STANDARD

American National Standard

ANSI/ISA-5.1-2024

This standard is intended for worldwide use in all

industries as applicable.

Instrumentation and
Control Symbols
and Identification

Approved: 17 JULY 2024

Second Printing: 05 SEPTEMBER 2024

© 2024 International Society of Automation. This is a copyright document and may not be copied or distributed in any form or manner
without the permission of ISA. This copy of the document was made for the sole use of the person to whom ISA provided it and is subject to
the restrictions stated in the Copyright of ISA Standards policy. It may not be provided to any other person in print, electronic, or any other form.
Violations of ISA’s copyright will be persecuted to the fullest extent of the law and may result in substantial civil and criminal penalties.
ANSI/ISA-5.1-2024, Instrumentation and Control Symbols and Identification

ISBN: 978-1-64331-250-7

Copyright © 2024 by the International Society of Automation (ISA). All rights reserved. Not for
resale. Printed in the United States of America. No part of this publication may be reproduced,
stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical,
photocopying, recording, or otherwise), without the pr ior written permission of the Publisher.

ISA
3252 S. Miami Blvd., Suite 102
Durham, NC 27703 USA
 -3- ANSI/ISA-5.1-2024

Preface
This preface, as well as the annex, is included for informational purposes and is not a normative part of
ANSI/ISA-5.1-2024.

This document has been prepared as part of the service of ISA toward a goal of uniformity in the field of
automation. To be of real value, this document should not be static but should be subject to periodic review.
Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the
Secretary, ISA Standards and Practices Board; standards@isa.org.

It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests
in the development of ISA standards, recommended practices, and technical reports. Participation in the
ISA standards-making process by an individual in no way constitutes endorsement by the employer of that
individual, of ISA, or of any of the standards, recommended practices, and technical reports that ISA
develops.

International System of Units (SI) are the preferred units of measurement in ISA standards.

CAUTION — ISA ADHERES TO THE POLICY OF THE AMERICAN NATIONAL STANDARDS INSTITUTE
WITH REGARD TO PATENTS. IF ISA IS INFORMED OF AN EXISTING PATENT THAT IS REQUIRED
FOR USE OF THE DOCUMENT, IT WILL REQUIRE THE OWNER OF THE PATENT TO EITHER GRANT
A ROYALTY-FREE LICENSE FOR USE OF THE PATENT BY USERS COMPLYING WITH THE
DOCUMENT OR A LICENSE ON REASONABLE TERMS AND CONDITIONS THAT ARE FREE FROM
UNFAIR DISCRIMINATION.

EVEN IF ISA IS UNAWARE OF ANY PATENT COVERING THIS DOCUMENT, THE USER IS
CAUTIONED THAT IMPLEMENTATION OF THE DOCUMENT MAY REQUIRE USE OF TECHNIQUES,
PROCESSES, OR MATERIALS COVERED BY PATENT RIGHTS. ISA TAKES NO POSITION ON THE
EXISTENCE OR VALIDITY OF ANY PATENT RIGHTS THAT MAY BE INVOLVED IN IMPLEMENTING
THE DOCUMENT. ISA IS NOT RESPONSIBLE FOR IDENTIFYING ALL PATENTS THAT MAY REQUIRE
A LICENSE BEFORE IMPLEMENTATION OF THE DOCUMENT OR FOR INVESTIGATING THE
VALIDITY OR SCOPE OF ANY PATENTS BROUGHT TO ITS ATTENTION. THE USER SHOULD
CAREFULLY INVESTIGATE RELEVANT PATENTS BEFORE USING THE DOCUMENT FOR THE
USER’S INTENDED APPLICATION.

HOWEVER, ISA ASKS THAT ANYONE REVIEWING THIS DOCUMENT WHO IS AWARE OF ANY
PATENTS THAT MAY IMPACT IMPLEMENTATION OF THE DOCUMENT NOTIFY THE ISA
STANDARDS AND PRACTICES DEPARTMENT OF THE PATENT AND ITS OWNER.

ADDITIONALLY, THE USE OF THIS DOCUMENT MAY INVOLVE HAZARDOUS MATERIALS,

OPERATIONS OR EQUIPMENT. THE DOCUMENT CANNOT ANTICIPATE ALL POSSIBLE
APPLICATIONS OR ADDRESS ALL POSSIBLE SAFETY ISSUES ASSOCIATED WITH USE IN
HAZARDOUS CONDITIONS. THE USER OF THIS DOCUMENT MUST EXERCISE SOUND
PROFESSIONAL JUDGMENT CONCERNING ITS USE AND APPLICABILITY UNDER THE USER’S
PARTICULAR CIRCUMSTANCES. THE USER MUST ALSO CONSIDER THE APPLICABILITY OF ANY
GOVERNMENTAL REGULATORY LIMITATIONS AND ESTABLISHED SAFETY AND HEALTH
PRACTICES BEFORE IMPLEMENTING THIS DOCUMENT.

THE USER OF THIS DOCUMENT SHOULD BE AWARE THAT THIS DOCUMENT MAY BE IMPACTED
BY ELECTRONIC SECURITY ISSUES. THE COMMITTEE HAS NOT YET ADDRESSED THE
POTENTIAL ISSUES IN THIS VERSION.

ISA (www.isa.org) is an international professional association that sets the standard for those who apply
engineering and technology to improve the management, safety, and cybersecurity of modern automation
and control systems used across industry and critical infrastructure. Founded in 1945, ISA develops widely
used international standards for industrial automation, safety, and cybersecurity; certifies industry

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 -4-

professionals; provides education and training; publishes books and technical articles; hosts conferences
and events; and provides networking and career development programs for its members and customers
around the world.

ISA owns Automation.com, a leading online publisher of automation-related content. In addition, through a
wholly owned subsidiary, ISA bridges the gap between standards and their implementation with the ISA
Security Compliance Institute (www.isasecure.org) and the ISA Wireless Compliance Institute
(www.isa100wci.org).

The following served as voting members of ISA5 for this 2024 revision of ISA-5.1:

NAME AFFILIATION

Thomas McAvinew, Chair Consultant

Ian Verhappen, Managing Director Willowglen Systems
James Federlein, ISA5.1 WG Chair Consultant
Yamei Chen Eli Lilly
David Hobart Consultant
Michel Ruel BBA

On behalf of the ISA Standards and Practices Board, we wish to recognize and thank Tom McAvinew for
his years of dedication and knowledge in leading the development and understanding of this widely used
ISA international standard.

Ian Verhappen
Jim Federlein
Charley Robinson

This standard was approved for publication by the ISA Standards and Practices Board on 5 April 2024:

NAME AFFILIATION

D. Lee, Chair UCDS

D. Brandl, Chair-Elect BR&L Consulting
D. Bartusiak Collaborative Systems Integration
P. Brett Honeywell
E. Cosman OIT Concepts
J. Federlein Federlein & Associates
S. Fluchs admeritia GmbH
J.-P. Hauet KB Intelligence
G. Lehmann NextEra Energy
T. McAvinew Consultant
V. Mezzano Fluor
C. Monchinski Automated Control Concepts
D. Reed Rockwell Automation
H. Sasajima FieldComm Group
I. Verhappen Willowglen Systems
D. Visnich Burns & McDonnell
W. Weidman Consultant
J. Weiss Applied Control Solutions
M. Wilkins Yokogawa
D. Zetterberg Consultant

Copyright © 2024 ISA. All rights reserved.

 -5- ANSI/ISA-5.1-2024

Contents
Introduction (informative) .............................................................................................................................. 7
1 Scope .................................................................................................................................................. 9
1.1 General............................................................................................................................................. 9
1.2 Application to industries ................................................................................................................... 9
1.3 Application to work activities ............................................................................................................ 9
1.4 Application to classes of instrumentation and to instrument functions .......................................... 10
1.5 Extent of loop and functional identification..................................................................................... 11
1.6 Extent of symbolization .................................................................................................................. 11
1.7 Inclusion and modification of this standard in user engineering and design documents ............... 12
2 Normative references ........................................................................................................................ 12
3 Terms, definitions, and abbreviated terms ........................................................................................ 13
3.1 Definitions ...................................................................................................................................... 13
3.2 Abbreviations – for terms not otherwise defined ............................................................................ 20
4 Identification letters ........................................................................................................................... 20
4.1 Identification letters table ............................................................................................................... 20
5 Graphic symbols ............................................................................................................................... 25
5.1 General........................................................................................................................................... 25
5.2 Tables to be used for common applications .................................................................................. 26
5.3 Instrumentation device and function symbols ................................................................................ 28
5.4 Measurement symbols ................................................................................................................... 32
5.5 Line symbols .................................................................................................................................. 40
5.6 Final control element symbols ....................................................................................................... 44
5.7 Functional diagramming symbols .................................................................................................. 52
5.8 Signal processing function block symbols ..................................................................................... 54
5.9 Binary logic symbols ...................................................................................................................... 62
5.10 Electrical schematic symbols ......................................................................................................... 73
5.11 Instrument loop diagrams............................................................................................................... 78
6 Graphic symbol dimensions .............................................................................................................. 79
6.1 Graphic symbols dimension tables ................................................................................................ 79
6.2 Measurement units ......................................................................................................................... 80
6.3 Dimensions for graphic symbol tables ........................................................................................... 80
6.4 Dimensions for measurement symbols .......................................................................................... 81
6.5 Dimensions for line symbols .......................................................................................................... 84
6.6 Dimensions for final control elements ............................................................................................ 85
6.7 Dimensions for functional diagramming symbols .......................................................................... 87
6.8 Dimensions for binary logic symbols .............................................................................................. 88
6.9 Dimensions for electrical schematic symbols ................................................................................ 89

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 -6-

7 References ........................................................................................................................................ 91
7.1 ISA publications ............................................................................................................................. 91
7.2 Other related information ............................................................................................................... 91
Annex A – Summary of changes to previous 2022 revision ....................................................................... 93

Tables
Table 1 – Identification letters ..................................................................................................................... 21
Table 2 – Instrumentation device and function symbols ............................................................................. 28
Table 3 – Instrumentation device or function symbols, miscellaneous ....................................................... 29
Table 4 – Measurement symbols: Primary elements and transmitters ....................................................... 32
Table 5 – Measurement symbols: Measurement notations ........................................................................ 33
Table 6 – Measurement symbols: Primary elements .................................................................................. 34
Table 7 – Measurement symbols: Secondary instruments ......................................................................... 37
Table 8 – Measurement symbols: Auxiliary and accessory devices .......................................................... 38
Table 9 – Line symbols: Instrument to process and equipment connections ............................................. 40
Table 10 – Line symbols: Instrument-to-instrument connections ............................................................... 41
Table 11 – Final control element symbols .................................................................................................. 44
Table 12 – Final control element actuator symbols .................................................................................... 46
Table 13 – Self-actuated final control element symbol ............................................................................... 49
Table 14 – Control valve failure and de-energized position indications ..................................................... 51
Table 15 – Functional diagramming symbols ............................................................................................. 52
Table 16 – Signal processing function block symbols ................................................................................ 54
Table 17 – Binary logic symbols ................................................................................................................. 62
Table 18 – Electrical schematic symbols .................................................................................................... 73
Table 19 – Instrument loop diagram symbols ............................................................................................. 78
Table 20 – Dimensions for Tables 2 and 3 ................................................................................................. 80
Table 21 – Dimensions for Tables 4, 5, 6, 7, and 8 .................................................................................... 82
Table 22 – Dimensions for Tables 9 and 10 ............................................................................................... 84
Table 23 – Dimensions for Tables 11, 12, 13, and 14 ................................................................................ 86
Table 24 – Dimensions for Table 15 ........................................................................................................... 87
Table 25 – Dimensions for Table 17 ........................................................................................................... 88
Table 26 – Dimensions for Table 18 ........................................................................................................... 90

Copyright © 2024 ISA. All rights reserved.

 -7- ANSI/ISA-5.1-2024

Introduction (informative)

General
This introduction is included for information only and is not a normative section of
ANSI/ISA-5.1-2024. It is highly recommended that users compare the changes listed in Annex A
against the 2022 revision of this standard to fully understand the exact changes made.

When integrated into a system, the designations of mandatory and nonmandatory symbols and
identification form a dedicated language that communicates concepts, facts, intent, instructions,
and knowledge about measurement and control systems that can be appl ied as appropriate in all
industries. Consistency is the one criterion that should govern the selection and application of
identification systems and graphic symbols.

The symbols and identification methods set forth in this standard are intended as conceptualizing
aids, as design tools, as teaching devices, and as a concise and specific means of communication
in all types and kinds of technical, engineering, procurement , construction, and maintenance
documents. This includes but is not restricted to instrumentation and control on piping and
instrumentation diagrams (P&IDs), functional diagrams, binary logic diagrams, electrical
schematics, and loop diagrams.

The ISA5.1 Working Group greatly appreciates and respects the work completed by the past
ISA5.1, ISA5.2, ISA5.3, and ISA77.40 Subcommittees/Working Groups and has tried to treat their
work with the great respect it deserves.

Background of 2024 revision

This revision changes the title to “Instrumentation and Control Symbols and Identification ” to
emphasize that symbols for control are also included. This revision is published with significant
changes to improve the readability of the document by organizing notes with corresponding tables,
by providing definition consistency and clarification, by elim inating redundant text, by adding new
symbols, notation, and identification for new automation technology, by adding new sections for
Normative Reference, Abbreviation, and Bibliography, and by adding a Loop Instrument Diagrams
Symbol Table.

This revision moves Annex A and Annex B from the 2022 revision into separate technical
reports for easier maintenance and to reduce the size of the standard. The new technical
reports are ISA-TR5.1.02, Instrumentation and Control Identification System Guidelines, and
ISA-TR5.1.03, Instrumentation and Control Graphic Symbols Guidelines. Users are
encouraged to read and utilize these TRs together with the standard.

Historical background
The symbols and identification systems described in this standard accommodate advances in
technology and reflect the collective industrial experience gained since the original ISA
Recommended Practice RP-5.1, published in 1949, was revised, affirmed, and subsequently
published as ANSI/ISA-5.1-1984. The 1949 recommended practice and the 1984 standard were
published as nonmandatory rather than as mandatory consensus documents.

The 1992 revision was published as a consensus standard. Function block and function
designation of SAMA PMC 22.1-1981, “Functional Diagramming of Instrument and Control
Systems,” were incorporated. Key elements of ISA-5.3-1983, Graphic Symbols for Distributed
Control/Shared Display Instrumentation, Logic and Computer Systems were incorporated.

The 2009 revision was published as a consensus standard and contains both mandatory and non -
mandatory statements that have been reviewed and approved by a large group of practitioners in
the field of instrumentation and control. Nonmandatory examples were move d to a new Annex B,
“Graphic symbol guidelines” (Informative), to provide some limited assistance in the application or

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 -8-

removed for inclusion into future technical reports to provide special practices and requirements
of particular interest groups and/or specific industries. The 2009 revision was published with
significant changes as technological advancement resulted in the evolution from a hardware
(instrument) centric standard to a hardware/software (automation) centric standard. Key elements
of ISA-5.2-1976, Binary Logic Diagrams for Process Operations, were incorporated. Binary logic
symbols of SAMA PMC 22.1-1981, “Functional Diagramming of Instrument and Control Systems,”
were incorporated. Graphic symbol dimension tables were incorporated to establish minimum
mandatory dimensions for the symbols.

The 2022 revision of ISA-5.1 was published as an interim revision with only known usage
clarifications and to correct technical and typographical errors so that the ISA5 Standards
Committee could begin a new revision of this widely used international standard.

For a summary of the changes made to ISA-5.1-2022 in this revision, refer to Annex A.

ISA-TR5.1.01/ISA-TR77.40.01, Functional Diagram Usage, issued in 2012 and reaffirmed in 2016,

was published as the first joint technical report under the ISA5 and ISA77 Committees. The
purpose of this technical report is to provide advice and guidance in documenting application
software through functional diagrams by expand ing on and illustrating usage of function block
symbols and functions and to prepare examples in development of complex function block symbols
and functions.

Copyright © 2024 ISA. All rights reserved.

 -9- ANSI/ISA-5.1-2024

1 Scope
This standard establishes a uniform means of depicting and identifying instruments or devices and
their inherent functions, instrumentation systems and functions, and application software functions
used for measurement, monitoring, and control, by presenting a designation system that includes
identification schemes and graphic symbols.

