC212 indicates an indication. Check the content of the indication from the
LTI211 is giving a status message and follow the instructions
fast flash instead of above.
a long flash of 1 Hz
Status LED inside LTC212 indicates a warning. Check the content of the warning from the
the LTI211 is giving status message and follow the instructions
two fast flashes above.
instead of blinking at
1 Hz constant rate.
Status LED inside LTC212 indicates an alarm. Check the content of the alarm from the
LTI211 is giving status message and follow the instructions
three fast flashes above.
instead of blinking at
a constant rate of
1 Hz.
Status LED inside LTC212 indicates an error. Check the content of the error from the
LTI211 is giving four status message and follow the instructions
fast flashes instead above.
of blinking at a
constant rate of
1 Hz.

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Chapter 7 ___________________________________________________________ Troubleshooting

Technical Support
For technical questions, contact the Vaisala technical support:

E-mail helpdesk@vaisala.com
Phone (int.) +358 9 8949 2789
Fax +358 9 8949 2790

Please proceed as follows before you contact the Vaisala technical

support:

1. In case of technical questions or concerns, please capture the LT31

response to the DUMP command as follows:
a. Connect a computer with terminal program to the LT31
maintenance interface (see section LT Maintenance Interface
on page 132).
b. Activate the logging respectively capture feature of the
terminal program (for example, in case of Microsoft
Hyperterminal use pull-down menu transfer and select
capture text). Use a suitable file name to identify the
respective LT31 later on (for example, installation site code +
date and time).
c. Use the OPEN respectively OPEN * to enter the command
mode (see section LT Maintenance Interface on page 127).
d. Execute the DUMP command and wait until the prompt (>0)
is shown again.
e. Execute the CLOSE command (see section LT Maintenance
Interface on page 132).
f. Stop the logging/capture function of the terminal program.
2. In case of a technical problem, also write a Problem Report with
the name and contact information of a technically competent
person who can provide further information on the problem. On the
Problem Report, please explain:
- Airport name and LT31 Installation site
- Contact Person for technical questions/phone number/email
- What failed (what worked/did not work)?
- Where did it fail (location and environment)?
- When did it fail (date, immediately/after a
while/periodically/randomly)?
- How many failed (only one defect/other same or similar
defects/several failures in one unit)?
- What was connected to the product and to which connectors?

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- Input power source type, voltage and list of other items

(lighting, heaters, motors etc.) that were connected to the same
power output.
- What was done when the failure was noticed?
3. Please submit the captured LT31 DUMP command response
together with the problem report and the following information:
- Airport name and LT31 Installation site
- Contact Person for technical questions/phone number/email

Return Instructions
If the product needs repair, please follow the instructions below to speed
up the process and avoid extra costs.

1. Read the warranty information.

2. Write a Problem Report with the name and contact information of a
technically competent person who can provide further information
on the problem.
3. On the Problem Report, please explain:
- What failed (what worked/did not work)?
- Where did it fail (location and environment)?
- When did it fail (date, immediately/after a
while/periodically/randomly)?
- How many failed (only one defect/other same or similar
defects/several failures in one unit)?
- What was connected to the product and to which connectors?
- Input power source type, voltage and list of other items
(lighting, heaters, motors etc.) that were connected to the same
power output.
- What was done when the failure was noticed?

4. Include a detailed return address with your preferred shipping

method on the Problem Report.
5. Pack the faulty product using an ESD protection bag of good
quality with proper cushioning material in a strong box of adequate
size. Please include the Problem Report in the same box.
6. Send the box to:
Vaisala Oyj
Contact person/Division
Vanha Nurmijärventie 21
FIN-01670 Vantaa
Finland

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Chapter 8 ____________________________________________________________ Technical Data

CHAPTER 8
TECHNICAL DATA

This chapter provides the technical data of the product.

Operational Specifications
Table 38 LT31 Operational Specifications
Property Description / Value
Baseline MOR measurement range

General:
- 30 ... 35 m baseline length 1/3 x baseline length to 10 000 m
- 35 ... 75 m baseline length 1/2 x baseline length to 10 000 m
Recommended baseline lengths:
- 30 m baseline length (optimal) 10 m to 10 000 m
- 50 m baseline length 25 m to 10 000 m
- 75 m baseline length 37.5 m to 10 000 m
Purchased option:
- 25 m baseline length 1/3 x baseline length to 10 000 m
Accuracy (RVR range) Exceeds the ICAO (Annex 3) recommendations
Resolution 0.1 m
Sampling rate 1.000/sec
Averaging 1 minute gliding average
Output data Automatic or polled data messages
- Standard message including MOR, LT31
status and BGL data (option)
- Message including present weather
data (option)
- MITRAS transmissometer compatible
message
- SKOPOGRAPH II Flamingo transmissometer
compatible message
- FD12 Visibility Meter compatible message

