1. TEMPERATURE MEASUREMENTS
17/09/2019
Bachelor Degree in Chemical Engineering
Course: Process Instrumentation and Control
Measuring devices of the main process variables
TEMPERATURE MEASUREMENTS
Rev. 2.6 – March 13, 2019

2. MEASURE OF TEMPERATURE
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MEASURE OF TEMPERATURE
The temperature control is fundamental in process engineer.
For this reason it is indispensable to have instrumentation able to measure this important process variable with accuracy, repeatability and short response time.
The common devices used in industrial plants operate based on thermomechanic and thermoelectric principles.
Among the thermomechanic sensors we have:
Liquid-in-glass column thermometer
Bourdon tube gauges thermometer
Bi-metal mechanic thermometer
Among the thermoelectric sensors (usually transducer) we have:
Thermocouples
Resistance thermometers
Thermistors
Solid state sensors
Thermal flux sensors
Pyrometers
Process Instrumentation and Control - Prof M. Miccio

3. Thermocouples Resistance thermometers Thermistors Solid state sensors
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4. Thermoelectric effect
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Thermoelectric effect
SEEBECK EFFECT (1822)
When a homogeneous conductor is submitted to a temperature gradient and thus there are two different temperatures between the conductor terminals, a direct voltage E is produced. 1 E 2 T1 T2
with T2 > T1
σ = SEEBECK coefficient (intrinsic property of any single conductor material) [=] mV/K
E = direct voltage between points “1” and “2” [=] mV
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5. Process Instrumentation and Control - Prof M. Miccio
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SEEBECK EFFECT 
PROBLEM !
Usually, we cannot measure the voltage E from the point “2” because it is hot!
If a voltmeter is connected, a new metal-to-metal junction is introduced, which negatively affects the measure. 
SOLUTION!
We use 2 conductors of different material connected in the “hot” point (hot junction)
We use the voltmeter and measure the voltage between the 2 “cold” metallic contacts (cold junction)
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6. SEEBECK EFFECT M1 Cold junction (2 measuring
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SEEBECK EFFECT E
Cold junction (2 measuring
terminals) at Tf
Hot junction at Tc > Tf M1 M2
M1 and M2 two different homogeneous conductors
In a non-closed circuit, with two junctions at different temperatures, the generated fem E is independent of the temperature distribution along the homogeneous wires
It is possible to use a voltmeter with each contact wire of the same metal of the terminal, without altering the measurement made by the thermocouple. Obviously, the wires must be homogeneous and without damage, which would lead to spurious voltages.
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7. THERMOELECTRIC LAWS SUMMARY
HOMOGENEOUS CIRCUIT LAW
In a "closed" circuit of completely homogeneous material there is no difference in potential, whatever the temperature distribution in it, and even if the thickness of the wires is not constant.
 V = 0
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8. MEASUREMENT SYSTEM WITH THERMOCOUPLE
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MEASUREMENT SYSTEM WITH THERMOCOUPLE 
PROBLEM !
The electromotive force E depends on the cold junction temperature Tf, which is not fixed and is subject to variability, even unpredictable. E
Cold junction (2 measuring
terminals) at Tf
Hot junction at Tc > Tf M1 M2
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9. "IDEAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE
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"IDEAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE 
SOLUTION !
We transform the cold junction into a reference junction and keep it at a reference temperature, e.g. Tref = 0°C.
The use of the two sections of copper (Cu) wire for the voltmeter, often present to limit the length of the TC wires, does not affect the VACQ reading, being subject to the same temperature difference (Homogeneous Circuit Law).
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10. “REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE
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“REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE 
PROBLEM !
In the measurement of temperature, both in the laboratory and in the industry, it is not practical to keep the "Cold junction” (measurement terminals) a TF = 0°C
Measuring device
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11. LAW OF INTERMEDIATE TEMPERATURES
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LAW OF INTERMEDIATE TEMPERATURES
The electromotive force E generated in a pair with junctions respectively at temperatures T1 and T3 is equal to the sum of electromotive force of two pairs, of material identical to the first, with junctions respectively at T1 and T2 and at T2 and T3, whatever T2
EC = EB+ EA
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12. “REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE
SOLUTION !
The problem is solved by observing that for TF ≠ 0°C :
 INTERMEDIATE TEMPERATURES LAW
NOTE:
Actually, TC measures the difference of temperature between 2 points, but NOT an absolute temperature
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13. Process Instrumentation and Control - Prof M. Miccio
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“PRACTICAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE software COLD JUNCTION COMPENSATION
MEASURING DEVICE
(Voltmeter)
The measuring device “reads” VACQ = E(TC,TF) from the hot joint through copper (Cu) wires for the voltmeter
TF is measured separately by a different additional (small) sensor
A special software (software compensation) or dedicated integrated circuit (hardware compensation) calculates E(TF,0) from the static characteristic of the TC, applies the intermediate temperature law
and calculates E(TC ,0).