1.1 General
1.1.1 This standard is intended to meet the different procedures of various users who need to
identify and graphically depict measurement and control equipment and systems. These
differences are recognized when they are consistent with the objectives of this standard, by
providing alternative symbols and identification methods.

1.1.2 This document provides graphic symbols for representations of the instrumentation and
control functions required for process or equipment measurement, indication, and control of
variables used in any of the following applications.

a) Instrumentation and control diagrams, which illustrate and identify instruments and
related devices and their inherent functions, instrumentation systems and functions,
and application software functions used for measurement, monitoring, and control.
These illustrations can appear as part of process flow diagrams (PFDs) and piping
and instrumentation diagrams (P&IDs), which include process equipment,
mechanical equipment, and piping information.
b) Functional diagrams, which depict application software functions (analog and
binary signal processing) for implementation into the selected control systems.
c) Binary logic diagrams, which depict interlocking and stepwise logic programming.
d) Electrical schematics, which depict electrical circuitry for on -off control. These
include ladder diagrams.
e) Instrument loop diagrams, which depict a composite representation of instrument
loop information, containing associated electrical, piping, and process equipment
connections as well as other pertinent information, including ranges, alarms, and
device configuration settings.
1.1.3 Examples of identification and symbol applications are available in associated technical
reports ISA-TR5.1.02, Instrumentation and Control Identification System Guidelines and
ISA-TR5.1.03, Instrumentation and Control Graphic Symbols Guidelines and are intended to
illustrate basic concepts in the construction of the identification systems and diagrams covered by
this standard that are applicable to all industries.

1.2 Application to industries

1.2.1 This standard is suitable for use in chemical, petroleum, power generation, metal refining,
pulp and paper, and numerous other continuous, batch, and material -handling industries. These
industries and others require the use of some or all of the diagrams listed in 1.1.2 to describe the
relationship between processing equipment and the functionality of measurement and control
equipment.

1.3 Application to work activities

1.3.1 This standard is suitable for use whenever reference to measurement and control
instrumentation, control devices and functions, and application software and functions is required
for identification and symbolization, such as:

a) design sketches;

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 10 -

b) teaching examples;
c) technical papers, literature, and discussions;
d) instrumentation, loop, logic, and functional diagrams;
e) function descriptions;
f) conceptual drawings including, but not limited to:
1) PFDs; and
2) utility flow diagrams (UFDs).
g) construction drawings including, but not limited to:
1) engineering flow diagrams (EFDs);
2) mechanical flow diagrams (MFDs);
3) P&IDs; and
4) system flow diagrams (SFDs).
h) specifications, purchase orders, manifests, and other lists ;
i) identification and tag numbering of instruments and control functions; and
j) installation, operating and maintenance instructions, drawings, and records.
1.3.2 This standard provides sufficient information to enable anyone who has a reasonable
amount of process and instrumentation knowledge, and who is reviewing documents depicting
measurement and control, to understand the means and purpose of the instrumentatio n and
controls shown.

1.3.3 The detailed knowledge of a specialist in instrumentation and/or control systems is not a
prerequisite to understanding this standard.

1.4 Application to classes of instrumentation and to instrument functions

1.4.1 The identification and symbolism methods provided in this standard are applicable to all
classes and types of measurement and control instruments and/or functions.

1.4.2 The methods can be used for, but are not limited to, describing and identifying:

a) standalone instruments and their functions ;

b) shared display and shared control functions ;
c) higher level control functions; and
d) application software display and control functions.
1.4.3 Classification of instrumentation:

1.4.4 Instrumentation can be classified as primary, secondary, auxiliary, or accessory for

assigning loop and functional identities and symbols. Also, refer to Clause 3 in ISA-TR5.1.02.

1.4.5 Primary instrumentation consists of measuring, monitoring, controlling, or calculating

devices and hardware and their inherent functions and application software functions that include,
but are not limited to, transmitters, recorders, controllers, control valves, self -actuated safety and
control devices, and application software functions that require or allow user -assigned
identifications.

Copyright © 2024 ISA. All rights reserved.

 - 11 - ANSI/ISA-5.1-2024

1.4.6 Secondary instrumentation consists of measuring, monitoring, or controlling devices and

hardware that include, but are not limited to, l evel glasses, pressure gauges, thermometers, and
pressure regulators.

1.4.7 Auxiliary instrumentation consists of devices and hardware that measure, control, or
calculate and that are needed for effective operation of primary or secondary instrumentation; they
include, but are not limited to, calculating devices, purge meters, sam ple handling systems, and
instrument air sets.

1.4.8 Accessory instrumentation consists of devices and hardware that do not measure or control
but are needed for effective operation of the measuring, monitoring, or control system; they
include, but are not limited to, flowmeter tube run, straightening vanes, and seal pots.

1.5 Extent of loop and functional identification

1.5.1 This standard and ISA-TR5.1.02 provide identification codes and methods for the
alphanumeric identification of monitoring and controlling loops, instruments, and functions.

1.5.2 These identification methods depend on tagging according to function and not according
to construction or form. For example, a differential pressure transmitter is identified as a:

a) flow transmitter when connected to an orifice plate to measure flow;

b) level transmitter when connected to the side of a vessel to measure liquid level; or
c) pressure differential transmitter when connected across a filter to measure pressure
difference.
1.5.3 The user is free to apply added identification by serial, equipment, unit, area, or plant
number, or any other additional means required for the unique identification of a loop, instrument,
or function.

1.5.4 A unique function identification number shall be assigned to identify each:

a) loop instrument or device and its integral and/or inherent functions ; and
b) loop-configurable function, which requires or allows a unique user -assigned
microprocessor or computer address.
1.6 Extent of symbolization
1.6.1 This standard provides symbol sets for the graphic depiction of a limited or total
functionality of instruments and devices, entire monitor or control loops, or control circuits.

1.6.2 The amount of detail to be shown by the use of symbols depends on the purpose and
audience for which the document is being prepared.

1.6.3 Sufficient symbols shall be used to show the functionality of the instrumentation and/or
control loop being depicted. It is not considered necessary to provide a symbol for each instrument
device and each function required by a loop.

1.6.4 Additional construction, fabrication, installation, and operation details of an instrument are
better described in a suitable specification, data sheet, installation/wiring drawing, sketch, or other
document intended for those requiring such details rather than the drawings covered in this
standard.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 12 -

1.7 Inclusion and modification of this standard in user engineering and design
documents
1.7.1 This standard can be used and, if used, shall be credited by a user/owner in the preparation
of engineering, design, or project standards, guidelines, specifications, and other documents,
either without exception or with exception per the following :

a) without exception, in which case this standard in its entirety shall be mandatory
with respect to:
1) letters assigned a specific meaning in Table 1;
2) symbols and their assigned meanings in Tables 2 through 19; and
3) symbol dimensions in Tables 20 through 26.
b) with exceptions, in which case the parts of this standard for which exception is:
1) taken shall be fully described and detailed in the user/owner ’s standards,
guidelines, or specifications and in drawing cover sheet legends and notes ;
and
2) not taken shall be mandatory.
1.7.2 Symbols different from those given in this standard and user’s choice letters when used,
shall be fully described, and detailed in the user/owner’s standards, guidelines, or specifications
and in drawing cover sheet legends and notes.

1.7.3 A user shall choose a numbering scheme, graphic symbol, and other options where
required, and document those choices.

1.7.4 When a previous issue of this standard is included by reference with or without exception
in the user/owner’s engineering and design standards, guidelines, or specifications, that issue, in
part or in its entirety, shall be mandatory until such time as the user/owner’s guidelines or
standards are revised.

1.7.5 Symbols and the meanings of letters and symbols from previous versions of this standard
that are different from those contained in this version can continue to be used provided they are
clearly referenced in the user’s engineering and design standards, practices, and/or guidelines.

2 Normative references
2.1.1 The following documents, in whole or in part, are normatively referenced in this document
and are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.

ISA-5.4, Instrument Loop Diagrams

Copyright © 2024 ISA. All rights reserved.

 - 13 - ANSI/ISA-5.1-2024

3 Terms, definitions, and abbreviated terms

3.1 Definitions
For the purposes of this standard, the following definitions apply. Applicable abbreviations are
included. For additional information, see ISA-TR77.00.01-2019, Fossil Fuel Power Plant
Definitions and Basic Control Concepts. Terms italicized in a definition are themselves defined
elsewhere in this clause.

3.1.1
accessible
feature of a device or function, or a feature of an interactive shared system function, that can be
used or seen by an operator for the purpose of performing control operations, such as setpoint
changes, auto-manual transfer, or on-off operations

3.1.2
alarm
indicating device or function that provides a visible and/or audible indication if and when the value
of a measured or initiating variable is outside of set limits, has changed from a safe to an unsafe
condition, or has changed from a normal to an abnormal operating state or condition
Note 1 to entry: Actuation can be by a discrete or analog signal or function.
Note 2 to entry: Indication can be by any or all the following: annunciator panels, flashing lights, printers, buzzers, bells,
horns, sirens, or shared graphic display systems.

3.1.3
analog signal
signal representing a variable that can be continuously observed and continuously represented

3.1.4
application software
software specific to a user application that is configurable and in general contains logic sequences,
interlocks and limit expressions, control algorithms, and other code required to control the process
using the appropriate inputs, outputs, calculations, and decisions

3.1.5
basic process control system
BPCS
instrumentation and systems that are installed to monitor and control normal production operations
using, but not limited to, combinations of single-loop pneumatic and electronic monitors and
controllers, programmable logic controllers and distributed control systems
Note 1 to entry: A BPCS is necessary to operate a plant or process.
Note 2 to entry: The BPCS can take the form of a DCS, PLC, SCADA, PC, etc. (See 3.2 for abbreviations.)

3.1.6
behind the panel
location that in a broad sense means “not normally accessible to an operator ,” such as the rear of
an instrument or control panel, an enclosed instrument rack or cabinet, or an instrument rack room
within an area that contains a panel

3.1.7
binary logic
programming based on two-state or Boolean logic

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 14 -

3.1.8
binary signal
signal that has only two individual positions or states, and when used in its simplest form, the term
denotes an “on-off” or “high-low” state

3.1.9
bubble
balloon
circle-based symbols (Table 2) used to denote and identify the purpose of an instrument or function
that can contain a tag number

3.1.10
communication link
data link
wire, cable, or electromagnetic network or bus system that connects dedicated microprocessor-
based systems so that they share a common database and communicate according to a rigid
protocol in a hierarchical and/or peer-to-peer relationship
Note 1 to entry: Wire or cable networks can be twisted pair, coaxial, telephone, or fiber optic.
Note 2 to entry: Electromagnetic networks can be radio or microwave.

3.1.11
computing device
computing relay
device that performs one or more calculations or logic operations, or both, and transmits one or
more resultant output signals

3.1.12
configurable
devices or systems whose functional and/or communication characteristics can be selected or
rearranged through the setting of program switches, application software, fill-in-the-blank forms,
pull-down menus, entered values or text, and/or other methods, other than rewiring /reprogramming
as a means of altering the configuration

3.1.13
controller
device that operates automatically to regulate a controlled variable

3.1.14
control valve
device, other than a common, hand-actuated process block valve or self-actuated check valve,
that directly manipulates the flow of one or more fluid process streams
Note 1 to entry: The designation “hand control valve” is limited to hand-actuated valves that when used for process
throttling require identification as an instrument or control device.

3.1.15
controlled variable
quantity, property, or condition that is measured and controlled
Note 1 to entry: Controlled variables are a subset of measured variables.
3.1.16
converter
device that receives information as one form of an instrument signal and transmits an output signal
as another form, such as a current-to-pneumatic signal converter
Note 1 to entry: An instrument that changes a sensor’s output to a standard signal is properly designated as a transmitter,
not a converter; typically, a temperature element (TE) connects to a transmitter (TT), not to a converter (TY).

Copyright © 2024 ISA. All rights reserved.

 - 15 - ANSI/ISA-5.1-2024

Note 2 to entry: A converter is sometimes referred to as a transducer, a completely general term not recommended for
signal conversion.

3.1.17
detector
device that is used to detect the presence of something, such as flammable or toxic gases or
individual parts; see also primary element and sensor

3.1.18
device
piece of instrument hardware that is designed to perform a specific action or function, such as a
controller, indicator, transmitter, annunciator, or control valve

3.1.19
digital communication
signal that consists of binary digits transmitted to represent analog values or other information

3.1.20
discrete signal
signal that has any number of noncontinuous distinct or defined states or positions, but most
typically having two distinct states

3.1.21
final control element
device, such as a control valve, that directly controls the value of the manipulated variable of a
control loop

3.1.22
function
purpose of, or the action performed by, a device or application software

3.1.23
hardware
physical equipment directly involved in performing measuring, monitoring, and controlling functions

3.1.24
higher level control system
HLCS
system that provides sophistication above that of the BPCS , with functions typically based in
hardware and software outside of the BPCS
Note 1 to entry: Control functions in the HLCS include, but are not limited to, statistical process control and model
predictive control.
Note 2 to entry: A HLCS is not necessary to operate a plant or process.

3.1.25
identification
sequence of letters or numerals, or both, used to designate an individual instrument, function, or
loop

3.1.26
initiating variable
quantity, property, or condition that is not measured or calculated but serves as the measured
variable of a loop, a change of which initiates loop action
Note 1 to entry: For example, “hand,” as in push button or switch, is an initiating variable.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 16 -

3.1.27
instrument
device used for direct or indirect measurement, monitoring, and/or control of a process variable,
including primary elements, indicators, controllers, final control elements, computing devices, and
electrical devices such as annunciators, switches, and push buttons
Note 1 to entry: The term does not apply to a device’s internal components or parts, such as receiver bellows or resistors.

3.1.28
instrument, field
instrument, local
instrument that is not mounted on a panel or console or in a control room but commonly in the
vicinity of its primary element or final control element

3.1.29
instrument, standalone
device or hardware that is a separate entity, such as a single-case controller or recorder

3.1.30
instrumentation
collection of instruments, devices, hardware, or functions or their application for the purpose of
measuring, monitoring, or controlling an industrial process, or any combination of these

3.1.31
interlock
device or group of devices (hardware) or function or group of functions (application software)
arranged to sense a limit or off-limit condition, or improper sequence of events and take a desired
action
3.1.32
light, pilot
light, status
light that indicates which of one or more normal conditions of a system or device exists
Note 1 to entry: It is not an alarm light, which indicates an abnormal condition.

3.1.33
local panel
panel that is not a central or main panel and is commonly located in the vicinity of plant subsystems
or subareas
Note 1 to entry: The term “local panel instrument” should not be confused with “local instrument.”

3.1.34
loop
instrumentation arranged as a combination of two or more instruments or functions so that signals
pass from one to another for the purpose of measurement and indication or control of a process
variable
Note 1 to entry: Term includes a self-contained device that measures and controls a process variable.

3.1.35
loop identification number
unique combination of letters and numbers that are assigned to each monitoring and control loop
in a facility to identify the process variable that is being monitored or controlled

Copyright © 2024 ISA. All rights reserved.

 - 17 - ANSI/ISA-5.1-2024

3.1.36
manipulated variable
quantity or condition that is varied by a controller’s output to change the value of the controlled
variable

3.1.37
measured variable
quantity, property, or condition that is measured or calculated
Note 1 to entry: Measured variables are a subset of process variables.

3.1.38
measurement
determination of the existence and/or magnitude of a process variable

3.1.39
monitor (noun)
instrument or device used for indicating or recording of a process variable or quantity

3.1.40
monitor (verb)
to measure or sense the status or magnitude of one or more variables for the purpose of deriving
useful information, and that sometimes means analyzer, indicator, or alarm

3.1.41
owner
company that operates or owns the production facilities where the instrumentation and control
system is or will be installed

3.1.42
panel
freestanding or built-in structure consisting of one or more sections, cubicles, consoles, or desks
that has groups of instrument hardware mounted on it, houses the operator-process interface, and
is given a unique designation

3.1.43
primary element
external or internal instrument or system element that quantitatively converts the measured
variable into a form suitable for measurement; see also detector and sensor
Note 1 to entry: An orifice plate is an external primary element .
Note 2 to entry: The sensor of a pressure transmitter is an internal primary element.