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Property Description / Value

Special features: - Auto-calibration
- Automated fine alignment
- Alignment quality control
- Window contamination measurement
and compensation
- Window blower system
- Double mast structure
- Sophisticated self-diagnostics
Present Weather Reporting Option: - 7 different types of precipitation (rain, freezing
Identifies rain, drizzle, freezing drizzle, mixed rain/snow,
snow, ice pellets)
Reports - 49 different codes from WMO 4680 code table

Optical Specifications
Table 39 LT31 Transmitter Optical Specifications
Property Description / Value
Light source - White Light Emitting Diode
- Certified class 1 product according to
EN 60 825-1
Modulation frequency 1 kHz
Lens diameter 40 mm
Automatic fine alignment span 3.5°
Optical monitoring Light source stability control
Window contamination measurement and
compensation circuitry

Table 40 LT31 Receiver Optical Specifications

Property Description / Value
Spectral response Adapted to human eye spectral sensitivity
Lens diameter 40 mm
Automatic fine alignment span 3.5°
Optical monitoring Photodiode DC level measurement
Window contamination measurement and
compensation
Active self-test circuitry

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Chapter 8 ____________________________________________________________ Technical Data

Electrical Specifications
Table 41 LT31 Electrical Specifications
Property Description / Value
AC supply 100/115/230 VAC + 10 %, 50-60 Hz
Power consumption 800 VA maximum (for complete LT31 system)
with all options
Battery backup option Battery 2 Ah, backup time 60 min at 20 °C when
a proper and fully charged battery is used
Outputs Serial data line RS-232 or opto-isolated
RS-485 (2-wire) or optional data modem
Separate Maintenance line RS-232

Mechanical Specifications
Table 42 LT31 Mechanical Specifications
Property Description / Value
Dimensions (h  w  d) 1) 2.685 mm  420 mm  1022 mm
Weight
Transmitter Unit LTT111 85 kg
Receiver Unit LTR111 82 kg
Mounting On concrete foundation with three 16 mm
diameter bolts.
Mounting bolt pattern equal with MITRAS and
SKOPOGRAPH II Flamingo.
Material
- Mast, hoods, support units, Anodized/alodined aluminum, paint-coated
and all enclosures
- air duct and interface unit UV-resistant plastics (ASA)
cover parts
Frangibility Mounting kit contains frangible bolts
1) Height  width  depth

Environmental Specifications
Table 43 LT31 Environmental Specifications
Property Description / Value
Operating temperature -40 ... +65 °C standard; -55 ... +65 °C hood
heater option
Operating humidity 0 ... 100 %
Wind speed Up to 60 m/s

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Electromagnetic Compatibility
LT31 is CE compliant. This compliance has been verified according to
the following directives.

Table 44 LT31 CE Compliance

Verification Subject Standard
Electrical Safety IEC 60950-22/ EN 60950-22/A11
IEC 60950-1/ EN 60950-1/ UL 60950-1
Radiated Emissions CISPR 22/ EN 55022
Conducted Emissions AC CISPR 22/ EN 55022
Harmonics Current Emissions IEC 61000-3-2/ EN 61000-3-2
Electrostatic Discharge IEC 61000-4-2/ EN 61000-4-2
RF Field Immunity IEC 61000-4-3/ EN 61000-4-3
11 V/m (80 MHz-1 GHz)
4 V/m (1 GHz-4 GHz)
Electric Fast Transient IEC 61000-4-4/ EN 61000-4-4
Surge IEC 61000-4-5/ EN 61000-4-5
Conducted RF Immunity IEC 61000-4-6/ EN 61000-4-6

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Appendix A ________________________________________________________ Options Installation

APPENDIX A
OPTIONS INSTALLATION

Extension Poles Installation

LT31 Transmissometer can be installed on extension poles for added
height. The LTEP100/200 extension poles are an option that needs to be
ordered separately.

LTEP100/200 Extension Pole Unit

The LTEP100/200 extension pole unit includes the following:

- Extension pole LTEP100 (1 m extension) or LTEP200 (2 m

extension)
- One grounding cable
- Three frangible threaded mounting rods/foundation screws with
contraction area according to the extension pole length:
- Mounting rod contraction diameter 8 mm for usage with
LTEP100
- Mounting rod contraction diameter 9 mm for usage with
LTEP200
- Screws, washers and nuts to mount the LT31 to the extension pole

Prerequisites

CAUTION Before starting the installation of LT31 on extension poles, keep in mind
the total weight (approximately 85 kg) of the LT31 transmitter/receiver.
Plan the installation procedure accordingly.

