Temperature Measurement
The Thermocouple
e.m.f.
Temperature T
Learning Outcomes
After this lecture you should be able to…
• Explain the thermoelectric effect
• Explain how a thermocouple works
• Write the equation that relates the millivolt output
from the thermocouple and temperature
• List the different types of thermocouples
available
• Explain the laws of thermocouples
• Show how a thermocouple is used to measure
temperature
• Reference the thermocouple tables
Thermocouple
The thermoelectric effect – when two different metals are
joined together an electromotive force (e.m.f.) is generated.
The amount of e.m.f. is proportional to the junction
temperature e.m.f.
e.m. f . = aT
Temperature T
Linear between temperature and e.m.f.
Thermocouple is a couple of these junctions (both the same)
Overall e.m.f. is the difference between the two individual
e.m.f.s
Thermcouple, i.e. 2 junctions
e.m.f.1
T1
T1
e.m.f.2
An e.m.f. is generated at each junction
We want to know the difference in the e.m.f.s
V = e.m.f.1 – e.m.f.2
V = aT1 – aT2
V = a(T1 – T2)
T2
T2
Equation
If we know one temperature and measure the e.m.f. we can
predict the other temperature
e.m. f . = a (T1 − T2 )
Cold junction – ideally we keep one junction at 0 °C
Measuring junction – placed in the unknown temperature
a = sensitiviy. It depends on the particular pairing of
metals.
For Chromel and Alumel, a = 41 µV/°C.
For Platinum and Platinum+Rhodium Alloy, a = 8 µV/°C.
Thermcouple Types
Type
Materials
Range °C
a. µV/°C
Cost
E
Chromel Constantan
0 to 980
63
Cheap
J
Iron Constantan
-180 to 760
53
Cheap
K
Chromel Alumel
-180 to 1260
41
Cheap
R
Platinum
Platinum/Rhodium
Copper Constantan
0 to 1750
8
Dear
-180 to 370
43
Cheap
T
History
Thermocouple operation (I.e. generation of e.m.f.) is a
physical phenomenon known as the Thermoelectric effect or
also called the Seebeck effect.
It is named after Thomas Johann Seebeck (1770 – 1831)
who was born in Estonia and educated in Germany. In 1821
he discovered that joining semicircular pieces of bismuth
and copper together caused a magnetic disturbance on a
nearby compass needle.
He also discovered that a voltage existed between two ends
of a metal bar that had a temperature gradient, i.e. the
temperature of the bar increased along its length.
He did not make the connection that Faraday made 10 years
later (1831, the year Seebeck died) that the needle
disturbance was caused by a magnetic field created by an
electric current due to the generation of the e.m.f.
Instead he incorrectly attributed the behaviour to an effect
he called thermomagnetism.
Thermcouple Table – Type K
°C
0
1
2
3
4
5
6
7
8
9
10
°C
0
0.000
0.039
0.079
0.119
0.158
0.198
0.238
0.277
0.317
0.357
0.397
0
10
0.397
0.437
0.477
0.517
0.557
0.597
0.637
0.677
0.718
0.758
0.798
10
20
0.798
0.838
0.879
0.919
0.960
1.000
1.041
1.081
1.122
1.163
1.203
20
30
1.203
1.244
1.285
1.326
1.366
1.407
1.448
1.489
1.530
1.571
1.612
30
40
1.612
1.653
1.694
1.735
1.776
1.817
1.858
1.899
1.941
1.982
2.023
40
Activity - Thermocouples
Activity 1 - Read the handout on the Laws of Thermocouples and identify which
laws apply to the following scenario.
Steam Supply
120 - 160 degC = T1
Control Room
15 - 30 degC = T3
Plant Room
30 - 50 degC = T2
e.m.f. = VT3, 0
T3
V
T1
T2
e.m.f.
Copper
Alumel
Chromel
Extension leads
through building
varying temp
Activity - Thermocouples
Use the thermocouple tables to solve this problem.
In a similar situation to the above example, the voltmeter in the control displays a
reading of 5.4 mV. A thermometer on the wall of the control room displays a
temperature of 22 °C. What is the temperature of the steam in the pipe
Steam Supply
= ? degC
Control Room
= 22 degC
e.m.f. = 5.4 mV
Alumel
Chromel
Type K
Conversion Table
To
K (Kelvin)
°C (Celsius)
°F (Fahrenheit)
°R (Rankine)
TK K
1
TK - 273.15
9/5TK – 459.67
5/9TK
TC °C
TC + 273.15
1
9/5TC + 32
5/9TC + 151.75
TF °F
5/9TF + 255.37
5/9TF – 17.8
1
0.308TF + 141.9
TR °R
9/5TR
9/5TR – 273.15
3.24TR – 459.67
1
From
For example, what is 46 °C in Kelvin? TC = 46 so Kelvin = TC + 273.15 = 319.15 K
Exercise, what is 46 °C in Fahrenheit?