Thermocouples are temperature measurement devices that produce a voltage when two different conductors contact each other at different temperatures. The voltage is proportional to the temperature difference and relies on the Seebeck effect where a temperature gradient along conductors generates an electric current. Common thermocouple types use different metal combinations like chromel-iron and alumel-constantan wired into a circuit to measure temperature in various applications such as steel production, gas appliances, and vacuum gauges.

1. Thermocouple
Mechanical Engineering Department
Selection of an Instrument for a Need….
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2. Thermocouples
A thermocouple is a temperature-measuring device
consisting of two dissimilar conductors that contact each
other at one or more spots. It produces a voltage when the
temperature of one of the spots differs from the reference
temperature at other parts of the circuit.

3. Misconception in Thermoelectricity
The EMF generated by the Seebeck effect is due to the temperature
gradient along the wire.
The EMF is not generated at the junction between two dissimilar wires.

4. Principle of operation

6. Sheath options

7. Thermocouples: Seebeck effect
The Seebeck effect is the conversion of thermal energy/temperature differences directly
into electrical energy or electricity.
This effect measures the ease at which excess electrons will circulate in an electrical circuit
under the influence of thermal difference.
The change in the voltage is proportional to the temperature difference between the
junctions when the ends are connected to form a loop.

8. A thermoelectric circuit composed of materials of different Seebeck coefficient
(p-doped and n-doped semiconductors), configured as athermoelectric generator.
If the load resistor at the bottom is replaced with avoltmeter the circuit then
functions as a temperature-sensing thermocouple.

9. Seebeck Effect

10. Circuit construction

11. Standard Calibration Setup
Ice-point
The result of the calibration is a table of EMF versus T values.
The integral is never directly evaluated.
Instead a polynomial curve fit to the calibration data gives:
( ) n
j j j j n j E = F T = b + bT + b T 2 +.....+ b T
0 0 1 2

12. Material EMF versus Temperature
With reference to
the characteristics
of pure Platinum
emf
Temperature
Chromel
Iron
Copper
Platinum-Rhodium
Alumel
Constantan

13. Thermocouple temperature vs.Voltage graph

14. Seebeck coefficient vs. Temperature

15. applications
1.Steel industry
2.Gas appliance safety
3.Thermopile radiation sensors
4.Manufacturing
5.Power production
6.Thermoelectric cooling
7.Process plants
8.Thermocouple as vacuum gauge

16. A thermocouple (the right most tube) inside the burner
assembly of a water heater.

17. Thermocouple connection in gas appliances. The end ball (contact) on the left is insulated
from the fitting by an insulating washer. The thermocouple line consists of copper wire,
insulator and outer metal (usually copper) sheath which is also used as ground.

18. Type

20. Thermocouple insulation

21. Advantages

23. The end