Use only appropriate utilities to lift the LT31 for installation on

extension poles.

Take all necessary precautions to avoid any harm to personnel involved

when lifting and mounting LT31.

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CAUTION The LT31 transportation box contains the mounting kit including nuts,
washers and frangible mounting rods/foundation screws for installation
without extension poles. Do not use these standard frangible mounting
rods when installing extension poles.

The transportation box of each extension pole contains frangible

mounting rods which are adapted to the specific extension length.

Be sure to use frangible mounting rods/foundation screws with a

contraction area of diameter 8 mm when a LTEP100 (1 m) extension is
used and frangible mounting rods/foundation screws with a contraction
area of diameter 9 mm when a LTEP200 (2 m) extension is used.

Installing LTEP100/200
In this section, LTEP100/200 specific installation instructions are given
which replace the sections Setting up Receiver Mast to Foundation and
Setting up Transmitter Mast to Foundation in the LT31 User´s Guide in
case receiver and/or transmitter unit is installed using extension poles.
All other information given in the LT31 User´s Guide remains valid also
for cases where extension poles are used.

Drilling Instructions

NOTE The following instruction replaces the instruction in section Dowel

mounting on page 87 in a case where LTEP100/200 extension poles are
used.

1. Orientate the drilling template so that one of the drilling holes

points exactly towards the opposite installation place. On both
foundations, the instrument should be mounted afterwards in the
center of the foundation surface. The distance between the single
holes which point towards the opposite place, must match exactly
the desired baseline length of typically 30 m (50 m and 75 m
optional; see Figure 152 on page 283).

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Appendix A ________________________________________________________ Options Installation

0401-068

Figure 152 Positioning Drilling Holes

2. Drill three 20 mm holes with a depth of 70 mm using the drilling

template. Make sure that the holes are drilled perfectly
perpendicular.
3. Remove the template and clean the holes.
4. Insert the dowels into the holes and hammer them down until their
edges are on the foundation level. Recommended dowels: HILTI
HKD-S M16 x 65.
5a. Use the HILTI tool HSD-G M16 x 65 respectively a 14 mm 
mandrel bar and hammer the expanding mechanism of the wedge
bolts down until the bolts are completely fixed inside the holes.
5b. Fasten the frangible mounting rods/foundation screws to the
inserted dowels by hand.
6. Check the right orientation of the foundation screws. The breaking
area (area with smaller diameter → contraction area) just above the
welded nut on the frangible mounting rods/foundation screws must
point upwards.
7. Place a washer between the foundation surface and the welded nut
(see Figure 153 on page 284).

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0401-069

Figure 153 Frangible Mounting Rod and Foundation

Screws

8. Insulate the area between the frangible mounting rod/foundation

screw and foundation surface below the washer with silicon rubber,
so that water cannot get to the drilling hole.
9. Tighten the frangible mounting rod/foundation screws using the
welded nut (60 Nm; maximum 80 Nm).
10. Make sure that the screw positions fit by using the drilling
template.

Setting Up Receiver/Transmitter Mast to

Foundation
NOTE In case the LT31 Receiver Unit is installed using an extension pole
LTEP100/200, the following instruction replaces the section Setting up
Receiver Mast to Foundation on page 94.

In case the LT31 Transmitter Unit is installed using an extension pole

LTEP100/200, the following instruction replaces Section Setting up
Receiver Mast to Foundation on page 110.

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Appendix A ________________________________________________________ Options Installation

1. Screw one nut on the three frangible mounting rods/foundation

screws, until the distance between the foundation surface and the
nut top is 5 cm.
2. Place a washer on top of the nuts.
3. Place the extension pole LTEP100/200 on the prepared frangible
mounting rods/foundation screws so that the grounding screws and
the cut-out in the extension pole base and top plate is oriented
backwards from the baseline between the two installation places.
See Figure 157 on page 288.
4. Secure this preliminary extension pole position. Use a washer and
nut on each of the three foundation screws and fix it only hand-
tight (see Figure 154 below).

3 - washer for M16

4 - nut M16
6 - threaded frangible mounting rod 1104-118

Figure 154 Extension Pole Mounting

5. Ensure that the base plate of the extension pole has settled on the
nuts and washers mounted below.
6. Use a level and find the perfect vertical alignment for the extension
pole by loosening the respective securing nut and aligning the
extension pole by turning the nut below the base plate. See Figure
155 on page 286.

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0401-051

Figure 155 Vertical Extension Pole Alignment

7. When the extension pole is aligned perpendicular, secure the base

plate again and fix the nuts to ensure a stable mast position.
8. If the level of receiver and transmitter foundation surfaces is
identical in the frame of +/- 10cm, tighten the nuts as tight as
possible. A torque of 60 ... 80 Nm is recommended for the final
fixture. See Figure 156 on page 287.

