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## LO1-UD2 - TEMPERATURE MEASUREMENT

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Auteur Author: cext104
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Mis en ligne Uploaded: 15/09/2021 - 18:47:46
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### Description

UNIVERSITÁ DEGLI STUDI DI
SALERNO

Bachelor Degree in Chemical Engineering

Course:
Process Instrumentation and Control

Measuring devices of the main process variables

TEMPERATURE MEASUREMENTS

Rev. 3.3 of March 24, 2021
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

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 2
THERMOCOUPLES
RESISTANCE THERMOMETERS
THERMISTORS
SOLID STATE SENSORS

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 3
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

T2
E   (T)dT
T1

σ = SEEBECK coefficient (intrinsic property of any single conductor material)
[=] mV/K
E = direct voltage between points “1” and “2” [=] mV

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 4
THERMOELECTRIC LAWS SUMMARY

E≠
0

E=
0
HOMOGENEOUS CIRCUIT LAW

In a "closed" circuit of completely homogeneous
material there is no difference in potential,
E= whatever the temperature distribution in it, and
even if the thickness of the wires is not constant.
0
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 5
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 instead of one, but they are made of different
material and connected in the “hot” point (hot junction)
 We use the voltmeter and measure the voltage between the 2 “cold”
metallic contacts (cold junction)
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 6
SEEBECK EFFECT
M1
Cold junction
(2 measuring Hot
E junction
terminals)
at Tf at Tc > Tf

M2 Tc
E   (T )   (T ) dT
2 1
Tf

• 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

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 7
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.
M1
Cold junction
Hot
E (2 measuring
junction
terminals)
at Tc > Tf
at Tf
M2

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 8
"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.

E TC ,0  VACQ

The use of the two sections of copper (Cu) wire for the voltmeter, often used to limit the length
of the TC wires, does not affect the VACQ reading, because the copper wires are subject to the
same temperature difference (Homogeneous Circuit Law).
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 9
“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

TF Cu TL
M1
TC Dispositivo
Measuring
VACQ
di device
misura
M2
TF Cu TL

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 10
 SOLUTION !
LAW OF INTERMEDIATE TEMPERATURES
The electromotive force E generated in a pair with junctions respectively at temperatures T 1 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

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 11
“REAL" MEASUREMENT SYSTEM WITH THERMOCOUPLE

SOLUTION !
The problem is solved by observing that for TF ≠ 0°C :

TC TC TF

E TC , TF    2 T   1 T dT   2 T   1 T dT   T   T dT
2 1
TF 0 0

E TC ,0  E TF ,0 
 INTERMEDIATE TEMPERATURES LAW

NOTE:
Actually, TC measures the difference of temperature between 2 points, but NOT
an absolute temperature
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 12
“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

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 13
THERMOCOUPLE WIRES
www.tc-srl.it
• The extension wires consist of wires of the same material or the same
thermoelectrical behavior of the thermocouple materials. They come from
wastes 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.
COPPER
M1...

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