Principle of Thermocouple

Types of thermocouples

Earlier, we saw a temperature sensor based on change in resistance. This sensor, thermocouple, is based on Seebeck effect (1821).

Temperature Range: - 200 deg. C to 1450 deg. C (-328 deg. F to 2640 deg. F)

(Depends on type)

Seebeck effect: When heat is applied to a junction formed by joining two dissimilar metals, a measurable emf is generated at the corresponding cold junction.

The emf, thus generated is proportional to the change in temperature and the relationship between temperature and emf generated is different for different metal combinations, and is purely non-linear.

An electrical circuit is formed by the two dissimilar metals and a current flows as a result of the generated emf. The current continues to flow as long as there is a difference in the temperature of the junctions.

Seebeck effect
Seebeck effect as used in a thermocouple.
 Temperature of hot junction T1 > Temperature of cold junction T2

A device for measuring the change in voltage produced can be installed in a thermocouple circuit as shown in the image below. The produced voltage can be converted into the corresponding temperature using proper signal conditioning circuit.

There are various laws of thermo-electricity which are followed by the thermocouple. Some of them are advantageous while some prove to be limitations of thermocouple.
These laws are as follows:

  1. Single metal cannot be used to form the two junctions, as they cannot produce an electric current by the Seebeck effect.
  2. The thermal emf  is produced only as a result of temperature difference between the two junctions. Therefore, any change in temperature along the length of the two conductors will not affect the current passing through them; as long as the temperature is well within the conductors' capable range.
  3. The thermal emf produced when a thermocouple is formed by keeping a junction at a certain temperature, say T1, and other junction at temperature T2. We form another thermocouple, keeping the same conductors in same position as in the earlier circuit, with one junction at the same temperature T2 and other at certain random temperature T3. Then the sum of emfs generated in both the thermocouples is equivalent to the emf generated when a thermocouple is formed by keeping its one junction at T1 and other at T3. This law can be bettter understood by studying the figure below:
    Seebeck effect
    The hot and the cold junctions of the thermocouple so formed will depend upon the temperature T1 and T3.
    The direction of the current will also be decided by the same.
  4. In a thermocouple formed by joining two dissimilar metals and having its junctions at different temperatures, the emf generated will not be affected when a third homogeneous metal is introduced in the circuit; provided that temperatures and the conductors which form the junctions are kept constant. This law is known as the law of dissimilar metals and is the most important law as far as thermocouples are concerned.
    This helps us in reducing cost of long distance measurements as we can use cheaper extension metal wires like, for a Pt-Rh thermocouple, we can use Copper wires for transmission from junction to junction, which is very cheap as compared to Pt or Rh.
    This law also enables us to introduce a measuring device in the circuit, without which, measurement would have been impossible.
    This law also enables us to solder or braze the metals at the junction.
  5. The algebraic sum of emfs produced in a circuit containing 2 or more thermocouples all set at a satire temperatures is zero.
  6. The net emf of a circuit containing two thermocouples is unaffected by introduction of the third thermocouple at a temperature equal to that of any of the two thermocouples.

    For further information about Thermocouple:

    Types and Specifications of Thermocouples

    Related Articles:

    Temperature Units & Conversions                                       RTD