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Appendix A ________________________________________________________ Options Installation

0401-053

Figure 156 Tightening Nuts

9. Mount the LT31 receiver/transmitter unit mast on the installed

extension pole using M16x80 bolts, washers and nuts. Keep in
mind that the interface unit at the mast base must be oriented
backwards from the baseline between the two installation places
(see Figure 156 above).

WARNING Use only appropriate utilities to lift the LT31 for installation on
extensions poles.

Take all necessary precautions to avoid any harm to involved personnel

when lifting and mounting the LT31.

10. Install the grounding connection between LT31 receiver/transmitter

unit using the grounding cable as shown in Figure 157, Detail A on
page 288.
11. Continue the LT31 installation by following the directions given in
Section Setting up Weather Protection Hood to Support Unit on
page 110 of the User's Guide.
12. Continue the LT31 installation by following the directions given in
section Setting Up Weather Protection Hood to Support Unit on
page 95 of the User's Guide.

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1001-099

Figure 157 LTEP100/200 Installation

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Appendix A ________________________________________________________ Options Installation

The following numbers refer to Figure 157 on page 288:

1 = Extension pole LTEP100/200
2 = Bolt M16x80
3 = Washer for M16
4 = Nut for M16
5 = Grounding cable
6 = Threaded frangible mounting rod

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 ___________________________________________________________________________INDEX

INDEX

Interface unit LTI211 55

A
Introduction to LT31 25
Alignment 225
L
Alignment Quality Control 43
Auto-alignment 42 Linearity test 245
Fine alignment 231 LT maintenance interface 132
Raw and fine alignment 226 LT31 key features 23
B M
Background luminance sensor LM21 61 Main Menu 136, 156
baseline 23, 41, 70, 86, 87, 94, 110, 152, 157, Main receiver module LTD112 50
165, 191, 226, 234, 277, 282, 285, 287 Main transmitter module LTL112 49
Battery backup LTBB111 63 Maintenance 193
Battery charger QBR101 36, 37 Master CPU LTC212 58
Measurement head CPU LTC112 53
C
Measurement range 165, 277
Calibration 232 Menu-driven operation 156
Auto-calibration 41 Message formats 161
Visibility calibration 238 METAR messaging 152
Commands 145 MOR 40
Communication options 78
O
D
Obstruction light LT31OBS 62
Disassembly for transportation 144 Operating principles 39
Operation 145
E Optical filter set LTOF111 245
Equipment grounding 72 Optics unit receiver LTO212 48
Equipment nomenclature 35 Optics unit transmitter LTO112 48
Event log 160
P
F Periodic maintenance 193
Fine alignment 24, 193, 206, 207, 208, 213, Cleaning instructions 194
215, 218, 220, 222, 225, 226, 232, See Power supply FSP102 56
Foundation level differences 123 Preparing installation 67
Functional description 39 Cable selection 73
Equipment grounding and lightning
H protection 72
Hardware description 45 Selecting location 70
Heated weather protection hood 110, 193, 200 Unpacking instructions 70
Host interface 59 Product overview 23
PWD forward scatter sensor 59
I PWD sensor 241
Installation 67 R
Installation procedure 85
Interface unit LTI111 55 Receiver measurement unit LTM212 47

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User's Guide ______________________________________________________________________

Receiver unit LTR111 27, 85, 126 Calibration 159

Recycling 20 Configuration 157
Repair instructions 201 Linearity test 159
Replacing battery in case of optional PWD sensor 160
LTBB111 222 Service 160
Replacing blower unit LTB111 220 Simulation 159
Replacing main receiver module LTD112 System status 158
218 Support unit 29, 30, 95, 110, 123
Replacing main transmitter module LTL112 System configuration 184
216
T
Replacing PWD sensor 223
Replacing window transmitter module Technical data 277
LTL212 213 Technical support 275
Return instructions 276 TERMBOX-1200/TERMBOX-9000 62
RVR system test commands 186 Transmittance 23, 41, 191, 245, 246, 249
Transmitter measurement unit LTM112 47
S
Transmitter unit LTT111 28, 120
Safety 16 Troubleshooting 261
Service 250
W
Software 60
Spare parts 36 Warranty 21
Spare parts for options 37 Weather protection hood 30, 31, 44, 93, 95, 96,
Startup procedure 132 109, 110, 194, 287
Status messages 261 Window transmitter module LTL212 52
Submenus 157
Alignment 159

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___________________________________________________________________________INDEX

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www.vaisala.com

*M210667EN*