3.1.44
process
any operation or sequence of operations involving a change of energy, state, composition,
dimension, or other properties, e.g., change in pressure, temperature, speed, electrical potential
Note 1 to entry: A process can be small or large in scale, e.g. a water heater or a refinery.

3.1.45
process variable
any property of a process; used in this standard to apply to all variable properties of the process
and includes measured variables and controlled variables

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 18 -

3.1.46
program
repeatable sequence of actions that defines the state of outputs as a relationship to the state of
inputs

3.1.47
relay
device, whose function is to pass on information in an unchanged form or in some modified form
Note 1 to entry: Relay is a term applied specifically to an electric, pneumatic, or hydraulic switching device that is
actuated by a signal, and to functions performed by a relay.

3.1.48
safety instrumented system
SIS
system composed of sensors, logic solvers, and final control elements for the purpose of taking
the process to a safe state when predetermined conditions a re violated

3.1.49
scan
to sample, in a predetermined manner, each of several variables periodically or intermittently
Note 1 to entry: A scanning device is often used to ascertain the state or value of a group of variables and can be
associated with other functions such as recording and alarming.
3.1.50
sensor
separate or integral part or function of a loop or an instrument that first senses the value of a
process variable, that assumes a corresponding predetermined and intelligible state, and/or
generates an output signal indicative of or proportional to the process variable; see also detector
and primary element

3.1.51
setpoint
input variable that sets the desired value of the controlled variable manually, automatically, or by
means of a program in the same units as the controlled variable

3.1.52
shared control
feature of a control device or function that contains several preprogrammed algorithms, which are
user retrievable, configurable, and connectable, and allows user-defined control strategies or
functions to be implemented
Note 1 to entry: Term often used to describe the control features of a distributed control system, programmable logic
controller, or other processor-based system.
Note 2 to entry: Control of multiple process variables can be implemented by sharing the capabilities of a single device
of this kind.

3.1.53
shared display
operator interface device, a video, light-emitting diode, liquid crystal, or other display unit, used to
display process control information from several sources at the command of the operator
Note 1 to entry: Term often used to describe the visual features of a distributed control system, programmable logic
controller, or other processor-based system.

Copyright © 2024 ISA. All rights reserved.

 - 19 - ANSI/ISA-5.1-2024

3.1.54
station, auto-manual
station, control
device or function that provides switching between manual and automatic control modes of a
control loop

3.1.55
station, manual loading
device or function that has a manually adjustable output, and can also have indicators, lights,
and/or other functions, that is used to actuate and/or modulate one or more devices but does not
provide switching between auto-manual modes of a control loop

3.1.56
switch
device that connects, disconnects, selects, or transfers one or more circuits and is not designated
as a controller, a relay, or a control valve; the term is also applied to the functions performed by
switches

3.1.57
tag number
unique combination of letters and numbers that are formed by adding letters to the loop
identification number to define the purpose of each loop device and/or function that comprises a
monitoring or control loop
Note 1 to entry: Synonymous terms include: instrument identification, instrument identification number, instrument tag
number, instrument number, and tag name.

3.1.58
test point
process connection to which no instrument is permanently connected, but which is intended for
the temporary or intermittent connection of an instrument

3.1.59
transducer
device, which can be a primary element, transmitter, relay, converter, or other device, that receives
information in the form of one or more physical quantities, modifies the information or its form, or
both if required, and produces a resultant output signal

3.1.60
transmitter
device that senses a process variable through the medium of a sensor or measuring element and
has an output whose steady-state value varies only as a predetermined function of the process
variable
Note 1 to entry: The sensor can be an integral part, as in a direct -connected pressure transmitter, or a separate part, as
in a thermocouple.

3.1.61
user
entity that applies this standard for their company, or on behalf of their customer/client.
Note 1 to entry: Examples include engineering/consulting firms, operating process companies, manufacturing
companies, etc.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 20 -

3.2 Abbreviations – for terms not otherwise defined

DCS distributed control system

PC personal computer

PFD process flow diagram

P&ID piping (or process) and instrument diagram

PLC programmable logic controller

SCADA supervisory control and data acquisition

4 Identification letters
4.1 Identification letters table
4.1.1 This subclause provides in tabular form and accompanying notes the alphabetic building
blocks of the instrument and function identification system in a concise, easily referenced manner.

4.1.2 Table 1, together with 4.1.5, defines and explains the meanings of the individual letters
when used to identify loop and device functions.

4.1.3 The letters in Table 1 shall have the mandatory meanings assigned except the user shall
assign:

a) variables to the user’s choice letters in column 1 and functions to the user’s choice
letters in columns 3, 4, and 5 when such letters are used; and
b) meanings to the blanks in columns 2, 3, 4, and 5 when additional functions or
modifiers are assigned.
4.1.4 When such assignments are made, they shall be documented in the user’s engineering and
design standards or guidelines and on drawing legend sheets.

Copyright © 2024 ISA. All rights reserved.

 - 21 - ANSI/ISA-5.1-2024

Table 1 – Identification letters

NOTE Numbers in parentheses refer to the explanatory notes in 4. 1.5.

First letters (1) Succeeding letters (13)

Column 1 Column 2 Column 3 Column 4 Column 5

Measured/initiating Variable Readout/passive Output/active Function

variable modifier (10) function function modifier

A Analysis (2)(3)(4) Alarm

B Burner, Combustion (2) User’s Choice (5) User’s Choice (5) User’s Choice (5)

C User’s Choice (3a)(5) Control (21a)(21e) Close (25b)

D User’s Choice (3a)(5) Difference, Differential Deviation (26)

E Voltage (2) Sensor, Primary Element

F Flow, Flow Rate (2) Ratio

Glass, Gauge, Viewing
G User’s Choice
Device (14)
H Hand (2) High (25a)( 26a)(27)

I Current (2) Indicate (15)

J Power (2) Scan (16)

K Time, Schedule (2) Time Rate of Change (11) Control Station (22)

L Level (2) Light (17) Low (25b)( 26)(27)

Middle, Intermediate
M User’s Choice (3a)(5)
(25c)(26)(27)
N User’s Choice (5) User’s Choice (5) User’s Choice (5) User’s Choice (5)

O User’s Choice (5) Orifice, Restriction Open (25a)

P Pressure (2) Point (Test Connection)

Q Quantity (2) Integrate, Totalize Integrate, Totalize

R Radiation (2) Record (18) Run

S Speed, Frequency (2) Safety (12) Switch (21b) Stop

T Temperature (2) Transmit

U Multivariable (2)(6) Multifunction (19) Multifunction (19)

Vibration, Mechanical Valve, Damper, Louver
V
Analysis (2)(4)(7) (21c)(21e)
W Weight, Force (2) Well, Probe
Accessory Devices (20),
X On/Off or Unclassified (8) X-axis Unclassified (8) Unclassified (8)
Unclassified (8)
Auxiliary Devices
Y Event, State, Presence (2)(9) Y-axis
(21d)( 23)( 24)

Z-axis, Safety Instrumented Driver, Actuator, Unclassified

Z Position, Dimension (2)
System (28) final control element

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 22 -

4.1.5 Notes for Table 1 – Identification letters:

The following notes, indicated in Table 1 by parentheses, are to be used as an aid in understanding
the meanings of the letters when they are used in certain positions in loop identification letter(s)
or functional identifications.

(1) First letters are a measured/initiating variable and, if required, a combination of a

measured/initiating variable and a variable modifier that shall be referred to by the
combined meaning.

(2) The specific meanings given for measured/initiating variables [A], [B], [E], [F], [H], [I], [J],
[K], [L], [P], [Q], [R], [S], [T], [U], [V], [W], [Y], and [Z] shall not be modified.

(3) Measured/initiating variable analysis [A] shall be used for all types of process stream
composition and physical property analysis. The type of analyzer, and for stream
component analyzers the components of interest, shall be defined outside the tagging
bubble.

(a) “User’s choice” measured/initiating variables [C], [D], and [M] can be assigned to
identify conductivity, density, and moisture analysis, respectively, when it is the
user’s common practice.

(4) Measured/initiating variable analysis [A] shall not be used to identify vibration or other
types of mechanical or machinery analysis, which shall be identified by measured/initiating
variable vibration or mechanical analysis [V].

(5) “User’s choice” letters [C], [D], [M], [N], and [O] that cover unlisted repetitive meanings that
can have one meaning as a measured or initiating variable and another as a succeeding
letter shall be defined only once. For example, [N] can be defined as “modulus of elasticity”
as a measured/initiating variable or “oscilloscope” as a readout/passive function.

(6) Measured/initiating variable multivariable [U] identifies an instrument or loop that requires
multiple points of measurement or other inputs to generate single or multiple outputs, such
as a controller that uses multiple pressure and temperature measurements to regulate the
switching of multiple on-off valves.

(7) Measured/initiating variable vibration or mechanical analysis [V] is intended to perform the
function in machinery monitoring that measured/initiating variable analysis [A] performs in
process monitoring, and except for vibration, it is expected that the variable of interest will
be defined outside the tagging bubble.

(8) First letter [X] default use is for on/off, for example on/off valve [XV]. Alternately, first letter
or succeeding letter for unclassified devices or functions [X] for nonrepetitive meanings
that are used only once or to a limited extent can have any number of meanings that shall
be defined outside the tagging bubble or by a note in the document. For example, [XR -2]
can be a stress recorder, and [XX-4] can be a stress oscilloscope.

(9) Measured/initiating variable event, state, or presence [Y] is intended for use when control
or monitoring responses are not driven by time or time schedule —but are driven by events,
presence, or state.

(10) Measured/initiating variable and variable modifier combinations shall be selected according
to how the property being measured is modified or changed.

Copyright © 2024 ISA. All rights reserved.

 - 23 - ANSI/ISA-5.1-2024

(11) Variable modifier time or time schedule [K] in combination with a measured/initiating
variable signifies a time rate of change of the measured or initiating variable; [WK]
represents a rate-of-weight-change loop.

(12) Variable modifier safety [S] is technically not a measured variable but is used to identify
self-actuated emergency protective primary and final control elements only when used in
conjunction with measured/initiating variables flow [F], pressure [P], or temperature [T].
And because of the critical nature of such devices, [FS, PS, and TS] shall be considered
as measured/initiating variables in all loop identification number construction schemes:

(a) Flow safety valve [FSV] applies to valves intended to protect against an emergency
excess flow or loss of flow condition. Pressure safety valve [PSV] and temperature
safety valve [TSV] apply to valves intended to protect against emergency pressure
and temperature conditions. This applies regardless of whether the valve
construction or mode of operation places it in the category of safety valve, relief
valve, or safety relief valve.

(b) A self-actuated pressure valve that prevents operation of a fluid system at a higher -
than-desired pressure by bleeding fluid from the system is a backpressure control
valve [PCV], even if the valve is not intended to be used normally. However, this
valve is designated a pressure safety valve [PSV] if it protects against emergency
conditions hazardous to personnel and/or equipment that are not expected to arise
normally.

(c) Pressure rupture disc [PSE] and fusible link [TSE] apply to all sensors or primary
elements intended to protect against emergency pressure or temperature
conditions.

(d) [S] shall not be used to identify safety instrumented systems (SISs) and
components; see (28).

(13) The grammatical form of succeeding letter meanings shall be modified as required; for
example, “indicate” [I] can be read as “indicator” or “ indicating,” and “transmit” [T] can be
read as “transmitter” or “transmitting.”

(14) Readout/passive function glass, gauge, or viewing device [G] should be used instead of
readout/passive function indicate [I] for self-contained instruments or devices that provide
a local-only view of the process variable, such as level gauges, pressure gauges,
temperature gauges, and flow gauges/sight glasses.

(a) Also used to identify devices that provide an uncalibrated view of plant operations,
such as television monitors.

(15) Readout/passive function indicate [I] applies to the analog or digital readout of an actual
measurement or input signal to a standalone instrument or a shared control system’s video
display unit.

(a) In the case of a manual loading station, it should be used for the dial or setting
indication of the output signal being generated, [HIC] or [HIK].

(16) Readout/passive function scan [J] when used shall indicate a noncontinuous periodic
reading of two or more measured/initiating variables of the same or different kinds, such
as multipoint temperature and pressure recorders .

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 24 -

(17) Readout/passive function light [L] identifies devices or functions that are intended to
indicate normal operating status, such as motor on-off or actuator position, and is not
intended for alarm indication.

(18) Readout/passive function record [R] applies to any permanent or semi-permanent

electronic or paper media storage of information or data in an easily retrievable form.

(19) Readout/passive and output/active function multifunction [U] is used to:

(a) identify control loops that have more than the usual indicate/record and control
functions; and

(b) save space on drawings by not showing tangent bubbles for each function.

A note describing the multiple functions should be on the drawing if needed for clarity.

(20) Readout/passive function accessory [X] is intended to identify hardware and devices that
do not measure or control but are required for the proper operation of instrumentation.

(21) There are differences in meaning to be considered when selecting between output/active
functions for control [C], switch [S], valve, damper, or louver [V], and auxiliary device [Y] .

(a) Control [C] means an automatic device or function that receives an input signal
representing a measured/initiating variable and generates a variable output signal
that is used to modulate or switch a valve [V] or auxiliary device [Y] at a
predetermined setpoint for ordinary process control.

(b) Switch [S] means a device or function that connects, disconnects, or transfers one
or more air, electronic, electric, or hydraulic signals, that can be manually actuated
or automatically actuated directly by a measured or initiating variable, or indirectly
by a measured or initiating variable transmitter.

(c) Valve, damper, or louver [V] means a device that modulates, switches, or turns
on/off a process fluid stream after receiving an output signal generated by a
controller [C], switch [S], or auxiliary device [Y].

(d) Auxiliary device [Y] means an automatic device or function actuated by a controller
[C], transmitter [T], or switch [S] signal that connects, disconnects, transfers,
computes, and/or converts air, electronic, electric, or hydraulic signals or circuits.

(e) The succeeding letters CV shall not be used for anything other than a self-actuated
control valve.

(22) Output/active function control station [K] shall be used for:

(a) designating an operator-accessible control station used with an automatic controller

that does not have an integral operator-accessible auto-manual and/or control
mode switch; or

(b) split architecture or fieldbus control devices where the controller functions are
located remotely from the control station.

(23) Output/active function auxiliary devices and functions [Y] include, but are not limited to,
solenoid valves, relays, and computing and converting devices and functions .

Copyright © 2024 ISA. All rights reserved.

 - 25 - ANSI/ISA-5.1-2024

(24) Output/active function auxiliary devices [Y] for signal computing and converting when
shown in a diagram or drawing shall be defined outside their bubbles with an appropriate
symbol from Table 16 – Signal Processing Function Blocks and when written in text shall
include a description of the mathematical operation from Table 16.

(25) Function modifiers high [H], low [L], and middle or intermediate [M] when applied to
positions of valves and other open-close devices, are defined as follows :

(a) high [H], the valve is in or approaching the fully open position ; open [O] can be used
as an alternative;

(b) low [L], the valve is in or approaching the fully closed position; closed [C] can be
used as an alternative; and

(c) middle or intermediate [M], the valve is traveling or located in between the fully
open or closed position.

(26) Function modifier deviation [D] when combined with readout/passive function [A] (alarm)
or output/active function [S] (switch) indicates a measured variable has deviated from a
controller or other setpoint more than a predetermined amount.

(a) Function modifiers high [H] or low [L] shall be added if only a positive or negative
deviation, respectively, is of importance.

(27) Function modifiers high [H], low [L], and middle or intermediate [M] when applied to alarms
correspond to values of the measured variable, not to values of the alarm -actuating signal,
unless otherwise noted.

(a) A high-level alarm derived from a reverse-acting level transmitter signal is an LAH,
even though the alarm is actuated when the signal falls to a low value.

(b) The terms shall be used in combination as appropriate to indicate multiple levels of
actuation from the same measurement, for example high [H] and high -high [HH],
low [L] and low-low [LL], or high-low [HL].

NOTE “HL” is used for an alarm having two setpoints (limits). For example, FAHL is one alarm indicating the
flow is outside a normal operating range, either high or low, while FAH and FAL are two separate alarms , each
having its own setpoint.

(28) Variable modifier [Z] is technically not a measured variable but is used to identify the
components of SISs.

(a) [Z] shall not be used to identify the safety devices noted in (1 2).

5 Graphic symbols
5.1 General
5.1.1 The graphic symbols shown in the tables are generally drawn full size for use in full-size
sketches or drawings. Actual symbol full-size dimensions are provided in Clause 6.