The software evaluates TC from the static characteristic of the TC
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14. “REAL” MEASUREMENT SYSTEM WITH THEMOCOUPLE
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“REAL” MEASUREMENT SYSTEM WITH THEMOCOUPLE 
PROBLEM!
For the common laboratory and industrial application the cold junction is far from the voltage measurement device (voltmeter, etc.).
The TC should be too “long”! 
SOLUTION!
In the practice, special Thermocouple Wires are used in laboratories and plants
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15. THERMOCOUPLE WIRES www.tc-srl.it
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THERMOCOUPLE WIRES
The extension wires consist of wires of the same material or the same thermoelectrical behaviour of the thermocoulple materials. They come from waste of TC production. They are not cheap, but less expensive than thermocouple. They have a good accuracy.
The compensation wires are made of a material different but equivalent to the constructive metals of TC. They are cheaper, but have a worse accuracy.
These cables are commonly used as bipolar wires with an external insulator (PVC, silicon rubber, enameling) selected up to the temperature range. Tc M1
Extension or compensation wires
TERMINALS
COPPER
VOLTMETER M2
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16. THERMOCOUPLE PROPERTIES
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THERMOCOUPLE PROPERTIES
For TC construction, only those metals should be considered, the properties of which meet, at least partially, the following requirements:
High thermoelectric voltage, with quite linear variation in relation to the temperature;
Constancy of the thermoelectric characteristic over time;
Ability to preserve the thermoelectric characteristics unchanged due to mechanical deformations;
Resistance to aggressive, oxidizing or reducing environments;
Ductility, in order to obtain even thin threads;
Low cost;
Usability even at high temperatures;
Very large T range.
Unable to combine all these features into a single thermocouple, different types of thermocouples were introduced.
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17. THERMOCOUPLE TYPES
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International standards define different TC types characterizing them with the relative E(Tc,0)
type
metals/alloy pairs
normal range [°C]
extended range [°C] J
iron/costantan K
chromel/alumel E
chromel/costantan T
copper/costantan B
Pt80-Rh20/Pt94-Rh6 S
Pt90-Rh10/Pt R
Pt87-Rh13/Pt N
Nicrosil/nisil
From Magnani, Ferretti e Rocco (2007)
costantan = Cu-Ni alloy; chromel = Ni-Cr alloy; alumel = Ni-Al-Si alloy;
nicrosil = Cr, 1.40 Si, bal. Ni alloy; nisil = 4.40 Si, 0.10 Mg, bal. Ni alloy;
materials are given in the order: positive leg/negative leg
E(Tc,0) are quite linear, con sensitivity of V/°C
Measuring range max: -200 ÷ 2750 °C
Absolute accuracy 0.1 ÷ 4ºC depending on measuring range
Long response times (slow dynamic characteristic) except in the case of exposed hot joint.
Process Instrumentation and Control - Prof M. Miccio

18. TC STATIC CHARACTERISTIC
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TC STATIC CHARACTERISTIC
Each thermocouple is characterized by the static characteristic E(Tc,0) which is not difficult to measure in the laboratory, just placing the cold junction at temperature TF = 0 °C J K T
TC type K, J and T are the most used
Sensitivity of about di V/°C for J,
52 di V/°C for K,
50 di V/°C for T
Accuracy between 0.5 and 2.5 °C
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19. CONSTRUCTIVE FEATURES OF TCs
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CONSTRUCTIVE FEATURES OF TCs
A- Bare hot junction
It has a very short response time because of the hot junction is in direct contact with the environment in which the measurement is performed. However, it is not recommended in corrosive environment.
B- Grounded hot junction
In this type of realization the hot junction is an integral part of the protection sheath. The response time is reduced. This meets the standard ASTM-E-235. It is recommended for high pressure applications (up to 3500 kg/cm2).
C- Insulated hot junction
The hot junction is completely insulated form the protection sheath. It is particularly recommended when Eddy currents could compromise the measurement. This meets the standard ASTM-E-235.
The hot junction selection depends on the operating condition of the thermocouple.
It is important to bear in mind that type B and C joints are typical of thermocouples with compact ceramic insulation.
For the thermocouples with the two wires isolated simply in PVC or fiber we are always in the case A.
