Week 5 Temperature Measurement
After time, temperature is the second most measured physical
unit.
Temperature Sensing Techniques (see Table 16.1)
Changes in Physical Dimensions
Bimetallic Thermometers
Filled-Bulb and Glass-Stem Thermometers
Changes in Electrical Properties
Integrated Circuitry (IC) Transistors and Diodes
Resistance Temperature Detectors (RTDS)
Thermistors
Thermocouples
Changes in Chemical Phase
Quartz Crystal Thermometry
Changes in Emitted thermal radiation
Radiation and Infrared Pyrometers
Temperature Sensor Range
Comparison
Bimetal Thermometer
Two different metals with different coefficients of thermal expansion are bonded
together. As the temperature changes from the bonding temperature the unequal
expansion of the two metals will cause the bimetal strip to curl. If one end is fixed
the other end displaces in response to temperature changes.
Bimetal strips can be fabricated into coils, spirals, and disks.
Frequently used in on-off temperature control (thermostats)
Range of use:
-65 to 430 C
Accuracy varies with range
0.5 to 12C
Advantages:
Low cost
Negligible maintenance
Stable operation over time
Bimetal Thermometer
R = Radius of curvature
t=total thickness
m = ratio of thicknesses
Low/high expansion materials
n=ratio of Young moduli of
elasticity
Low/high expansion materials
1=lower coefficient of
thermal expansion, 1/C
2=higher coefficient of
thermal expansion, 1/C
T=Temperature, C
To=Initial bonding
temperature, C
* Problem 16.4 in Text
* Table 6.3 material properties
Liquid Expansion Thermometer
The volumetric expansion of liquids and solids is
used for temperature measurement.
Note: Expansion registered is actually the difference
between the liquid and the glass.
Mercury filled thermometers (best)
Range= -37 to 320 C , Accuracy 0.3 C
Alcohol filled thermometers.
Range =-75 to 120 C , Accuracy 0.6C
Upper range determined by boiling point.
Advantages/Disadvantages
Low cost
No remote readings
Liquid in Glass Thermometers
Greatest sensitivity to temperature is at the bulb, where the largest
volume of liquid exists. However, all portions of the thermometer are
temperature sensitive.
Immersion depth is important consideration. This type of thermometer is
calibrated at a specific immersion depth (usually total immersion).
Equation 16.1
where
T=T1+kT(T1-T2)
T=correct temperature
T1=actual temperature reading
T2=ambient temperature surrounding emerged portion
T=see text for definition-correction related to exposed length.
K=differential expansion coefficient between liquid and glass
(mercury in glass, =0.00009 F scale and = 0.00016 C scale)
Fluid Expansion Thermometer
A fluid filled bulb is connected to a pressure
measuring device via a capillary tube. As fluid
is heated it expands thus pressure increases.
Pressure is linked to temperature.
Accuracy and range depends on fluid.
Advantages/Disadvantages
Low cost
Stable in operation
Widely used in industrial applications.
Remote readings are possible.
Transient response is a function of bulb size
and capillary tube length.
Liquid Crystals
Liquid crystals possess the mechanical properties of a liquid, but have the optical
properties of a single crystal. Temperature changes can affect the color of a liquid
crystal, which makes them useful for temperature measurement.
The range and resolution of liquid crystal thermometers is varied by adjustment of
the formulation.
They operated just below 0C to several hundred degrees Celsius.
The resolution of liquid crystal sensors is in the 0.1 C range.
Disposable liquid crystal thermometers have been developed for home and
medical use.
Infrared Thermometers (Non-contact sensors)
What is infrared?
IR portion of the spectrum spans wavelengths
between 0.7 to 1000 microns. Based on the
principle that all objects warmer than
absolute zero ( 0 K) emit energy somewhere
in that range.
Infrared thermometers are able to determine
an objects temperature without touching it
by measuring the amount of infrared energy
being emitted from the object. Modern IR
instruments can only measure between 0.7
and 20 microns because they are not
sensitive enough to measure the small
amounts of energy available outside this
range.
Measurement Principles
Energy transmitted as electromagnetic waves or photons traveling at the
speed of light. Radiation striking a surface is partially reflected, absorbed,
and transmitted.
(refectivity) + (emissivity) + (transmissivity) = 1
= (absorptivity) {Kirchoff' s Law}
Radiation Heat Transfer:
q=Q/A=FBA(TA4-TB4)
Where is the Stefan-Boltzmann constant,
= (5.6705 x 10-8 W/m2K4)
Basic Components of IR
Thermometer
Lens to collect energy emitted from an
object.
Detector to convert the thermal energy to
and electrical signal with signal conditioning
(filters and amplifiers).
Emissivity adjustment to correct for various
surface finishes and materials.
Temperature TB (internal temperature) must
be known or determined through prior
calibration. Ambient temperature
compensation.
TA, the unknown temperature, is determined
from the detector temperature.
Infrared Cameras
Operating Temperature
5 to 122 F (-15 to 50 C)
Measuring Temperature
-40 to 3630 F (-40 to 2000 C)
Accuracy
2% of range or 4 F ( 2 C)
Sensitivity
0.2 F (0.1 C)
Image Storage Capacity
700 (14 bit) on 100 MB Card
Digital Voice Recorder
for Active
Documentation
yes, 30 sec per image
Camera Weight
5 lbs.
Modern infrared cameras are light weight,
portable, and can accurately measure dynamic
temperature changes in equipment and
processes. They have the ability to measuring
thermal variations of less than 0.1 C and are
non-invasive.
Attaching thermocouples, RTDs, or
thermistors is seldom an option in moving
components. In addition, thermocouples
cannot be attached at every location on the
component.
The biggest advantage over conventional
temperature measurement techniques is that a
total picture of the component or system is
possible.
Infrared Camera Examples
Piping
Heat transfer coils
Valve operation
Electronics
Engines/Compressors
100.6F
100
Building/Structures
106.4F
100
95
80
92.2F
73.5F
Single-point Infrared Temperature Sensors
Non-contact infrared thermometers uses:
Facility maintenance
Utilities and electrical inspection
Medical industry
HVAC/R maintenance and inspection
Food safety
Automotive and diesel maintenance
Asphalt, cement, and construction materials.
Infrared thermometers measure the surface
temperature of objects within their field of view.
Focal length of instrument is important
consideration.
Material Emmisivity
Material emissivity is subject to a great amount of uncertainty
because it depends on surface finish, color, oxidation, aging,
and several other factors.
How do determine emissivity?
Use published values as a good start.
Table 16.8 in text contains certain material emissivity values
Heat transfer text books
ASHRAE Fundamentals Handbook
Internet/Web pages
Example
The energy emitted from a piece of metal is measured, and the
temperature is determined to be 1050 C, assuming an emissivity of 0.82.
It is later found that the true emissivity is 0.75. What is the error in the
temperature determination?
q / A = T 4
and
q/A = T 4
where
T = 1050C = 1323K
T = ???
= 0.82
= 0.75
and
(0.82)(1323)4 = (0.75)(T ) 4
solve
T = 1352 K
Infrarred Thermocouples (IRt/c)
New method of surface temperature measurement
Unpowered
Low cost
Non-invasive
They can be installed on conventional thermocouple
controllers.
How do they work?
Receives heat energy from the object that it is aimed at
and converts the heat to an electrical potential.
Millivolt output signal is produced. This signal is scaled to
the desired thermocouple characteristics.
Adhere to the same laws as other infrared thermometers.
IR Thermocouples
Even though the overall governing
equations are non-linear, the
infrared thermocouples output is
linear over a small enough range
Uses:
Monitoring process temperatures.
Web roller temperature control.
Asphalt temperature monitoring.
Processing oven temperatures.
Summarize IR Thermometer
Advantages
IR thermometers can measure objects that move, rotate, or
vibrate.
They can measure temperatures > 1500 C
They do not damage or contaminate the surface of the object
of interest (food, painted surfaces)
Response time is in the millisecond range.
Contact Sensors
RTDs
Thermistorss
Thermocouples
Resistance Temperature
Thermometers
The electrical resistance of various materials changes in a reproducible
way with temperature variations. Both RTDs and Thermistors operate on
this basis.
RTD (resistance temperature device or detector) is a precision
temperature sensing device that utilizes metal conductors (typically a fine
platinum wire winding or thin metallic layer applied to a substrate) and
has a positive coefficient of resistance. That is, as T increases, resistance
increases almost linearly. Positive temperature coefficient (PTC).
Thermistors are made from semiconductor materials which have a large
negative coefficient of resistance. That is, as temperature increases then
the resistance of the thermistor decreases. These are called negative
temperature coefficient (NTC) thermistors.
Resistance Temperature
Thermometers
Sensitivity=dR/d
T
RTDs Principle of Operation
Metals have linear relationship with temperature
Platinum (high cost, highly linear, most common)
Tungsten (highly linear)
Copper (lower temperature ranges)
Nickle (lower temperature, low cost, nonlinear)
Nickle alloys (lower temperature, low cost)
Each metal has a specific resistivity, , which varies with temperature
and is determined experimentally.
L
R=
A
L=metal wire length
A=cross-sectional area
T = o [1 + a(T To )]
RTD Response Relationship
[
R = R o 1 a (T To ) + b(T To )
] (derived from Callendar - Van Dusen equation)
where
R o = reference resistance measured at T o (To is typically 0 C)
T = measured temperature
a and b = calibration constants
simple linear verson is often used over smaller temperature interval
R = R o [1 a (T To )]
Coefficients a and b depend on the wire material and its purity.
Example: Platinum
a=3.9083 E -03
b=-5.775 E -07
RTDs
Low resistance
100 (most common) to 1000
Wide operating range (-200 C to 850 C)
High sensitivity
(compared to thermocouples)
High accuracy (0.0006 C to 0.1C)
High Repeatability and Stability
Low drift (0.0025 C/year)
Industrial models drift < 0.1 C/year
RTD Disadvantages/Issues
Lead wire resistance can be significant.
2-wire is well suited for applications where there
are no lead wires.
3-wire provides sufficient accuracy in accounting
for the lead wire resistance. However, best
accuracy is obtained with 4-wire RTD.
4-wire RTD has one pair of leads that supplies
excitation current and the other pair measure the
voltage drop across it.
Slower response time
Sensitive to shock and vibration
Internal/self-heating To measure resistance must pass current through
sensor. Joule Heating=I2R
Thermistors
High resistance 1 k to 100 k
Eliminates most lead resistance issues
Highly non-linear resistance-to-temperature
relationships.
Mostly NTC (metal oxides) but PTC (barium and
strontium titanate mixtures) models available
Small physical size
Fast response time
Lower cost than RTDs
Very high sensitivity and resolution
Up to 1000 times more sensitive than RTDs
Not sensitive to shock and vibration
Thermistor Disadvantages/Issues
Narrow operating range for a single unit
Operating range is between -200 C and 1000C
More susceptible to internal/self heating issues than
RTDs
Less stable than RTDs
Thermistor Calibration
R = R oe
1 1
T To
where
R o = reference resistance measured at T o
T
= measured temperature
= material constant
Integrate Circuit Thermometer (IC)
IC temperature sensors contain a temperature sensor and signal
conditioning circuits. Sometimes used in compensating temperature
circuits.
Voltage or current output that is nearly linear with temperature.
An input voltage must be applied to the sensor.
Some even digitize the signal and produce digital serial output;
IC temperature sensors may require you to design your own electronics to
digitize the analog output or to interpret the digital signal.
Accuracy is about 0.5 C
Low cost sensor
Not susceptible to voltage noise and lead-wire errors are minimal.
Thermocouples
Most frequently used method to measure
temperature with an electrical output signal.
What are thermocouples?
Thermocouples operate under the principle that the junction
of two dissimilar metals produces a measurable voltage (emfelectromotive force) when the two ends of the thermocouple
are at different temperatures.
They are inexpensive, small in size, rugged, and remarkably
accurate when used with an understanding of their
peculiarities.
Thermocouples Principle of
Operation
In, 1821 T.J. Seebeck observed the existence of and
electromotive force (EMF) at the junction formed
between two dissimilar metals (Seebeck effect).
Seebeck effect is actually the combined result of two other
phenomenon, Thomson and Peltier effects.
Thomson observed the existence of an EMF due to the contact of
two dissimilar metals and the junction temperature.
Peltier discovered that temperature gradients along conductors in a
circuit generate an EMF.
Thomson effect is normally much smaller in magnitude than the
Peltier effect and can be minimized and disregarded with proper
thermocouple design.
How TCs work
Equation 1, shows the emf is
generated in the wires not the
junction. Signal is generated when
dT/dx is not zero.
When materials are homogeneous
is not a function of position.
May not be true with old TCs
Two wires begin and end the same
two temperatures.
Reduced linear final equation
Homogeneous materials
Only two wires
Each begins at Tref and ends at Tjct
Generally, second order eqn. is
used.
2
E = (T To ) + (T To )
Material EMF versus Temperature
Thermocouple Effect
Any time a pair of dissimilar wires is joined
and a junction is formed an emf voltage will
be generated.
Twist and solder or welded junctions are most
common
Keep weld bead or solder bead within 10-15%
of wire diameter
Welding is generally quicker than soldering but
both are equally accurate
Voltage or EMF produced depends on:
Types of materials used, A and B
Temperature of the junctions, measuring and
reference
Reference Temperature Systems and Zone Boxes
Ice Baths
Accurate and inexpensive
Electronically Controlled References
Require periodic calibration and are generally not as
stable as ice bath but are more convenient.
Compensated Reference Temperature Systems
A dedicated temperature sensor inside the chassis
automatically accounts for the reference
temperature before calculating temperature.
Zone boxes
A zone of uniform temperature that insures all
connections made within the zone are ate the same
temperature.
What thermocouple materials should be used?
Depends on requirements:
Temperature range?
Required accuracy
Chemical resistance issues
Abrasion or vibration resistance
Installation requirements (size of wire)
Thermocouple Material Color Coding
Thermocouple Material vs EMF
Types T, J, and K are most commonly used
thermocouples.
Thermocouple Materials Types
Copper-Constantan (T Curve)
Iron-Constantan (J Curve)
The Copper-Constantan thermocouple, with a positive copper wire and a negative Constantan wire is
recommended for use in mildly oxidizing and reducing atmospheres up to 400C. They are suitable for
applications where moisture is present. This alloy is recommended for low temperature work since the
homogeneity of the component wires can be maintained better than other base metal wires. Therefore,
errors due to the non-homogeneity of wires in zones of temperature gradients is greatly reduced.
The Iron-Constantan thermocouple with a positive iron wire and a negative Constantan wire is
recommended for reducing atmospheres. The operating range for this alloy combination is 870C for
the largest wire sizes. Smaller wire sizes should operate in correspondingly lower temperatures.
Chromel-Alumel (K Curve)
The Chromel-Alumel thermocouple, with a positive Chromel wire and a negative Alumel wire, is
recommended for use in clean oxidizing atmospheres. The operating range for this alloy is 1260C for
the largest wire sizes. Smaller wires should operate in correspondingly lower temperatures.
Thermocouple Materials
Chromel-Constantan (E Curve)
The Chromel-Constantan thermocouple may be used for temperatures up to 870C in a vacuum or
inert, mildly oxidizing or reducing atmosphere. At sub-zero temperatures, the thermocouple is not
subject to corrosion. This thermocouple has the highest emf output of any standard metallic
thermocouple.
Platinum-Rhodium (S and R Curve)
Three types of noble-metal thermocouples are in common use. They are:
1.The S curve shows a positive wire of 90% platinum and 10% rhodium used with a negative wire of pure
platinum,
2.The R curve indicates a positive wire of 87% platinum and 13% rhodium used with a negative wire of pure
platinum, and
3.(not shown) a positive wire of 70% platinum and 30% rhodium used with a negative wire of 94% platinum
and 6% rhodium.
They have a high resistance to oxidation and corrosion. However, hydrogen, carbon and many metal
vapors can contaminate a platinum-rhodium thermocouple.
The recommended operating range for the platinum-rhodium alloys is 1540C although
temperatures as high as 1780C can be measured with the Pt-30% Rh vs Pt-6% Rh alloy combination.
Tungsten-Rhenium (C Curve)
Three types of tungsten-rhenium thermocouples are in common use for measuring temperatures up
to 2760C. These alloys have inherently poor oxidation resistance and should be used in vacuum,
hydrogen or inert atmospheres.
Grades of TC wire
Grade is based on calibration accuracy of the wire.
Precision grade - ( 0.5 % reading or 1C) , greater of the two
Standard grade -( 0.75 % reading or 2C), greater of the two
Extension or Lead-wire grade - ( 1 % reading or 4 C)
Based on cost of TC wire its hard to justify the purchase of any
material other than precision grade.
Wire diameter
0.001 inches and up is possible
Standard diameters are: 0.01, 0.02, 0.032, 0.040, 1/16, 1/8,
3/16, and 1/4
Smaller the better but fragility of wire becomes and issue
Type of probes
Ungrounded
Junction is detached from the probe. Electrical
isolation is obtained at the cost of response
time.
Grounded
Junction is physically attached to probe wall.
Good heat transfer to junction.
Exposed
Junction is exposed to the surrounding
environment. Best response time, but is
limited to noncorrosive and nonpressurized
applications.
How do we measure T and interpret the emf?
The signal from a thermocouple depends as much on
the reference junction as it does on the measuring
junction temperature. In addition, we have to connect
the circuit to a DVM, which in turn creates a new TC
circuit.
Thermocouple Tables (EMF-Temperature)
Thermocouple tables correlate temperature to emf voltage.
(Lab Manual)
Need to keep in mind that the thermocouple tables provide a voltage
value with respect to a reference temperature, usually it is 0C. If
measuring junction is not at reference temperature a correction must
be applied using law of intermediate temperatures.
Measuring
Junction
Reference
Junction
e reference-instrument+ e instrument - junction= e reference - junction
Simple TC Model EMF-Temperature Sketch
1
2
Two materials
3
Material A (+)
Material B (-)
Plus and minus refers to how the emf changes with temperature.
Example:
Number junctions around circuit and draw
3
emf
B
Measure
d Emf
2
A
1
T junction
T meter
Temperature
Thermocouple Laws or Rules
Practical application of the thermoelectric effect is based
on a few thermocouple principles:
1) A thermocouple must contain two dissimilar materials
and at least two junctions at different temperatures to
have an emf voltage output.
T reference
T junction
Law of Intermediate Metals
2) Insertion of an intermediate
metal into a thermocouple circuit
will not affect the emf voltage
output as long as the two
junctions are at the same
temperature.
Permits soldered and welded joints.
5
2
4
3
Fe (+)
P (+)
emf
C (-)
C
Measure
d Emf
1
T ref
F
e
5
3
2
T 2 and 4
Temperature
T measured
Tcandle
Law of Intermediate Temperatures
3) If a thermocouple circuit develops a net
T2
T1
T3
T2
emf1-2 for measuring junction
temperatures T1 and T2, and a net emf2-3
for temperatures T2 and T3, then it will
develop a net voltage of emf1-3 = emf1-2 +
emf2-3 when the junctions are at
temperatures T1 and T3.
emf1-2+ emf2-3= emf1-3
T3
T1
emf
emf1-2+ emf2-3= emf1-3
emf23
emf13
emf12
T1
F
e
T2
T3
4) If a thermocouple circuit of materials A and C generates a
net emfA-C when exposed to temperatures T1 and T2, and a
thermocouple of materials C and B generates a net emfC-B
for the same two temperatures T1 and T2, then a
thermocouple made from materials A and B will develop a
net voltage of
emfA-B = emfA-C + emfC-B
between temperatures T1 and T2.
Sometimes useful in the calibration of different thermocouple wires.
4) Temperature changes in the wiring between the input and
output ends do not affect the output voltage, provide the
wiring is of a thermocouple alloy.
Hot or Cold Zone
5
3
Hot Zone
emf
C
Measure
d Emf
1
T ref
F
e
T measured
T hot
Example problems
Ambient Temp. = 24 C
ice bath
?
Voltmeter +-
Cu
iron
Cu
constantan
-10 C
Ambient Temp. = 24 C
Isothermal Block
at Ambient Temp.
6.232 mV
Voltmeter +-
Cu
iron
Cu
constantan
? C
TC Diagnostics
What happens if:
1.
2.
3.
You use copper instead of extension cable
You use proper extension cable but you cross the pair of wires at
both ends.
You use the wrong extension cable.
More Examples
Specialty Thermocouple Circuits
Thermopile-Thermocouples connected in series between two temperature
zones.
Good for determining small temperature differences
Amplification affect
Ambient Temp. = 24 C
49.312 mV
Isothermal Block
at Ambient Temp.
Cu
Averaging Circuit- Thermocouple are connected
Voltmeter +- in parallel between two
temperature zones.
Cu
4 iron
constantan
TC's
Ambient Temp. = 24 C
Ice Bath
?
Voltmeter +-
iron
Cu
110 C
100 C
Cu
constantan
130 C
? C
Temperature Measurement Errors
Conduction
Convection
Radiation
Response Time
Noise
Grounding issues and shorts, especially on
metal surfaces
Conduction
L
Thermocouple acts like a fin.
Ttrue T junction =
Tm T j
4hconv
cosh L
d
k
wire wire
Tj
Tm
Decrease Tm - Tj , run wires along a mild temperature gradient
Longer wire length
Increase h conv
Decrease d w and k w
Errorconduction = TT TJ =
Convection Errors
Applies to high speed flow applications. Loss of kinetic energy is
converted to heat at stagnation point.
(
1 )[( 1) / 2]M 2
Ttrue T junction =
1 + [( 1) / 2]M 2
where
= recovery factor
= ratio of specific heats, c p /c v
M = Mach Number
=0.86
=0.68
Radiant Heat Transfer
Temperature Differences
Emissivity of members involved
Ttrue T junction =
Reduce error by:
Increase convection
Use thermal shielding
Special probe designs
Page 706, Figure 16.25
(T j4 Tw4 )
hconvection
where
Tw = Wall Temperature