5.1.2 The device and function symbols shown in Table 2 are based on the 7/16-inch or 11-mm
circle format. Refer to 6.1.

5.1.3 When selecting symbol sizing, consideration shall be given to the size and readability of
symbols and text if the drawing will be used in a reduced -size format.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 26 -

5.1.4 All symbols shall maintain the size ratios shown in the tables when reduced or increased
in size.

5.1.5 Illustrations in these tables can include representation of relative process piping or
equipment in addition to the symbol being defined. This representation is shown in grey (“ghosted”)
to communicate that it is not part of the symbol but illustrates how the symbol is applied in context
in diagrams.

5.1.6 Illustrations in these tables can include representation of instrumentation or components

mounted inside of a tank, vessel or other mechanical equipment by the use of dashed rather than
solid lines.

5.1.7 Symbols are shown in this document as they would be oriented in a diagram having flow
from left to right. For instances where flow is other than left to right, the symbol s that have a
process flow directional aspect, e.g., flow measurement instrumentation and self-contained
regulators, should be rotated appropriately.

5.2 Tables to be used for common applications

5.2.1 Instrumentation and control diagrams shall be constructed from the symbols shown in:

a) Tables 2 and 3 – Instrumentation devices and functions;

b) Tables 4, 5, 6, 7, and 8 – Measurement elements and transmitters;
c) Tables 9 and 10 – Lines, instrument to process or instrument to instrument ;
d) Tables 11, 12, 13, and 14 – Final control elements; and
e) Table 16 – Signal processing function blocks.
5.2.2 Functional diagrams shall be constructed from the symbols shown in:

a) Table 2 – Instrument device and function symbols;

b) Table 10 – Line symbols;
c) Table 15 – Functional diagramming symbols;
d) Table 16 – Signal processing function block symbols;
e) Table 17 – Binary logic symbols; and
f) For special symbols, reference ISA-TR5.1.01/ISA-TR77.40.01, Functional Diagram
Usage.
5.2.3 Binary logic diagrams shall be constructed from the symbols shown in:

a) Table 2 – Instrumentation devices and functions;

b) Table 10 – Line symbol; and
c) Table 17 – Binary logic symbols.
5.2.4 Electrical schematics shall be constructed from the symbols shown in:

a) Table 2 – Instrumentation devices and functions; and

b) Table 18 – Electrical schematic symbols.
5.2.5 Instrument loop diagrams shall be constructed from the symbols shown in:

Copyright © 2024 ISA. All rights reserved.

 - 27 - ANSI/ISA-5.1-2024

a) Tables 2 and 3 – Instrumentation devices and functions;

b) Tables 4, 5, 6, 7, and 8 – Measurement elements and transmitters;
c) Tables 9 and 10 – Lines, instrument to process or instrument to instrument ;
d) Tables 11, 12, 13, and 14 – Final control elements; and
e) Table 19 – Instrument loop diagrams.
5.2.6 Symbols can be developed to show devices and functions not covered by this standard or
to simplify the depiction of frequently used instrumentation. Such uses shall be fully detailed by
sketches or notes on the drawing legend and detail sheets.

5.2.7 If new or revised symbols are developed, they should be submitted to standards@isa.org
for consideration in the next revision of this standard.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 28 -

5.3 Instrumentation device and function symbols

5.3.1 This subclause provides symbols for instrumentation devices and functions in tabular form
with accompanying notes.

Table 2 – Instrumentation device and function symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.3.2.

Shared display,
Shared control (1)
High level
Basic control
Safety systems
No. process Standalone Location and accessibility (6)
instrumented and
control
system application (5)
system
or software
or
alternate
primary (4)
choice
choice
(3)
(2)

1 • Located in field.
• Not panel, cabinet, or console mounted.
• Visible at field location.
• Normally operator accessible.

• Located in or on front of central or main panel

2 or console.
• Visible on front of panel or on video display.
• Normally operator accessible at panel front or
console.

• Located in rear of central or main panel.

3
• Located in cabinet behind the panel.
• Not visible on front of panel or on video display.
• Not normally operator accessible at panel or
console.

• Located in or on front of secondary or local

4 panel or console.
• Visible on front of panel or on video display.
• Normally operator accessible at panel front or
console.

• Located in rear of secondary or local panel.

5
• Located in field cabinet.
• Not visible on front of panel or on video display.
• Not normally operator accessible at panel or
console.

Copyright © 2024 ISA. All rights reserved.

 - 29 - ANSI/ISA-5.1-2024

Table 3 – Instrumentation device or function symbols, miscellaneous

NOTE Numbers in parentheses refer to explanatory notes in 5.3.2.

No Symbol Description

• Signal processing function:

1 • Locate in upper right or left quadrant of Table 2 symbols
just outside the symbol.
• Attach to Table 2 symbols where affected signals are
connected.
• Insert signal processing symbol from Table 16.
• Expand symbol by 50% increments for larger function
symbols.

• Panel-mounted patchboard plug-in point.

2 C • Console matrix point.
12 • C-12 equals patchboard column and row, respectively, as
an example.

(7) (8) • Generic interlock logic function.

3 • Undefined interlock logic function.
I
• If a number is added, e.g., I-9, inside or immediately
outside of the diamond, it refers to a separate documented
description of the interlock function.

(7) (8) • “AND” interlock logic function.

4
AND

(7) (8) • “OR” interlock logic function.

5
OR

a) a) Standalone instruments
6 • Instruments or functions sharing a common housing.
• It is not mandatory to show a common housing.
• Notes shall be used to identify instruments in common
housings not using this symbol.

b) b) Shared display, shared control

• Functions within the same shared display, shared control
system (BPCS or SIS).
• Use applicable symbols from Table 2 “Shared display,
shared control” columns.
• This is an alternate to showing separate, unattached
symbols, perhaps to reduce signal line clutter on a drawing.
• Should not be used where the signal flow to/from symbols
can become unclear.
a) and b)
• Horizontal orientations of two symbols are shown.
Orientation can be vertical, and more than two adjoining
symbols can be used as needed.

• Pilot light.
7 • Circle shall be replaced with any symbol from the
standalone column in Table 2 if location and accessibility
need to be shown.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 30 -

Table 3 – Instrumentation device or function symbols, miscellaneous

NOTE Numbers in parentheses refer to explanatory notes in 5.3.2.

No Symbol Description
• User can choose to also indicate color of light adjacent to
the bubble.
(9) • Remote setpoint (SP) to a standalone controller (a) or a
8 shared display, shared control controller (b).
a)
• “SP” to be located wherever the corresponding signal
SP connects with the bubble.

b)

SP

• Alarm, hardware-based, implemented in a standalone

9 device or function.
_A_

(9) • Shared display, shared control alarms can be shown:

10 a) Individually, or
a)
_A_ b) Applicable alarm levels shown as notations outside the
symbol for the associated function (e.g. indicator,
controller). (Four alarm levels are shown in the example.)
b)
HH
H • Typical alarm abbreviations are:
L HH – high high DH – deviation from setpoint high
LL H – high DL – deviation from setpoint low
L – low D – deviation from setpoint, either direction
LL – low low

(9) • Notes regarding a standalone instrument (a) or a shared

11 display, shared control function (b) are typically placed in
a) the lower right quadrant surrounding the applicable bubble.
• Depending on the available space on the drawing, the note
can be moved to another location surrounding the bubble,
however, locations with other designated purpose should be
Note
avoided.
b) • A note can provide information about the corresponding
hardware or the function performed. For example, a bubble
representing a switch could have notes such as “Start ,”
“Stop,” “H-O-A” (Hand-Off-Auto), “FWD” (Forward), “REV”
(Reverse), “JOG.”
Note

5.3.2 Notes for Tables 2 and 3 – Instrumentation device and function symbols:

The following notes, indicated in Tables 2 and 3 by parentheses, are to be used as an aid in
understanding the entries in those tables.

(1) Devices and functions that are represented by these bubble symbols are:

Copyright © 2024 ISA. All rights reserved.

 - 31 - ANSI/ISA-5.1-2024

(a) used in shared display, shared control, configurable, microprocessor -based, and
data-linked instrumentation where the functions are accessible by the operator via
a shared display or monitor; and

(b) configured in control systems that include, but are not limited to, distributed control
systems (DCSs), programmable logic controllers (PLCs), personal computers
(PCs), and intelligent transmitters and valve positioners.

(2) The user shall select and document one of the following for use of these symbols:

(a) basic process control system (BPCS); or

(b) primary shared-display, shared-control system.

(3) The user shall select and document one of the following for use of these symbols:

(a) SIS; or

(b) alternate shared-display, shared-control system.

(4) Devices and functions represented by these bubble symbols are configured in higher level
control systems (HLCS), not part of the BPCS.

(5) Standalone devices or functions that are hardware-based and are either standalone or are
connected to other instruments, devices, or systems that include, but are not limited to,
transmitters, switches, relays, controllers, and control valves.

(6) Accessibility includes viewing, setpoint adjustment, operating mode changing, and any
other operator actions required to operate the instrumentation.

(7) Functions represented by these symbols are used for simple interlock logic.

(a) A description of the logic should be shown nearby or in the notes section of the
drawing or sketch if the intended logic is not clearly understandable.

(b) These symbols are not recommended for depicting complex DCS, PLC, or SIS
applications that require other than “AND” and “OR” signal gates.

(8) A logic number, letter, or number/letter combination identification shall be used if more
than one logic scheme is used on the project by:

(a) replacing [I], [A], and [O] with the logic identification; or

(b) appending the logic identification outside the symbol.

(9) Examples shown are for shared display, shared control BPCS/primary choice. This is also
applicable to SIS/alternate choice from Table 2.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 32 -

5.4 Measurement symbols

5.4.1 This subclause provides symbols for measurements in tabular form with accompanying
notes.

Table 4 – Measurement symbols: Primary elements and transmitters

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description
(1a) (2) • Generic primary element, bubble format.
1
• Notation (*) from Table 5 should be used to identify measurement type.
?E • Connect to process or other instruments by symbols from Tables 9 and 10.
• Insert in or on process flow line, vessel, or equipment.
(*)
• ? = Applicable first letter from Table 1.

(1a) (2) (3) • Transmitter with integral primary element, bubble format.
2
• Notation (*) from Table 5 should be used to identify measurement type.
?T
• Connect to process or other instruments by symbols from Tables 9 and 10.
(*) • Insert in or on process flow line, vessel, or equipment.
• ? = Applicable first letter from Table 1.

(1a) (2) (3) • Transmitter with close coupled primary element, bubble format.
3 ?T
• Notation (*) from Table 5 should be used to identify measurement type.
• Connecting line shall be equal to or less than 0.25 inches (6 millimeters).
• Connect to process or other instruments by symbols from Tables 9 and 10.

?E • Insert element in or on process flow line, vessel, or equipment.

• ? = Applicable first letter from Table 1.
(*)
(1a) (2) (3) • Transmitter with remote primary element, bubble format.
4 ?T
• Notation (*) from Table 5 should be used to identify measurement type.
• Connecting line shall be equal to or greater than 0.5 inches
?E (12 millimeters).
(*) • Connect to process or other instruments by symbols from Tables 9 and 10.
• Insert element in or on process flow line, vessel, or equipment.
• ? = Applicable first letter from Table 1.

(1b) (2) (3) • Transmitter with integral primary element inserted in or on process flow
5 line, vessel, or equipment, bubble/graphic format.
?T
• Insert primary element symbol from Table 6 in place of the square and #.
• Connect to other instruments by symbols from Table 10.
# • ? = Applicable first letter from Table 1.

(1b) (2) (3) • Transmitter with close-coupled primary element inserted in or on process
6 ?T flow line, vessel, or equipment, bubble/graphic format.
• Insert primary element symbol from Table 6 in place of the square and #.
• Connecting line shall be equal to or less than 0.25 inches (6 millimeters).
# • Connect to other instruments by symbols from Table 10.
• ? = Applicable first letter from Table 1.

Copyright © 2024 ISA. All rights reserved.

 - 33 - ANSI/ISA-5.1-2024

Table 4 – Measurement symbols: Primary elements and transmitters

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description
(1b) (2) (3) • Transmitter with remote primary element inserted in or on process flow line,
7 ?T vessel, or equipment, bubble/graphic format.
• Insert primary element symbol from Table 6 in place of the square and #.
• Connecting line can be any signal line from Table 10.
#
• Connecting line shall be equal to or greater than 0.5 inches
(12 millimeters).
• Connect to other instruments by symbols from Table 10.
• ? = Applicable first letter from Table 1.

Table 5 – Measurement symbols: Measurement notations

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2
Analysis
AIR = Excess air GC = Gas chromatograph MS = Mass spectrometer RI = Refractive index
CO = Carbon monoxide H2O = Water NIR = Near infrared TC = Thermal conductivity
CO2 = Carbon dioxide H2S = Hydrogen sulfide O2 = Oxygen TDL = Tunable diode laser
spectrometer
COL = Color HUM = Humidity OP = Opacity UV = Ultraviolet
COMB = Combustibles IR = Infrared ORP = Oxidation reduction VIS = Visible light
COND = Elec. conductivity LC = Liquid chromatograph pH = Hydrogen ion VISC = Viscosity
DEN = Density MOIST = Moisture REF = Refractometer
Flow
CFR = Constant flow regulator MAG = Magnetic OP-VC = Vena contracta taps TTS = Transit time sonic
CONE = Cone OP = Orifice plate Flow PD = Positive displacement TUR = Turbine
COR = Coriolis OP-CT = Corner taps PT = Pitot tube US = Ultrasonic
DOP = Doppler OP-CQ = Circle quadrant PV = Pitot venturi VENT = Venturi tube
DSON = Doppler sonic OP-E = Eccentric Flow SNR = Sonar VOR = Vortex shedding
FLN = Flow nozzle OP-FT = Flange taps SON = Sonic WDG = Wedge
FLT = Flow tube OP-MH = Multi-hole Flow TAR = Target
LAM = Laminar OP-P = Pipe taps THER = Thermal
Level
Flow
CAP = Capacitance GWR = Guided wave radar NUC = Nuclear US = Ultrasonic
d/p = Differential pressure LSR = Laser RAD = Radar
DI = Dielectric constant MAG = Magnetic RES
Flow = Resistance
DP = Differential pressure MS = Magnetostrictive SON = Sonic
Pressure
Flow
ABS = Absolute DRF = Draft P-V = Pressure-vacuum VAC = Vacuum
AVG = Average MAN = Manometer SG = Strain gauge
Flow
Temperature
BM = Bi-metallic RTD = Resistance temp detector TCK = Thermocouple type K TRAN = Transistor
IR = Infrared TC = Thermocouple TCT
Flow = Thermocouple type T
RAD = Radiation TCE = Thermocouple type E THRM = Thermistor
RP = Radiation pyrometer TCJ = Thermocouple type J Flow TMP = Thermopile
Miscellaneous
Burner, Combustion Position Quantity Speed
Flow
FR = Flame rod/flame ionization CAP = Capacitance PE = Photoelectric ACC = Acceleration
IGN = Igniter EC = Eddy current TOG
Flow = Toggle EC = Eddy current
IR = Infrared IND = Inductive PROX = Proximity
TV = Television LAS = Laser Radiation VEL = Velocity
UV = Ultraviolet MAG = Magnetic α = Alpha radiation Weight, Force
MECH = Mechanical βRadiation = Beta radiation LC = Load cell
OPT = Optical γ = Gamma radiation SG = Strain gauge
RAD = Radar nradiatio = Neutron radiation WS = Weigh scale
n

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 34 -

Table 6 – Measurement symbols: Primary elements

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description
Analysis Analysis Analysis

• Conductivity, moisture, etc.

1
• Single element sensing probe.

• pH, ORP, etc.

2
• Dual element sensing probe.

FO • Fiber optic (FO) sensing probe.

3

* • Ultraviolet (UV) flame detector.

4
Burner

• Television (TV) flame monitor.

• Infrared (IR) flame detector.
• Replace * with applicable two-letter type abbreviation.

•
Burner

Flame rod flame detector.

5

• Generic orifice plate.

Flow

6
• Restriction orifice.

• Orifice plate in quick-change fitting.

Flow

• Concentric circle orifice plate.

Flow

8
• Restriction orifice.

• Eccentric circle orifice plate.

Flow

• Circle quadrant orifice plate.

Flow

10

• Multi-hole orifice plate.

Flow

11

• Generic flow (venturi tube, flow nozzle, or flow tube).

Flow

12
(*) • Notation from Table 5 required at (*) if used for more than one type.

• Venturi tube.
Flow

13

• Flow nozzle.
Flow

14

Copyright © 2024 ISA. All rights reserved.

 - 35 - ANSI/ISA-5.1-2024

Table 6 – Measurement symbols: Primary elements

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description

• Flow tube.
Flow

15

• Integral orifice plate.

Flow

16

• Standard pitot tube.

Flow

17

• Averaging pitot tube.

Flow

18
• One or two axis. Optional to add “2 axis” note to symbol if applicable.

• Turbine flowmeter.
19
•
Flow

Propeller flowmeter.
• User can add a note next to the symbol to indicate an insertion or
probe flowmeter.

(3) • Vortex shedding flowmeter.

Flow

20 a) b) • User can add a note next to the symbol to indicate an insertion or

probe flowmeter.

• Target flowmeter.
Flow

21

(3) • Magnetic flowmeter.

Flow

22 a) M b)

(3) • Thermal mass flowmeter.

Flow

23 a) ΔT b)

(3) • Positive displacement flowmeter, including oval gear, progressive

24 cavity, nutating disc, rotary lobes, etc.
Flow

a) b)
• Symbols shown are alternates but a) is preferred to avoid confusion
with symbol 19.

• Cone meter.
Flow

25
• Annular orifice meter.

• Wedge meter.
Flow

26

• Coriolis flowmeter.
Flow

27

• Sonic flowmeter.
Flow

28
• Ultrasonic flowmeter.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 36 -

Table 6 – Measurement symbols: Primary elements

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description

(3) • Variable area flowmeter (rotameter).

29
• Symbol a) is preferred for instrumentation and control diagrams.
Flow

a)

b)

• Open channel weir plate.

Flow

30

• Open channel flume.

Flow

31

• Displacer internally mounted in vessel.

32
Level

• Dashed line represents mounted inside vessel.

• Ball float internally mounted in vessel.

33
Level

• Can be shown installed through top of vessel.

• Dashed line represents mounted inside vessel.

• Nuclear radiation, including source (square on left) and detector

34 (rectangle on right), multipoint or continuous.
Level

• Size of source and detector symbols should be proportional to size of

respective tank/vessel on the drawing.

• Dip tube or other primary element and stilling well.

35
• Can be shown installed through side of vessel.
Level

• Can be shown installed without stilling well.

• Dashed line represents mounted inside vessel.

• Float with guide wires.

36
• Location of readout should be noted, at grade, at top, or accessible
Level

from a ladder.
• Guide wires can be omitted.
• Dashed line represents mounted inside vessel.

• Generic insertion (vertical or horizontal) probe.

37
• Can be used for capacitance, inductance, or any other level
Level

measurement not having a specific symbol elsewhere in this table.

• User can add notes to drawing to communicate type or specifics .
• Dashed line represents mounted inside vessel.

Copyright © 2024 ISA. All rights reserved.

 - 37 - ANSI/ISA-5.1-2024

Table 6 – Measurement symbols: Primary elements

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description

• Radar – free space.

38
Level

• Radar – guided wave.

39
Level

• Dashed line represents mounted inside vessel.

• Strain gauge or other electronic type sensor.

PE
40
•
Pressure

Notation (*) from Table 5 should be used to identify type of element.

(*) • Connection symbols 6, 7, 8, or 9 in Table 9 are used if connection type
is to be shown.
• Bubble can be omitted if connected to another instrument.

TE • Generic element without thermowell.

Temperature

41
• Notation (*) should be used to identify type of element; see Table 5.
(*) • Connection symbols 6, 7, 8, or 9 in Table 9 are used if connection type
is to be shown.
• Bubble can be omitted if connected to another instrument.

Table 7 – Measurement symbols: Secondary instruments

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description

• Sight glass.
FG
Flow

FO • Generic flow restriction.

2
• Single stage orifice plate as shown.
Flow

• Note required for multistage or capillary tube types.

• Restriction orifice hole drilled in valve plug or other closure

3 FO components.
Flow

• Tag number shall be omitted if valve is otherwise identified.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 38 -

Table 7 – Measurement symbols: Secondary instruments

NOTE Numbers in parentheses refer to explanatory notes in 5.4.2.

No Symbol Description

• Gauge integrally mounted on vessel.

4 LG • Sight glass.
Level

• Gauge glass externally mounted on vessel or standpipe.

5
LG • Multiple gauges can be shown as one bubble or one bubble for each
Level

section.
• Use process connection 6, 7, 8, or 9 in Table 9 if connection type is to
be shown. (None shown in this example)

• Pressure gauge.
Pressure

6 PG
• Use process connection 6, 7, 8, or 9 in Table 9 if connection type is to
be shown. (None shown in this example)
Temperature

• Thermometer.
7 TG
• Use process connection 6, 7, 8, or 9 in Table 9 if connection type is to
be shown. (None shown in this example)

Table 8 – Measurement symbols: Auxiliary and accessory devices

NOTE Numbers in parentheses refer to explanatory notes in 5. 4.2.

No Symbol Description

• Sample insert probe, flanged.

AW
Analysis

1
• Sample well, flanged.
• Use connection 7, 8, or 9 in Table 9 if flange is not used.

AX • Sample conditioner or other analysis accessory, flanged.

Analysis

2
• Represents single or multiple devices.
• Use connection 7, 8, or 9 in Table 9 if flange is not used.

FX • Flow straightening vanes.

3
•
Flow

Flow conditioning element.

• Instrument purge or flushing fluid.

4
•
Flow

P Instrument purge or flushing device or devices.

• Show assembly details on drawing legend sheet.

Copyright © 2024 ISA. All rights reserved.

 - 39 - ANSI/ISA-5.1-2024

Table 8 – Measurement symbols: Auxiliary and accessory devices

NOTE Numbers in parentheses refer to explanatory notes in 5. 4.2.

No Symbol Description

• Diaphragm pressure seal, flanged, threaded, socket welded, or

5 welded.
Pressure

• Diaphragm chemical seal, flanged, threaded, socket welded, or

welded.
• Use connection 6, 7, 8, or 9 in Table 9 if connection type is to be
shown.

• Diaphragm pressure seal, welded.

Pressure

6
• Diaphragm chemical seal, welded.

• Thermowell, flanged.
Temperature

7 TW
• Test well, flanged.
• Bubble can be omitted if connected to another instrument.
• Use connection 7, 8, or 9 in Table 9 if flange is not used.

5.4.2 Notes for Tables 4, 5, 6, 7, and 8 – Measurement symbols:

The following notes, indicated in Tables 4 through 8 by parentheses, are to be used as an aid in
understanding the entries in those tables.

(1) Measurements are depicted by:

(a) bubbles, as shown in Table 1, for:

1) generic primary elements;
2) primary elements that do not have a graphic symbol in Table 6; or
3) users who elect not to use graphic symbols from Table 6.
(b) graphic symbols from Table 6. User can also add bubble with measurement tag.

(2) These symbols shall be used for process or equipment measurements if:

(a) a graphic symbol does not exist; or

(b) the user does not use graphic symbols.

(3) User engineering and design standards, practices, and/or guidelines shall document which
choices are selected.

(4) Flow direction for flow-related symbols shown in Tables 6, 7, and 8 is from left to right.
Unsymmetrical symbols should be mirrored when showing flow from right to left on a
drawing. When illustrating vertical flow, symbol should be rotated 90 degrees in the
appropriate direction and mirrored if necessary.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 40 -

5.5 Line symbols

5.5.1 This subclause provides symbols for lines in tabular form with accompanying notes.

Table 9 – Line symbols: Instrument to process and equipment connections

NOTE Numbers in parentheses refer to explanatory notes in 5.5. 2.

No Symbol Application

• Instrument connections to process and equipment.

• Process impulse lines.
1
• Analyzer sample lines.

• Heat [cool] traced impulse or sample line from process.

• Type of tracing indicated by: [ET] electrical, [ST] steam, [CW] chilled
2 (ST) water, etc.

• Generic instrument connection to process line.

• Generic instrument connection to equipment.
3

• Heat [cool] traced generic instrument impulse line.

• Process line or equipment may or may not be traced.
4

• Heat [cool] traced instrument.

• Instrument impulse line may or may not be traced.
5

• Flanged instrument connection to process line.

• Flanged instrument connection to equipment.
6

• Threaded instrument connection to process line.

• Threaded instrument connection to equipment.
7

• Socket welded instrument connection to process line.

• Socket welded instrument connection to equipment.
8

• Welded instrument connection to process line.

• Welded instrument connection to equipment.
9

Copyright © 2024 ISA. All rights reserved.

 - 41 - ANSI/ISA-5.1-2024

Table 10 – Line symbols: Instrument-to-instrument connections

NOTE Numbers in parentheses refer to explanatory notes in 5.5. 2.

No Symbol Application
(1) • Instrument pneumatic power supply.
IA
1 • IA can be replaced by PA [plant air], NS [nitrogen], or GS [any gas supply].
• Indicate supply pressure as required, e.g., PA -70 kPa, NS-150 psig, etc.

(1) • Instrument electric power supply.

ES
2 • Indicate voltage and type as required, e.g. , ES-220 VAC.
• ES can be replaced by 24 VDC, 120 VAC, etc.

(1) • Instrument hydraulic power supply.

3 HS • Indicate pressure as required, e.g., HS-70 psig.

(2) • Undefined signal.

4 • Use for PFDs.
• Use for discussions or diagrams where type of signal is not of concern.

(2) • Pneumatic signal: analog or discrete.

5

(2) • Electronic or electrical analog or discrete signal.

6
• Functional diagram binary signal.

(2) • Functional diagram analog signal.

7 • Electrical schematic ladder diagram signal and power rails.
• Binary logic diagram binary signal.

(2) • Hydraulic signal.

8

(2) • Filled thermal element capillary tube.

9
• Filled sensing line between pressure seal and instrument.

(2) • Guided electromagnetic signal.

10 • Guided sonic signal.
• Fiber optic cable.
(2)(3) • Unguided electromagnetic signals, light, radiation, radio, sound,
a) wireless, etc.
11 . • Wireless instrumentation signal.
b)
• Wireless communication link.

(2)(4) • Communication link and system bus, between devices and functions of a
12 shared display, shared control system.
• DCS, PLC, or PC communication link and system bus.

(2)(5) • Communication link or bus connecting two or more independent

13 microprocessor-based systems.
• DCS-to-DCS, DCS-to-PLC, PLC-to-PC, DCS-to-fieldbus, etc., connections.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 42 -

Table 10 – Line symbols: Instrument-to-instrument connections

NOTE Numbers in parentheses refer to explanatory notes in 5.5. 2.

No Symbol Application
(2)(6) • Communication link and system bus, between devices and functions of a
14 fieldbus system.
• Link from and to “intelligent” devices.

(2)(7) • Communication signal between a device and a remote calibration

15 adjustment device or system.
• Signal from and to “smart” devices.

• Mechanical link or connection.

16

(3) • Drawing-to-drawing signal connector, signal flow from left to right (a and c)
(#) or right to left (b and d).
a)
(##) • (#) = Tag number of the instrument on the referenced drawing sending or
receiving the signal.
(#) • (##) = Drawing or sheet number containing the referenced receiving or
b) sending instrument.
(##)
17 • c) and d) are alternates for a) and b), respectively.

c)
(#)
(##)

(#)
d)
(##)

(3) • Signal input to binary logic diagram.

a)
• Signal input to functional diagram.
(*)
18 • (*) = Input description, source, or tag number.
• Symbol b is an alternate to symbol a.
b)
(*)

(3) • Signal output from binary logic diagram.

a) (*) • Signal output from functional diagram
(*)
19 • (*) = Output description, destination, or tag number.
b)
• Symbol b is an alternate to symbol a.
(*)

5.5.2 Notes for Tables 9 and 10 – Line symbols:

The following notes, indicated in Tables 9 and 10 by parentheses, are to be used as an aid in
understanding the entries in those tables.

(1) Power supplies shall be shown when:

(a) different from those normally used, e.g., 120 VDC when normal is 24 VDC;

Copyright © 2024 ISA. All rights reserved.

 - 43 - ANSI/ISA-5.1-2024

(b) a device requires an independent power supply ; or

(c) affected by controller or switch actions.

NOTE Showing instrument air supply to a positioner or actuator is optional.

(2) Arrows can be used if needed to clarify direction of signal flow, primary information, or
control/action.

(3) Users’ engineering and design standards, practices , and/or guidelines shall document
which symbol has been selected.

(4) The line symbol connects devices and functions that are integral parts of dedicated
systems, such as DCSs, PLCs, PCs, and other microprocessor-based systems over a
dedicated communication link.

(5) The line symbol connects independent microprocessor-based systems to each other over
a dedicated communications link, using, but not limited to, the RS232 protocol.

(6) The line symbol connects “intelligent” devices, such as microprocessor-based transmitters
and control valve positioners that contain control functionality, to other such devices and
to the instrumentation system, using, but not limited to, fieldbus protocols.

(7) The line symbol connects “smart” devices, such as transmitters, to instrumentation system
input signal terminals and provides a superimposed digital communication signal that is
used for instrument diagnostics and calibration , using, but not limited to, HART protocol.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 44 -

5.6 Final control element symbols

5.6.1 This subclause provides symbols for final control elements in tabular form with
accompanying notes.

Table 11 – Final control element symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description
(1) (2) • Generic two-way valve.
1 a) • Straight globe valve.
b) • Gate valve.

(2) (3) • Generic two-way angle valve.

2
• Angle globe valve.
• Safety angle valve.

(2) • Generic three-way valve.

3
• Three-way globe valve.
• Arrow should be drawn to indicate failure or unactuated flow path for the
application.

(2) • Generic four-way valve.

4
• Four-way four-ported plug or ball valve.
• Arrows should be drawn to indicate failure or unactuated flow paths for the
application.

(2) • Butterfly valve.

5

(2) • Ball valve.

6

(2) • Plug valve.

7

(2) • Eccentric rotary disc valve.

8

(1) (2) • Diaphragm valve.

9 a)
b)

(2) • Pinch valve.

10

(2) • Bellows sealed valve.

11

(2) • Generic damper.

12
• Generic louver.

Copyright © 2024 ISA. All rights reserved.

 - 45 - ANSI/ISA-5.1-2024

Table 11 – Final control element symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description
(2) • Parallel blade damper.
13
• Parallel blade louver.

(2) • Opposed blade damper.

14
• Opposed blade louver.

(6) • Two-way on-off solenoid valve body.

15

(6) • Angle on-off solenoid valve body.

16

(6) • Three-way on-off solenoid valve body.

17
• Arrow should be drawn to indicate de-energized flow path for the
application.

(6) • Four-way plug or ball on-off solenoid valve body.

18
• Arrows should be drawn to indicate de-energized flow paths for the
application.

(6) • Five-ported on-off solenoid valve body.

19
• Arrows should be drawn to indicate de-energized flow paths for the
application.

(6) • Six-ported on-off solenoid valve body.

20
• Arrows should be drawn to indicate de-energized flow paths for the
application.

(4) (5) • Electric motor.

21

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 46 -

Table 12 – Final control element actuator symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description
(2) • Generic actuator.
1
• Spring-diaphragm actuator.

(2) • Spring-diaphragm actuator with positioner.

2
• Use of crosshatch on the line between positioner and actuator is optional.

(2) • Pressure-balanced diaphragm actuator.

3

(2) • Linear piston actuator.

4
• Single acting spring opposed.
• Double acting.

(2) • Linear piston actuator with positioner.

5
• Use of crosshatch on the line between positioner and actuator is optional.

(2) • Rotary piston actuator.

6
• Can be single acting spring opposed or double acting.

(2) • Rotary piston actuator with positioner.

7

(2) • Bellows spring opposed actuator.

8

(2) • Motor operated actuator.

9
M • Electric, pneumatic, or hydraulic.
• Linear or rotary action.

(2) • Solenoid actuator.

10 S
• Solenoid actuator for process on-off valve.

(2) • Actuator with side-mounted handwheel.

11
• User can add a note to clarify handwheel type ( e.g., clutch, lost motion).

Copyright © 2024 ISA. All rights reserved.

 - 47 - ANSI/ISA-5.1-2024

Table 12 – Final control element actuator symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description
(2) • Actuator with top-mounted handwheel.
12

(2) • Manual actuator.

13
• Hand actuator.

(2) • Electrohydraulic linear or rotary actuator.

E
14 H

(2) • Actuator with manual actuated partial stroke test device.

15

(2) • Actuator with remote actuated partial stroke test device.

16 S

(2) • Generic on/off actuator.

17

(6) • Automatic reset on-off solenoid actuator.

18 S • Non-latching on-off solenoid actuator.

(6) • Manual or remote reset on-off solenoid actuator.

19 S • Latching on-off solenoid actuator.
R

(6) • Manual and remote reset on-off solenoid actuator.

20 S
R R • Latching on-off solenoid actuator.

(3) • Spring or weight actuated relief or safety valve actuator.

21

(3) • Pilot actuated relief or safety valve actuator.

P
22
• Pilot pressure sensing line deleted if sensing is internal.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 48 -

Table 12 – Final control element actuator symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description
(4) • Permanent magnet variable speed coupling.
23

(4) • Variable speed drive.

24 VSD
• Alternate – ASD – adjustable speed drive.

Copyright © 2024 ISA. All rights reserved.

 - 49 - ANSI/ISA-5.1-2024

Table 13 – Self-actuated final control element symbol

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description
(7) • Automatic flow regulator.
1
XXX • XXX = FCV without indicator.
• XXX = FICV with integral indicator.

(7) • Constant flow regulator.

2
FICV

(7) • Level regulator.

3
• Ball float and mechanical linkage.
TANK
• Dashed line represents mounted inside vessel.

(7) • Backpressure regulator.

4
• Internal pressure tap.

(7) • Backpressure regulator.

5
• External pressure tap.

(7) • Pressure-reducing regulator.

6
• Internal pressure tap.

(7) • Pressure-reducing regulator.

7
• External pressure tap.

(7) • Differential pressure regulator.

8
• External pressure taps.

(7) • Differential pressure regulator.

9
• Internal pressure taps.

(7) • Pressure-reducing regulator with integral outlet pressure relief and

10 pressure gauge.
PG

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 50 -

Table 13 – Self-actuated final control element symbol

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Symbol Description

• Generic pressure safety valve.

11
• Pressure relief valve.

• Generic vacuum safety valve.

12
• Vacuum relief valve.

• Generic pressure – vacuum relief valve.

13
• Tank pressure – vacuum relief valve.

• Pressure safety element.

14
• Pressure rupture disk.
• Pressure relief.

• Pressure safety element.

15
• Vacuum rupture disk.
• Vacuum relief.

(7) • Temperature regulator.

16
• Filled thermal system.

• Thermal safety element.

17
• Fusible plug or disk.
TANK TSE

• Generic moisture trap.

18
• Steam trap.
T
• Note required for other trap types.

• Moisture trap with equalizing line.

19
TANK

Copyright © 2024 ISA. All rights reserved.

 - 51 - ANSI/ISA-5.1-2024

Table 14 – Control valve failure and de-energized position indications

NOTE Numbers in parentheses refer to explanatory notes in 5.6. 2.

No Method A (1) Method B (1) Definition

• Fail to open position.

1

FO

• Fail to closed position.

2

FC

• Fail locked in last position.

3

FL

• Fail at last position.

4
• Drift open.

FL/DO

• Fail at last position.

5
• Drift closed.

FL/DC

5.6.2 Notes for Tables 11, 12, 13, and 14 – Final control element symbols:

The following notes, indicated in Tables 11 through 14 by parentheses, are to be used as an aid
in understanding the entries in those tables.

(1) Users’ engineering and design standards, practices, and/or guidelines shall document
which symbols have been selected.

(2) Element symbols 1 through 14, when combined with actuator symbols 1 through 17,
represent process control valves.

(3) Element symbol 2, when combined with actuator symbols 21 and 22, represent pressure
safety valves.

(4) Element symbol 21, when combined with actuator symbols 23 or 24, represents a variable
speed control unit.

(5) Element symbol 21 represents a motor that operates equipment to manipulate or control a
process variable.

(6) Actuator symbols 18, 19, and 20, when combined with element symbols 15 through 20,
represent on-off solenoid valves (not for process manipulated variable).

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 52 -

(7) Flow direction for symbols shown in Table 13 is from left to right. Unsymmetrical symbols
should be mirrored when showing flow from right to left on a drawing. When illustrating
vertical flow, symbol should be rotated 90 degrees in the appropriate direction and mirrored
if necessary.

5.7 Functional diagramming symbols

5.7.1 This subclause provides symbols for functional diagramming in tabular form with
accompanying notes.

Table 15 – Functional diagramming symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.7. 2.

No Symbol (1) (2) Description

• Measuring, input, or readout device.

1
• [*] = Tag number.
[*]
• Symbols from Table 4 can be used.

(3) • Automatic single-mode controller.

2
 • The top box is the signal processing symbol 3 (difference) from Table 16.
• Insert signal processing symbol 8 (proportional), 10 (integral) or 11
(*) (derivative) from Table 16 at (*).

(3) • Automatic two-mode controller.

3
 • The top box is the signal processing symbol 3 (difference) from Table 16.
• Insert two of three signal processing symbols , 8 (proportional), 10
(*) (*) (integral), and 11 (derivative), from Table 16 at (*).

(3) • Automatic three-mode controller.

4
 • The top box is the signal processing symbol 3 (difference) from Table 16.
• Insert signal processing symbol 8 (proportional) in left box, symbol 10
(*) (*) (*) (integral) in center box, and symbol 11 (derivative) in right box from
Table 16 at (*).

• Automatic signal processor.

5
• Insert signal processing symbol from Table 16 at (*).
(*)

• Manual signal processor.

6
• Insert signal processing symbol 26 (analog signal generator) or symbol 28
(*) (signal transfer) from Table 16 at (*).

(3) • The output is the manual transfer to or from a manually adjusted signal
(auto-manual station).
7
(*) (*) • For (*), insert signal processing symbol 26 (analog signal generator) and
symbol 28 (signal transfer) from Table 16 in separate manual signal
processor symbol.

Copyright © 2024 ISA. All rights reserved.

 - 53 - ANSI/ISA-5.1-2024

Table 15 – Functional diagramming symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.7. 2.

(3) • The output is the manual transfer to or from a manually adjusted signal
8 with an operator-adjustable setpoint or bias (auto-manual station).
(*) (*) (*) • For (*), insert two signal processing symbols, symbol 26 (analog signal
generator) and one signal processing symbol, symbol 28 (signal transfer)
from Table 16 in separate manual signal processor symbols.

(3) • The output is the manual transfer to or from a manually adjusted signal
9 with a manual transfer of inner-loop setpoint between an operator-
adjustable setpoint or automatic outer-loop setpoint demand (cascade-
(*) (*) (*) (*) automatic control station).
• For (*), insert two signal processing symbols, symbol 26 (analog signal
generator), and two signal processing symbols, symbol 28 (signal
transfer), from Table 16 in separate and alternate manual signal processor
symbols.

• Final control element.

10
• Control valve.
(*) • Insert signal processing symbol 12 (unspecified function) or symbol 14
(conversion) from Table 16 at (*).

• Final control element with positioner.

11
• Control valve with positioner.
(*) • Insert signal processing symbol 12 (unspecified function) or symbol 14
(conversion) from Table 16 at (*).

5.7.2 Notes for Table 15 – Functional diagramming symbols:

The following notes, indicated in Table 15 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) Signal flow is assumed to be from top-to-bottom or from left-to-right.

(2) Symbols are shown in a vertical diagram format.

(3) Symbols shall be rotated 90 degrees counterclockwise in a horizontal diagram format.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 54 -

5.8 Signal processing function block symbols

5.8.1 This subclause provides symbols for signal processing function blocks in tabular form with
accompanying notes.

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

Summation M = X1 + X2  + Xn • Output equals algebraic sum

1 of inputs.

X M

Σ Xn
X2
X1

t t

Average M = X1 + X2 + Xn /n • Output equals algebraic sum

2 of inputs divided by number
of inputs.

X M
Xn

Σ/n X2
X1

t t

Difference M = X1 − X2 • Output equals algebraic

3 difference of two inputs.

X M
X1
∆
X2

t t

Copyright © 2024 ISA. All rights reserved.

 - 55 - ANSI/ISA-5.1-2024

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

Multiplication M = X1 x X2 • Output equals product of two

4 inputs.

X X1 M

X2
X

t1 t t1 t

Division M = X1  X2 • Output equals quotient of two

5 inputs.

X X1 M

X2


t1 t t1 t

n
Exponential M=X • Output equals nth power of
6 input.

X M

Xn

t t

Root extraction M = nX • Output equals nth root of

7 input.
• If “n” omitted, square root is
assumed.
X M

n
√‾

t t

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 56 -

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

Proportion M = KX or M = PX • Output proportional to input.

8
(3) • Replace “K” or “P” with “1:1”
for volume boosters.
a) K b) P • Replace “K” or “P” with “2:1,”
X M “3:1,” etc., for integer gains.

t1 t t1 t

Reverse proportion M = – KX or M = – PX • Output inversely proportional

9 to input.
(3)
• Replace “–K” or “–P” with
a) -K b) -P t1 t “–1:1” for volume boosters.
X • Replace “–K” or “–P” with
“–2:1,” “–3:1,” etc., for integer
gains.

M
t1 t

Integral M = (1/TI)∫Xdt • Output varies with magnitude

10 and time duration of input.
(3)
• Output proportional to time
a) ∫ b) I integral of input.
X M • T I = integral time constant.

t1 t2 t t1 t2 t

Derivative M = TD (dx/dt) • Output proportional to time

11 rate of change of input.
(3)
• T D = derivative time constant.
a) d/dt b) D
X M

∫ t1 t t1 t

Copyright © 2024 ISA. All rights reserved.

 - 57 - ANSI/ISA-5.1-2024

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

Unspecified function M = (x) • Output is a nonlinear or

12 unspecified function of the
input.
• Function defined in note or
X M other text.

(x)

t t

Time function M = X(t) • Output equals a nonlinear or

13 unspecified time function
times the input.
• Output is a nonlinear or
X M unspecified time function.
• Function defined in note or
other text.
(t)

t1 t t1 t

Conversion I = P, P = I, etc. • Output signal type different

14 from that of input signal.
• Input signal is on the left, and
output signal is on the right.
X M • Substitute any of the
following signal types for “P”
I/P or “I”:
A = Analog H = Hydraulic
B = Binary I = Current
D = Digital O= Electromagnetic
t t E = Voltage P = Pneumatic
F = Frequency R = Resistance

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 58 -

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

M = X1 for X1 >X2 • Output equals greater of two

High signal select
15 M = X2 for X1 ≤ X2 or more inputs.

X M
X1

X2

t1 t t1 t

M = X1 for X2 >X1 >X3 or X3 >X1 >X2 • Output equals middle value of

16 Middle signal select M = X2 for X1 >X2 >X3 or X3 >X2 >X1 three or more inputs.
M = X3 for X1 >X3 >X2 or X2 >X3 >X1

X X2 M

X1
M

X3

t t

M = X1 for X1 ≤ X2 • Output equals lesser of two

Low signal select
17 M = X2 for X1 ≥ X2 or more inputs.

X M
X1


X2

t1 t t1 t

M = X for X ≤ H • Output equals the lower of

High limit
18 M = H for X ≥ H the input or high limit values.

X M

 H

t1 t t1 t

Copyright © 2024 ISA. All rights reserved.

 - 59 - ANSI/ISA-5.1-2024

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

M = X for X ≥ L • Output equals the higher of

Low limit
19 M = L for X ≤ L the input or low limit values.

X M

 L

t1 t t1 t

M = X1 + b • Output equal to input plus an

Positive bias
20 M = [−]X2 + b arbitrary value.

X X2 X1 M

+
b

t1 t2 t t1 t2 t

M = X1 – b • Output equal to input minus

Negative bias
21 M = [-]X2 – b an arbitrary value.

X2 X1
X M

−
b

t1 t2 t t1 t2 t

dM/dt = dX/dt for dX/dt ≤ H, M = X • Output equals input as long

Velocity limiter
22 dM/dt = H for dX/dt ≥ H, M ≠ X as the input rate of change
(3) does not exceed the limit


value that establishes the
a) dX/dt>H dM/dt=H
b) X M output rate of change until
the output again equals the
input.

t1 t2,3 t t1 t2 t3 t

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 60 -

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

(State 1) M = 0 @ X < H • Output state is dependent on

High signal monitor
23 (State 2) M = 1 @ X ≥ H value of input.
• Output changes state when
input is equal to or higher
than an arbitrary high limit.
X M
H
H State State

t t1 t
t1

(State 1) M = 1 @ X ≤ L • Output state is dependent on

Low signal monitor
24 (State 2) M = 0 @ X > L value of input.
• Output changes state when
input is equal to or lower than
X M an arbitrary low limit.

State State
L

t1 t t1 t

(State 1) M = 1 @ X ≤ L • Output states are dependent

High/low signal
25 (State 2) M = 0 @ L < X < H on value of input.
monitor
(State 3) M = 1 @ X ≥ H
• Output changes state when
input is equal to or lower than
an arbitrary low limit or equal
X M to or higher than an arbitrary
H high limit.
State State State
HL
L

t1 t2 t t1 t2 t

Analog signal • Output equals an analog

26 No equation signal:
generator

Copyright © 2024 ISA. All rights reserved.

 - 61 - ANSI/ISA-5.1-2024

Table 16 – Signal processing function block symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.8. 2.

Function Equation
No Definition
Symbol (1) (2) Graph

A a. When used with automatic

signal processor, the analog
signal is not adjustable by
the operator.
No graph
b. When used with manual
signal processor, the analog
signal is adjustable by the
operator.

Binary signal No equation • Output equals an on-off

27 generator binary signal:

a. When used with automatic

signal processor, the binary
signal is not adjustable by
operator.
B No graph
b. When used with manual
signal processor, the binary
signal is adjustable by the
operator.

If X3 = 0, then M = X1 • The figure on the left shows

Signal transfer
28 If X3 = 1, then M = X2 inputs X 1, X 2, and X 3 and the
figure on the right shows the
X X1 M output (M).
• Output equals input that is
X2 X3 = 1 selected by transfer.
T
X3 = 0 • Transfer actuated by external
X3 signal X 3 .

t1 t t1 t
Analog signal transfer

5.8.2 Notes for Table 5.8 – Signal processing function block symbols:

The following notes, indicated in Table 5.8 by parentheses, are to be used as an aid in
understanding the entries in that table.

(1) Symbols are used with symbol 1 from Table 3 or with symbol 5 from Table 15.

(2) When symbols are used with symbol 5 from Table 15, their size can be adjusted larger to
fit within the available area.

(3) Users’ engineering and design standards, practices , and/or guidelines shall document
which symbol has been selected.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 62 -

5.9 Binary logic symbols

5.9.1 This subclause provides symbols for binary logic in tabular form with accompanying notes.

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

AND gate • Output true only if all inputs are true.

1 • Alternate symbol. (2) (3)
A
B A A
C N O B
D C A O
X
X

A B C X O
1 0 0 0 0 0
2 1 0 0 0 0 1
3 0 1 0 0 0 A 0
4 0 0 1 0 0
5 0 0 0 1 0 B
6 1 1 0 0 0
7 1 0 1 0 0 C
8 1 0 0 1 0
9 0 1 1 0 0 X
10 0 1 0 1 0
11 0 0 1 1 0
12 1 1 1 0 0 O
13 1 1 0 1 0
14 1 0 1 1 0 t
15 0 1 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
16 1 1 1 1 1

OR gate • Output true if any input is true.

2 • Alternate symbol. (2) (3)

A
A
B
C OR B
O C O O
X
X

Copyright © 2024 ISA. All rights reserved.

 - 63 - ANSI/ISA-5.1-2024

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

A B C X O
1 0 0 0 0 0
2 1 0 0 0 1 1
3 0 1 0 0 1 A 0
4 0 0 1 0 1
5 0 0 0 1 1 B
6 1 1 0 0 1
C
7 1 0 1 0 1
8 1 0 0 1 1
9 0 1 1 0 1 X
10 0 1 0 1 1
11 0 0 1 1 1
O
12 1 1 1 0 1
13 1 1 0 1 1 t
14 1 0 1 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
15 0 1 1 1 1
16 1 1 1 1 1

NAND gate • Output true if any input or combination of inputs is false.

3 • Output false if all inputs are true.

A
N
B A
C O
N
D
X

A B C X O
1 0 0 0 0 1
2 1 0 0 0 1
3 0 1 0 0 1
4 0 0 1 0 1
5 0 0 0 1 1
6 1 1 0 0 1
7 1 0 1 0 1
8 1 0 0 1 1
9 0 1 1 0 1
10 0 1 0 1 1
11 0 0 1 1 1
12 1 1 1 0 1
13 1 1 0 1 1
14 1 0 1 1 1
15 0 1 1 1 1
16 1 1 1 1 0

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 64 -

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

NOR gate • Output true if all inputs are false.

4 • Output false if any input is true.
A
B
C NOR O

A B C X O
1 0 0 0 0 1
2 1 0 0 0 0
3 0 1 0 0 0
4 0 0 1 0 0
5 0 0 0 1 0
6 1 1 0 0 0
7 1 0 1 0 0
8 1 0 0 1 0
9 0 1 1 0 0
10 0 1 0 1 0
11 0 0 1 1 0
12 1 1 1 0 0
13 1 1 0 1 0
14 1 0 1 1 0
15 0 1 1 1 0
16 1 1 1 1 0

• Output true if number of true inputs is greater than or equal to “n.”

Qualified OR gate
5 • Truth table and graph are for n = 2.
Greater or equal to “n”

A
B
C n O

Copyright © 2024 ISA. All rights reserved.

 - 65 - ANSI/ISA-5.1-2024

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

A B C X O
1 0 0 0 0 0
2 1 0 0 0 0
3 0 1 0 0 0 1
4 0 0 1 0 0 A 0
5 0 0 0 1 0
6 1 1 0 0 1 B
7 1 0 1 0 1
8 1 0 0 1 1 C
9 0 1 1 0 1
10 0 1 0 1 1 X
11 0 0 1 1 1
12 1 1 1 0 1
13 1 1 0 1 1 O
14 1 0 1 1 1 t
15 0 1 1 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
16 1 1 1 1 1

Qualified OR gate • Output true if number of true inputs is greater than “n.”
6 Greater than “n” • Truth table and graph are for n = 2.

A
B
C n O

A B C X O
1 0 0 0 0 0
2 1 0 0 0 0 1
3 0 1 0 0 0 A 0
4 0 0 1 0 0
5 0 0 0 1 0 B
6 1 1 0 0 0
7 1 0 1 0 0 C
8 1 0 0 1 0
9 0 1 1 0 0 X
10 0 1 0 1 0
11 0 0 1 1 0
12 1 1 1 0 1 O
13 1 1 0 1 1
14 1 0 1 1 1 t
15 0 1 1 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
16 1 1 1 1 1

Qualified OR gate • Output true if number of true inputs is less than or equal to “n.”
Less or equal to “n”

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 66 -

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

• Truth table and graph are for n = 2.

A
7 B
C n O

A B C X O
1 0 0 0 0 1
1
2 1 0 0 0 1
A 0
3 0 1 0 0 1
4 0 0 1 0 1
B
5 0 0 0 1 1
6 1 1 0 0 1
C
7 1 0 1 0 1
8 1 0 0 1 1
X
9 0 1 1 0 1
10 0 1 0 1 1
11 0 0 1 1 1 O
12 1 1 1 0 0
13 1 1 0 1 0 t
14 1 0 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
15 0 1 1 1 0
16 1 1 1 1 0

Qualified OR gate • Output true if number of true inputs is less than “n.”
8 Less than “n” • Truth table and graph are for n = 2.

A
B
C n O

A B C X O
1 0 0 0 0 1
2 1 0 0 0 1
3 0 1 0 0 1 1
4 0 0 1 0 1 A 0
5 0 0 0 1 1
6 1 1 0 0 0 B
7 1 0 1 0 0
8 1 0 0 1 0 C
9 0 1 1 0 0
10 0 1 0 1 0 X
11 0 0 1 1 0
12 1 1 1 0 0
13 1 1 0 1 0 O
14 1 0 1 1 0 t
15 0 1 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
16 1 1 1 1 0

Copyright © 2024 ISA. All rights reserved.

 - 67 - ANSI/ISA-5.1-2024

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

Qualified OR gate • Output true if number of true inputs is equal to “n.”

9 Equal to “n” • Truth table and graph are for n = 2.

A
B
C =n O

A B C X O
1 0 0 0 0 0
2 1 0 0 0 0
3 0 1 0 0 0 1
4 0 0 1 0 0 A 0
5 0 0 0 1 0
6 1 1 0 0 1 B
7 1 0 1 0 1
8 1 0 0 1 1 C
9 0 1 1 0 1
10 0 1 0 1 1 X
11 0 0 1 1 1
12 1 1 1 0 0
13 1 1 0 1 0 O
14 1 0 1 1 0 t
15 0 1 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
16 1 1 1 1 0

Qualified OR gate • Output true if number of true inputs is not equal to “n.”
10 Not equal to “n” • Truth table and graph are for n = 2.

A
B
C n O

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 68 -

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

A B C X O
1 0 0 0 0 1
2 1 0 0 0 1
3 0 1 0 0 1
1
4 0 0 1 0 1
A 0
5 0 0 0 1 1
6 1 1 0 0 0
B
7 1 0 1 0 0
8 1 0 0 1 0 C
9 0 1 1 0 0
10 0 1 0 1 0 X
11 0 0 1 1 0
12 1 1 1 0 1
13 1 1 0 1 1 O
14 1 0 1 1 1
15 0 1 1 1 1 t
16 1 1 1 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

NOT gate • Output false if input true.

11 • Output true if input false.
A NOT O

A O 1
1 0 1 A 0
2 1 0
3 1 0 O 1
4 1 0 0
5 1 0
6 0 1 t
7 0 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
8 0 1
9 0 1
10 0 1
11 1 0
12 1 0
13 1 0
14 1 0
15 1 0
16 1 0

Basic memory • Outputs [C] and [D] are always opposite.

12 • If input [A] equals (1), then output [C] equals (1) and output [D] equals (0).
• If input [A] changes to (0), output [C] remains (1) until input [B] equals (1), then
A S C output [C] equals (0) and output [D] equals (1).
• If input [B] equals (1), then output [D] equals (1) and output [C] equals (0).
B R D
• If input [B] changes to (0), output [D] remains (1) until input [A] equals (1), then
output [D] equals (0) and output [C] equals (1).
• If inputs [A] and [B] are simultaneously equal to (1) , then outputs [C] and [D]
change state.

Copyright © 2024 ISA. All rights reserved.

 - 69 - ANSI/ISA-5.1-2024

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

1
A 0
A B C D
1 0 0 0 1 B
2 1 0 1 0
3 0 0 1 0 C
4 0 1 0 1
5 0 0 0 1 D
6 1 1 1 0
7 0 0 1 0
t
8 1 1 0 1 1 2 3 4 5 6 7 8

Set dominant memory • Outputs [C] and [D] are always opposite.
13 • If input [A] equals (1), then output [C] equals (1) and output [D] equals (0).
• If input [A] changes to (0), output [C] remains (1) until input [B] equals (1), then
A So C output [C] equals (0) and output [D] equals (1).
• If input [B] equals (1), then output [D] equals (1) and output [C] equals (0).
B R D
• If input [B] changes to (0), output [D] remains (1) until input [A] equals (1), then
output [D] equals (0) and output [C] equals (1).
• If inputs [A] and [B] are simultaneously equal to (1) , then output [C] equals (1)
and output [D] equals (0).

1
A B C D A 0
1 0 0 0 1
B
2 1 0 1 0
3 0 0 1 0 C
4 0 1 0 1
5 0 0 0 1 D
6 1 1 1 0
7 0 0 1 0
8 1 1 1 0 t
1 2 3 4 5 6 7 8

Reset dominant • Outputs [C] and [D] are always opposite.

memory
14 • If input [A] equals (1), then output [C] equals (1) and output [D] equals (0).
• If input [A] changes to (0), output [C] remains (1) until input [B] equals (1), then
output [C] equals (0) and output [D] equals (1).
A S C • If input [B] equals (1), then output [D] equals (1) and output [C] equals (0).
• If input [B] changes to (0), output [D] remains (1) until input [A] equals (1), then
B Ro D output [D] equals (1) and output [C] equals (0).
• If inputs [A] and [B] are simultaneously equal to (1) , then output [C] equals (0)
and output [D] equals (1).
1
A B C D A 0
1 0 0 0 1
2 1 0 1 0 B
3 0 0 1 0
C
4 0 1 0 1
5 0 0 0 1
D
6 1 1 0 1
7 0 0 0 1
8 1 1 0 1 t
1 2 3 4 5 6 7 8

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 70 -

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

• Output [O] changes from (0) to (1) and remains (1) for prescribed time duration
Pulse duration – fixed (t) when input [I] changes from (0) to (1).
15

I t PD O

1
I 0

O
NONE
t t

Time delay – off • Output [O] changes from (0) to (1) when input [I] changes from (0) to (1).
16 • Output [O] changes from (1) to (0) after input [I] changes from (1) to (0) and has
been equal to (0) for time duration (DT).

I t DT O

1
I 0

O
NONE

t t

Time delay – on • Output [O] changes from (0) to (1) after input [I] changes from (0) to (1) and [I]
remains (1) for prescribed time duration (t).
17
• Output [O] remains (1) until input [I] changes to (0) or optional reset [R] changes
I to (1).
t GT O

1
I 0

NONE O
t t t

R
t

Copyright © 2024 ISA. All rights reserved.

 - 71 - ANSI/ISA-5.1-2024

Table 17 – Binary logic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.9. 2.

Function Definition (1)

No
Symbol

Truth table (1) Graph

Pulse duration – • Output [O] changes from (0) to (1) when input [I] changes from (0) to (1).
variable • Output [O] changes from (1) to (0) when input [I] has been equal to (1) for time
18
duration (t), input [I] changes from (1) to (0), or optional reset [R] changes to (1).

I t LT O

1
I 0

O
NONE t t t

R
t

Transfer – binary
19 Signal A
The output equals the input logic state, which has been selected by transfer. The
T
state of the transfer is established by an external condition. True input can be shown
Signal B Signal D by signal convention or by external indicator (e.g., T for true) as shown in the example
T-Bin
symbol.

Condition C
D = A for C = zero, or
D = B for C = one

B
NONE
D

State 1 State 2

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 72 -

5.9.2 Notes for Table 17 – Binary logic symbols:

The following notes, indicated in Table 17 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) True signals are equal to binary one, and false signals are equal to binary zero.

(2) Alternate symbols shall be used only for “AND” and “OR” gates.

(3) Users’ engineering and design standards, practices , and/or guidelines shall document
which symbol has been selected.

Copyright © 2024 ISA. All rights reserved.

 - 73 - ANSI/ISA-5.1-2024

5.10 Electrical schematic symbols

5.10.1 This subclause provides symbols for electrical schematics in tabular form with
accompanying notes.

Table 18 – Electrical schematic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.10. 2.

No Symbol (1) Description

• Device wiring point.

1
• Device wiring terminal.

• Normally open single circuit momentary push -button switch.

2
• Form A switch contact.
• Stack symbols to form multi-pole switches.
• Actuated by hand (shown) or symbols 5 or 6 to form toggle or rotary
actuated switches.

• Normally closed single circuit momentary push -button switch.

3
• Form B switch contact.
• Stack symbols to form multi-pole switches.
• Actuated by hand (shown) or symbols 5 or 6 to form toggle or rotary
actuated switches.

• Normally closed/normally open double circuit momentary push -button

4 switch.
• Double pole single throw (DPST) switch contact, Form Z.
• Stack symbols to form multi-pole switches.
• Actuated by hand (shown) or symbols 5 or 6 to form toggle or rotary
actuated switches.

• Two-position toggle or rotary maintained position push-button switch

5 actuator.
• Combine with symbols 2, 3, or 4 to form single or multi-pole switches.

• Three-position toggle or rotary maintained position push-button switch

6 actuator.
• Combine with symbols 2, 3, or 4 to form single or multi-pole switches.

(1)(3) • Single pole single throw (SPST) normally open toggle switch.
7 a)
• Form A switch contact.
b) • Select a) or b) depending on actuating device
• Combine with symbols 11 through 16, 21, and 22.
(2)(3) • Single pole single throw (SPST) normally closed toggle switch.
8 a)
• Form B switch contact.
• Select a) or b) depending on actuating device.
b)
• Combine with symbols 11 thru 16, 21 and 22.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 74 -

Table 18 – Electrical schematic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.10. 2.

No Symbol (1) Description

(2) • Single pole double throw (SPDT) normally closed/normally open toggle
9 switch.
• Form C switch contact.
• Combine with symbols 11 through 16, 21, and 22.

• Rotary selector switch.

10

(2) • Pressure switch actuator.

11

(2) • Differential pressure switch actuator.

12

(2) • Liquid level switch actuator.

13

(2) • Temperature switch actuator.

14

(2) • Flow switch actuator.

15

(2) • Foot switch actuator.

16

• Relay operating coil.

17
• (*) = Relay designator, such as:
(*)
a. Tag number if assigned.
b. RO1, RO2, R4, R5, MR10, etc.

• Normally open relay contact.

18
• Form A contact.

Copyright © 2024 ISA. All rights reserved.

 - 75 - ANSI/ISA-5.1-2024

Table 18 – Electrical schematic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.10. 2.

No Symbol (1) Description

• Normally closed relay contact.

19
• Form B contact.

• Normally open, normally closed relay contact.

20
• Form C contact.

(2) • On time delay.

21
(*) • Moves after relay coil is energized and set time has elapsed.
• (*) = Set time.

(2) • Off time delay.

22
(*) • Moves after relay coil de-energizes and set time has elapsed.
• (*) = Set time.

• Transformer.
23
(*) • (*) = Rating, 220/120 VAC, etc.

(3) • Fuse, non-resettable.

24
• (*) = Rating, 2 A, 5 A, etc.
a) (*) b) (*)

• Thermal overload.
25

• Magnetic trip.
26

• Circuit breaker, 1-pole, manual reset.

27
(*) • (*) = Rating, 10 A, 15 A, etc.
• Manual operator shown.

• Circuit breaker, 3-pole, manual reset.

28
(*) • (*) = Rating, 15 A, 20 A, etc.
• Manual operator shown.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 76 -

Table 18 – Electrical schematic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.10. 2.

No Symbol (1) Description

• Circuit breaker, 1-pole, manual reset, with thermal trip.

29 (*)
• (*) = Rating, 20 A, 30 A, etc.
• Manual operator shown.

• Circuit breaker, 3-pole, manual reset, with thermal trip.

30 (*)
• (*) = Rating, 20 A, 25 A, etc.
• Manual operator shown.

• Circuit breaker, 1-pole, manual reset, with magnetic trip.

31 (*)
• (*) = Rating, 10 A, 15 A, etc.
• Manual operator shown.

• Circuit breaker, 3-pole, manual reset, with magnetic trip.

32 (*)
• (*) = Rating, 20 A, 25 A, etc.
• Manual operator shown.

• Bell.
33

• Horn or siren.
34

• Buzzer.
35

• Solenoid coil.
36

• Pilot light.
37

• Battery.
38

Copyright © 2024 ISA. All rights reserved.

 - 77 - ANSI/ISA-5.1-2024

Table 18 – Electrical schematic symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.10. 2.

No Symbol (1) Description

• Ground.
39

(3) • Connection conventions, alternatives a) and b):

40 Left side of illustration = Not connected.
a)
Right side of illustration = Connected.

b)

• Electric motor.
41

• Camera.
42 Z
• Optional “Z” to indicate zoom function.

• Camera with pan and tilt.

43
• Optional “Z” to indicate zoom function.
Z

5.10.2 Notes for Table 18 – Electrical schematic symbols:

The following notes, indicated in Table 18 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) All devices are shown in the unactuated or de-energized condition.

(2) Switch symbols 7, 8, and 9 shall be actuated by:

(a) hand;

(b) actuator symbols 11 through 16, 21, and 22; or

(c) bubble symbol for device or function assigned to actuate the switch symbol.

