Instrumentation

Thermocouple

In this lesson, we will study about Thermocouple.

  • A thermocouple is also a temperature-sensing device.
  • As the name Indicates Thermo means temperature and couple means a pair that means Thermocouple is used to measure Temperature and it has 2 wires.
  • In thermocouples, only two wires are there.
  • These two wires are short-circuited at one end.

  • A thermocouple consists of two conductors of different materials (usually metal alloys) that produce a voltage in the vicinity of the point where the two conductors are in contact as shown in the above figure.
  •  the above line can be better understand by-

In thermocouples, two junctions are formed, one is hot junction in which the thermocouple is inserted and the other one is a cold junction -the part of thermocouple remains in  environment.

Thermocouple Gives output as milli-volt.

So, we can say that thermocouple is a Temperature Sensor or temperature sensing device which gives output as milli volt.

  • The voltage produced is dependent on, but not necessarily proportional to, the difference of temperature of the junction to other parts of those conductors.
  • a thermocouple works for higher temperature ranges.
  • A thermocouple works on the Seebeck effect.

Working Principle

  •     when two junctions of differenent  temperatures are formed, then due to this temperature difference an emf is generated. This emf is produced in milli volts (mv).
  • In, 1821 T. J. Seebeck observed the existence of an electromotive force (EMF) at the junction formed between two dissimilar metals (Seebeck effect).
  • Seebeck effect is actually the combined result of two other phenomena, Thomson and Peltier effects.
  • Thomson observed the existence of an EMF due to the contact of two dissimilar metals at the junction temperature.
  • Peltier discovered that temperature gradients along conductors in a circuit generate an EMF.

How to check a Thermocouple

  • In thermocouple millivolts (mv) are checked. As the temperature increases, millivolts are also increased. 1 millivolt is approximately equal to 25 Degree Celsius for Type K (Type K is the most common general-purpose thermocouple). A table is also used for calculating temperature.
  • Two wires of thermocouple are short-circuited at one end. So we also check the continuity.

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