A thermocouple is a temperature measurement device consisting of two dissimilar metals joined at one end, producing a voltage when there is a temperature difference. It works based on the Seebeck effect where electrons flow due to different energy potentials in the alloys, generating a small voltage. Common types include J, K, E, and T, which can measure temperatures from -250°C to 3500°C depending on the type. A thermocouple has two wires insulated along their length and joined at the measuring point, with an outer protective sheath. It provides a rugged and cost-effective way to measure a wide range of temperatures, though accuracy can be impacted by noise, thermal shunting, and

2. Introduction
 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.
 Thermocouples can measure temperature at a point in a range of -250C to
+3500C.

3.  Principle of operation is based on the See beck effect, discovered by Thomas Seebeck in 1822,
 Electrons flow from one wire to other, due to different energy potentials of alloys
 As temperature changes, current flows
 Voltage is measured between the two alloys (Small voltage, less than 10 mV)
 See beck effect: when any conductor subjected to thermal gradient, generates a voltage.
 A temperature gradient along a conductor creates an EMF.
PRINCIPLE OF OPERATION

4. Thermocouple: the operating principle
 A thermocouple is a device made by two different wires joined at one end, called junction
end or measuring end. The two wires are called thermoelements or legs of the thermocouple: the two
thermoelements are distnguished as positive and negative ones. The other end of the thermocouple is
called tail end or reference end . The junction end is immersed in the environment whose temperature
T2 has to be measured, which can be for instance the temperature of a furnace at about 500°C, while the tail
end is held at a different temperature T1, e.g. at ambient temperature.
Because of the temperature difference between junction end and tail end a voltage difference can be
measured between the two thermoelements at the tail end: so the thermocouple is a temperature-
voltage transducer.

5.  A thermocouple construction consist of two conductors, welded together at the
measuring point and insulated from each other long the length.
 It will usually have an outer protection sheath
Practical Thermocouple Construction:

6. TYPES OF THERMOCOUPLES

7. TYPES OF THERMOCOUPLES
Type J:
 Type J is useable up to 720°C.
 It is not very susceptible to aging up to about 540°C.
 It is very cost effective and is the thermocouple of choice in the plastics processing industry where
temperatures rarely exceed 400 °C.
 The iron conductor is subject to oxidation (due to the iron wire) at higher temperatures and when unprotected.
 generates about 50 µV/°C (28 µV/°F)
Type K:
 Type K is useable up to 1150°C in an oxidizing atmosphere.
 Metallurgical changes can cause a calibration drift of 1 to 2°C in a few hours, increasing to 5 °C over time.
 generates about 40 µV/°C (22 µV/°F)

8. CONTINUE…..
Type E:
 Type E is useable up to 820 °C.
 It has the highest mV output of all the thermocouples and has similar calibration drift to that of Type K so the same
precautions are recommended.
Type T:
 Type T oxidization of the copper limits the useable temperature to about 370 °C.
 Type T is thermocouple of choice for applications down to –200 °C.
 generates about 40 µV/°C (22 µV/°F).
Type N :
 Type N is useable up to1260°C.
 It was developed to overcome several problems inherent in Type K TCs.
 Aging in the 300 to 600 °C range is considerably less.
 Also Type N has also been found to be more stable than Type K in nuclear environments, where Type K has been
the sensor of choice.

9. APPLICATION
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

10. THERMOCOUPLE CAPABILITIES
 It is rugged in construction
 Covers a wide range of temperature
 Comparatively cheaper in cost
 Calibration can be easily checked
 Offers good reproducibility
 High speed of response
 Satisfactory measurement accuracy

11. THERMOCOUPLES LIMITATIONS
 Accuracy
 Often between 0.5 and 2.2ºC, depending on TC type
 Noise
 Long leads can attract electrical signals
 Already low signal from thermocouple
 Thermal shunting
 Heating of wire mass can affect measurements by absorbing energy
 Corrosion
 High alkali or water environments can modify calibration

12. BY
SAUMYA PANDEY 113BT0579