(3) Users’ engineering and design standards, practices , and/or guidelines shall document
which symbol has been selected.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 78 -

5.11 Instrument loop diagrams

5.11.1 This subclause provides symbols for instrument loop diagrams in tabular form with
accompanying notes.

Table 19 – Instrument loop diagram symbols

NOTE Numbers in parentheses refer to explanatory notes in 5.11. 2.

No Symbol Description

(1)(2) XXXX • General terminal or bulkhead symbol.

1
1 • XXXX – Identify junction box or termination assembly symbols.
2 • Show the quantity of terminals necessary.
3
4
5
6

(1)(2)(3) • Instrument terminals or ports with identifying letters, numbers ,

2 or both.
7 • Bubble represents the instrument or device.
8 FIC 1 • Device input terminals can be shown on one side and output
15 103 2 terminals on the other.
16 • Show the quantity of terminals necessary.
• Orientation of terminals around the bubble can be changed to
suit the drawing.

(1)(2)(3) • Electrical power supply.

• Bubble represents the instrument or device.
L1
• Show the quantity of terminals necessary.
3
TR ES 115 V, 60 Hz • Orientation of terminals around the bubble can be changed to
L2 Panel A, Circuit 12
105 suit the drawing.
L2
G
G

(1)(2)(3) • Air supply.

4
• Identify air supply followed by air supply pressure.
TY • Orientation of terminal around the bubble can be changed to
S IA 20 psig suit the drawing.
104

(1)(2)(3) • Pneumatic output.

5
• Orientation of terminal around the bubble can be changed to
suit the drawing.
TY OO
104

(1)(2)(3) • Hydraulic fluid supply.

6
WT • Identify hydraulic fluid followed by the fluid supply pressure.
S HS 50 psig
103 • Orientation of terminal around the bubble can be changed to
suit the drawing.

Copyright © 2024 ISA. All rights reserved.

 - 79 - ANSI/ISA-5.1-2024

(1)(2)(3)(4) • Identification of instrument action.

7
• Locate with respect to bubble as space on drawing permits .
TC PC
104 104
DIR REV

REV
LT
104

5.11.2 Notes for Table 19 – Instrument loop diagrams:

The following notes, indicated in Table 19 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) Identifying terminal numbers or letters should be the manufacturer’s designations.

(2) Identify each connection used. Terminal numbers shown are for example only.

(3) Electrical/air/hydraulic supply and terminal and instrument tag numbers shown are for
example only.

(4) Show the direction of the instrument output signal, relative to the input or process
measurement, by placing appropriate letters close to the instrument bubble. Identify an
instrument in which the value of the output signal increases or changes to its maximum
value as input (measured variable) increases by the letters “DIR .” Identify an instrument in
which the value of the output signal deceases or changes to its minimum value as the value
of the input (measured variable) increases by the letters “REV .” However, since most
transmitters are direct-acting, the designation DIR is optional for them.

6 Graphic symbol dimensions

6.1 Graphic symbols dimension tables
6.1.1 The following tables provide measurement units for dimensioning the geometric shapes
that are required to construct the graphic symbols.

6.1.2 The shapes in the tables are drawn larger than suggested size for clarity.

6.1.3 Symbols shall be drawn to a:

a) larger size, by increasing the dimensional unit, when required reduction of an

original drawing or document results in an illegible diagram ; or
b) smaller size, by decreasing the dimensional unit, when required by space
limitations of an original drawing or document.
6.1.4 All the symbols shown in Clause 5 are not individually dimensioned, but the geometric
shapes required to construct all the symbols from the graphic symbol tables are included.

6.1.5 The minimum size for device and function symbols from Table 20 is a 7/16-inch (10.5-
millimeter) circle. This is based on use of a D-size (24 inch × 36 inch) or A1-size (594 millimeter ×
841 millimeter) drawing. Symbols can be scaled larger or smaller in cases described in 6.1.3.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 80 -

6.2 Measurement units

6.2.1 The dimensions are represented by measurement units (m.u.) that, as a minimum, shall
have equivalent dimensions equal to:

a) one-sixteenth inch (1/16 inch or 0.0625 inch); or

b) one and one-half millimeters (1.50 millimeters).
6.2.2 Symbols drawn in any full-size diagram shall be the product of the symbol’s geometric
shape m.u. times a selected equivalent dimension equal to or greater than the minimum equivalent
dimension.

6.2.3 Lettering in general should be of sufficient size to be readable when the drawing is reduced
for printing or other purposes. Lettering specifically dimensioned per the tables in Clause 6 is the
minimum size for full-size symbols, so that when drawings and other documents are reduced, no
more than 50%, lettering will still be readable without magnification .

6.3 Dimensions for graphic symbol tables

6.3.1 This subclause provides dimensions for symbols found in Tables 2 and 3 in tabular form
with accompanying notes.

Table 20 – Dimensions for Tables 2 and 3

NOTE Refer to 6.2.1 for unit dimensions. Numbers in parentheses refer to explanatory notes in 6.3. 2.

7[8] (1)(3)
7[8] (1)

1 7[8]
(1)(2)
1 7[8] (1)
0.5[0.75]

6
5.5
6 3 4.5
3
6
4 3 7
9

2
0.52

6.3.2 Notes for Table 20 – Dimensions for measurement and control instrumentation device or
function symbols, Tables 2 and 3:

The following notes, indicated in Table 20 by parentheses, are to be used as an aid in

understanding the entries in that table.

Copyright © 2024 ISA. All rights reserved.

 - 81 - ANSI/ISA-5.1-2024

(1) Dimension in brackets is for 1/2-inch (12-millimeter) option for generic circle symbol.

(2) Dimension for hex symbol alternate size.

(3) Bubble symbols can be broken or elongated to accommodate longer tag numbers.

ABCDEFGHJKLM ABCDEFGHJKLM
9876543210 9876543210

6.4 Dimensions for measurement symbols

6.4.1 This subclause provides dimensions for symbols found in Tables 4, 5, 6, 7, and 8 in tabular
form with accompanying notes.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 82 -

Table 21 – Dimensions for Tables 4, 5, 6, 7, and 8

NOTE Refer to 6.2.1 for unit dimensions. Numbers in parentheses refer to explanatory notes in 6.4.2.

7[8] (1) 4 6
2
3 5

4 1.5
3
1
4
2 1
2
4
1 1 4
2 3
1.25 1
1.5
4 5 1.25
1
1.5
2
0.5

6 6 4 6
1 2

4
4 0.5
2 2 2 2 3 3.5

3
1.5

1.5
1 2
2 1 2
2

4 2 2 3 2
1 2
1 2

2 4
4 2
3 1
1.5
2
1.5
1.5 2
6 3
1.5 6 (3) 7
(2)
0.75 1

1
4
6
6 6

7 3 1
5 2
4
4 2

Copyright © 2024 ISA. All rights reserved.

 - 83 - ANSI/ISA-5.1-2024

6.4.2 Notes for Table 21 – Dimensions for measurement symbols: primary elements and
transmitters, in Tables 4, 5, 6, 7, and 8:

The following notes, indicated in Table 21 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) Dimension in parentheses is for 1/2-inch (12-millimeter) option for generic circle symbol.

(2) Size as required by size of vessel as drawn or depth of application.

(3) Dip tube shown, show as required for other devices.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 84 -

6.5 Dimensions for line symbols

6.5.1 This subclause provides dimensions for symbols found in Tables 9 and 10 in tabular form
with accompanying notes.

Table 22 – Dimensions for Tables 9 and 10

NOTE Refer to 6.2.1 for unit dimensions. Numbers in parentheses refer to explanatory notes in 6.5. 2.

Min = 8 2
1 1 1
Max = 32 0.4 (2)
0.2 (1)
2
1

Min = 1 Min = 0.5

Max = 1.5 Max = 0.75

2 2 1 1 1 4 4

0.5 (3) 0.5 0.5

1 1 2 2

9[10]
1
6
Min = 1 Min = 0.5 As req’d
Max = 1.5 Max = 0.75
7[8]

(ST) 1.5 4

Max = 16 Max = 16
Min = 8 Min = 8

6.5.2 Notes for Table 22 – Dimensions for line symbols in Tables 9 and 10:

The following notes, indicated in Table 22 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) Recommended maximum signal line thickness.

Copyright © 2024 ISA. All rights reserved.

 - 85 - ANSI/ISA-5.1-2024

(a) Signal lines shall never be thicker than process and equipment lines.

(2) Recommended minimum process and equipment line thickness.

(3) Clearance around symbol shall be equal to half the width of the symbol.

6.6 Dimensions for final control elements

6.6.1 This subclause provides dimensions for symbols found in Tables 11, 12, 13, and 14 in
tabular form with accompanying notes.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 86 -

Table 23 – Dimensions for Tables 11, 12, 13, and 14

NOTE Refer to 6.2.1 for unit dimensions. No additional explanatory notes for this table.

(1) 2 1.5 2 1
2R 1.5R
2 1.5
VSD 2
3
2

4 4
2 4
1 3
1 2

0.5

1 3 4
3
3 3 2 0.5

1
2 1.5
4.5 5 3
5

2 1
4 2
3

2.5 1
3 4 3.5 1
4.5 2
1
2.8 1 1
3.5

6 3

1.5

4.5
3.5
3.5 3.5 3.5

3.5 1
0.5

0.75 0.5 4 1
2
0.5 1 M 4
3 4

1 5
P 2 4
0.75

Copyright © 2024 ISA. All rights reserved.

 - 87 - ANSI/ISA-5.1-2024

6.7 Dimensions for functional diagramming symbols

6.7.1 This subclause provides dimensions for symbols found in Table 15 in tabular form with
accompanying notes.

Table 24 – Dimensions for Table 15

NOTE Refer to 6.2.1 for unit dimensions. Numbers in parentheses refer to explanatory notes in 6.7. 2.

(1)
(2) (2) (2)
7
4

9
9 4.5 3

6 6

12

6.7.2 Notes for Table 24 – Dimensions for functional diagramming symbols in Table 15:

The following notes, indicated in Table 24 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) Symbols are shown for top-to-bottom signal flow.

(2) Rotate symbols 90 degrees counterclockwise for left-to-right signal flow.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 88 -

6.8 Dimensions for binary logic symbols

6.8.1 This subclause provides dimensions for symbols found in Table 17 in tabular form with
accompanying notes.

Table 25 – Dimensions for Table 17

NOTE Refer to 6.2.1 for unit dimensions. Numbers in parentheses refer to explanatory notes in 6.8.2.

(4)(5)

2 4.5
3

4
(1c)
(3) 2 6.5 5
8 (2)
10
(1b)
(1a) 1
(2) 1
(2)

3
9
4.5

4.5 (6)
4.5
9

4.5

6.8.2 Notes for Table 25 – Dimensions for binary logic symbols in Table 17:

The following notes, indicated in Table 25 by parentheses, are to be used as an aid in

understanding the entries in that table.

(1) Input connection line dimensions are the minimum for:

(a) five inputs,

(b) three inputs, and

(c) two inputs.

(2) Two m.u.’s shall be added for each additional input.

(3) Minimum spacing between inputs.

(4) Output signal line (shown greyed) shall be centered on symbol.

(5) Input signal line(s) (shown greyed) shall be centered on symbol.

Copyright © 2024 ISA. All rights reserved.

 - 89 - ANSI/ISA-5.1-2024

(6) Output signal shall be centered on the top square. As an option, a second output signal
can be shown centered on the bottom square.

6.9 Dimensions for electrical schematic symbols

6.9.1 This subclause provides dimensions for symbols found in Table 18 in tabular form with
accompanying notes.

Copyright © 2024 ISA. All rights reserved.

ANSI/ISA-5.1-2024 - 90 -

Table 26 – Dimensions for Table 18

NOTE Refer to 6.2.1 for unit dimensions. No additional explanatory notes for this table.

2
5 2
1 4
2 2

2
6 2

1 2 4
5 1
0.5
4
8 1 1
1
1
2
1 2 2
1.5 2
4 1
1.5 1
4 4
2
2

6 4
4 1 2 2
2

6
4 1
2
1
6 1
1
2 1 1 0.5
1 2

3 1
0.5
1 2
1 4 2
1 4 2

2 4
4

2 1 6
1 2 2 7
4
0.5
1 1
2
1
2 2
0.5
1

2 2

Copyright © 2024 ISA. All rights reserved.

 - 91 - ANSI/ISA-5.1-2024

7 References
7.1 ISA publications
ISA technical reports in support of ANSI/ISA-5.1:

[TR1] ISA-TR5.1.01/ISA-TR77.40.01-2012 (R2016), Functional Diagram Usage

[TR2] ISA-TR5.1.02-2024, Instrumentation and Control Identification System Guidelines
[TR3] ISA-TR5.1.03-2024, Instrumentation and Control Graphic Symbol Guidelines

ISA-51.1-1979 (R1993), Process Instrumentation Terminology

ANSI/ISA-5.06.01, Functional Requirements Documentation for Control Software Applications

ANSI/ISA-5.4, Instrument Loop Diagrams

Meier, Frederick A. and Meier, Clifford A. Instrumentation and Control Systems

Documentation (2nd Edition), ISA Books, 2011

Available from:
ISA
P.O. Box 12277
Research Triangle Park, NC 27709
Tel: (919) 549-8411
www.isa.org

7.2 Other related information

ISO 1219-1, Fluid power systems and components – Graphic symbols and circuit diagrams

Copyright © 2024 ISA. All rights reserved.

 This page intentionally left blank.

Copyright © 2024 ISA. All rights reserved.

 - 93 - ANSI/ISA-5.1-2024

Annex A – Summary of changes to previous 2022 revision

Below is a summary of changes made to the 2022 revision of ISA-5.1 to arrive at this 2024 revision:

• Standard was reorganized to place the notes for a table or group of tables immediately
following the respective table(s) rather than having all notes preceding all tables.
• Terms, Definitions and Abbreviations : Terms and abbreviations that were not used in the
body of the standard were removed. Synonyms were placed together rather than appearing
separately and referring to each other. A few definitions were changed to match and/or
reference other standards. Formatting was updated.
• Changed numbering of tables from subclause -based to simple sequential.
• Symbol changes:
o Table 3 (previously 5.1.2): Added symbols 6 b), 8, 9, 10, and 11.
o Table 4 (previously 5.2.1): Added clarifying note regarding “?” appearing in all
symbols.
o Table 5 (previously 5.2.2): Added “flame ionization” to FR.
o Table 6 (previously 5.2.3): Changed symbols 3, 4, and 19; added symbols 20 b),
24 a), 29 a), 34, 37 b), 39; deleted symbols 34 and 35.
o Table 7 (previously 5.2.4): Added symbols 2 and 3.
o Table 10 (previously 5.3.2): Made symbols 18 and 19 alternates to symbols 20
and 21, respectively.
o Table 11 (previously 5.4.1): Changed symbol 19; moved symbol 20 to Table 12;
inserted new symbol 20.
o Table 12 (previously 5.4.2): Added symbols 23 and 24.
o Table 13 (previously 5.4.3): Moved symbol 2 a) to Table 6 symbol 29 a); deleted
symbols 2, 4, 5, and 6.
o Table 15 (previously 5.5): Edited symbols 2, 3 and 4; added symbols 7, 8, and 9.
o Table 16 (previously 5.6): Deleted binary signal transfer from symbol 28 (moved
to Table 17).
o Table 17 (previously 5.7): Added symbol 19 from Table 5.6.
o Table 18 (previously 5.8): Changed symbol 16, added symbols 26, 31, 32, 41, 42,
43; edited symbols 27, 28, 29, 30.
o Table 19: New table with symbols for instrument loop diagrams.
o Table 20 (previously 6.1): Added alternate hexagon dimension; added note (3)
(moved from annex).
o Table 5.6: Deleted.
o Table 25 (previously 6.7): Edited all symbols by adding input and output lines in
phantom and clarifying dimensions of AND symbol.
• Content of annexes were moved to separate technical reports to facilitate future additions
of or changes to examples. Annex A was moved to TR-5.1.02. Annex B was moved to
TR-5.1.03.

Copyright © 2024 ISA. All rights reserved.

Developing and promulgating sound consensus standards, recommended practices, and technical
reports is one of ISA’s primary goals. To achieve this goal, ISA relies on the technical expertise and
efforts of volunteer committee leaders and members.

ISA is an American National Standards Institute (ANSI) accredited organization. To obtain additional
information, please visit www.isa.org/standards.

ISBN: 978-1-64331-250-7