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20. THERMOCOUPLES INTERNATIONAL COLOUR CODE
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THERMOCOUPLES INTERNATIONAL COLOUR CODE
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21. LABORATORY THERMOCOUPLE
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LABORATORY THERMOCOUPLE
Type J thermocouple
(iron– constantan)
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22. INSULATED HOT JUNCTION
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23. INDUSTRIAL THERMOCOUPLE
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INDUSTRIAL THERMOCOUPLE
TERMINALS BLOCK
COUPLING ELEMENT
CERAMIC INSULATOR
EXTENSION
PROTECTIVE TUBE
terminals
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24. Thermocouples Resistance thermometers Thermistors Solid state sensors
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25. RESISTANCE TEMPERATURE DETECTOR (RTD)
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RESISTANCE TEMPERATURE DETECTOR (RTD)
It is based on the temperature dependence
of the electrical resistance of a conductor
Materials:
Platinum, Nickel, Copper
A good approximation of the static characteristic is:
R(T)= R0 (1+α1T+α2T2+ α3T3) [=] Ω
where R0= resistance at T0
α1 temperature coefficient [=] 1/K
α1 > 0 <=> PTC = Positive Temperature Coefficient
The sensitivity is about R01
Accuracy ass ± 0.02°C (in the range -50 ÷ +150°C)  greater than TC
The transduction of the resistance value in voltage is performed by a Wheatstone bridge R
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26. 17/09/2019
WHEATSTONE BRIDGE
The transduction of the resistance-voltage is realized with a Wheatstone bridge.
Resistance-voltage transduction with Wheatstone bridge
from Magnani, Ferretti e Rocco (2007)
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27. MORE COMMON RTD from Magnani, Ferretti e Rocco (2007) Cu: 10 
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MORE COMMON RTD
Cu: 10 
from Magnani, Ferretti e Rocco (2007)
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28. STATIC CHARACTERISTIC
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STATIC CHARACTERISTIC
CALIBRATION CURVE OF A RESISTANCE TEMPERATURE DETECTOR
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29. RTD Constructive features
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RTD Constructive features
INSULATING DISK
CERAMIC
PLATINUM
PLATINUM WIRES
from Magnani, Ferretti e Rocco (2007)
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30. RTD Constructive features
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RTD Constructive features
SCHEMATIC DRAWING
INDUSTRIAL RTD
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31. Thermocouples Resistance thermometers Thermistors Solid state sensors
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32. Process Instrumentation and Control - Prof M. Miccio
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THERMISTORS
Measurement principle
Electrical resistance variation of a non-conductor with temperature:
semiconductor
metal oxides (e.g.: Mn oxides)
In this case, the dependence with the temperature is non-linear
α<0 <=> NTC (Negative Temperature Coefficient)
but they can be also PTC type
Max temperature range -50 ÷ 250 °C
Absolute accuracy εass = ± 0,1 °C
Disadvantage: non-linear static characteristic
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33. STATIC CHARACTERISTIC
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STATIC CHARACTERISTIC
NTC thermistor characteristic
from Magnani, Ferretti e Rocco (2007)
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34. Process Instrumentation and Control - Prof M. Miccio
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THERMISTORS Examples
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35. Process Instrumentation and Control - Prof M. Miccio
Thermocouples Resistance thermometers Thermistors non-contact temperature sensors
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36. Infrared Temperature Measurement
Contact-based temperature sensors, such as thermocouples and resistance temperature detectors (RTDs), have demonstrated accurate and cost-effective operation throughout the chemical process industries (CPI).
However, there are many applications and settings where they are simply not practical. In those cases, engineers can turn to a host of non-contact temperature measurement devices, many of which are based on measuring infrared (IR) radiation.
Pyrometer ⏎
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37. Process Instrumentation and Control - Prof M. Miccio
CONCLUSIVE REMARKS
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38. COMPARISON BETWEEN TCs AND OTHER SENSORS
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COMPARISON BETWEEN TCs AND OTHER SENSORS
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39. COMPARISON BETWEEN SENSORS
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COMPARISON BETWEEN SENSORS
From:
Hewlett-Packard Classroom Series
052 So we have already looked at the three types of absolute measuring devices: the RTD, the thermistor and the IC. Let's see how the thermocouple stacks up against these three. The thermocouple is useful in more types of atmospheres and over wider temperature ranges. Since you do not have to put current through the thermocouple to measure it, it has no self-heating and this can be extremely important.
Because it has a very small output voltage, the thermocouple is hard to measure and is susceptible to noise, both magnetic and electrostatic. It only measures relative temperature, not absolute temperature. It is non-linear, and it requires either special connectors that are built with the same metal as the thermocouple itself, or a special "isothermal" junction.
Process Instrumentation and Control - Prof M. Miccio

40. COMPARISON BETWEEN SENSORS
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COMPARISON BETWEEN SENSORS
From:
Dr. Chi-fu Wu, ME 4903-Special Problem in ME, GTREP, Savannah (USA), Oct. 21, 2004
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41. Process Instrumentation and Control - Prof M. Miccio
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TYPICAL COSTS
Type K thermocouple mineral insulated, inconel sheath, 3x3000mm       
RS Code  S X 1
          1+ 6+
12+
€ 46,32
€ 41,69
€ 39,37
Resistance temperature PT100 mineral insulated, 1000mm length wire, 6.0mm diam.   
RS code  S Y 1 1+ 6+
€ 91,98
€ 78,18
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