The working Principle of Thermocouples

When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small current is produced proportional to the difference in the temperature.

For example, we have the cold end joined at a sensor millivolt meter, and the hot junction forming the sensor end as shown below:

Peltier showed that the heat is absorbed at the hot end and rejected at the cold end. Thompson showed that part of the e.m.f. is due to temperature gradient in the wire as well as the temperature difference between the junctions.  Most of thermocouple metals produce a relationship between the two temperatures and the e.m.f. as follows:

The α and β are constants for the type of thermocouple. The relationship is nearly linear over the operating range. The actual characteristics and suitable operating temperatures depend upon the metals used in the wires. The various types are designated in international and national standards. Typical linear operating ranges are shown for standard types. Note, it is important for thermocouples to be standard so that the e.m.f. will always represent the same temperature.

Thermocouples come in several forms, they may be wires insulated from each other with plastic or glass fibre materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses, the sensor comes in a metal enclosure such as a stainless steel. 

You can also read: Temperature transducers as applied in Industrial Instrumentation

Example of typical thermocouple industrial probes is shown below: