As for Basic working principle of Thermocouple- Thermocouple is temperature sensor which measure temperature in terms of voltage according to the temperature difference between two end of the metal pair.
Thermocouple working principle:
Thermocouple made by two different metal wires joined at one end,this joint ends is placed in a temperature zone where temperature should be measured called “hot zone” and the other end of thermocouple where two metal wire are open(not connected or joined) placed in a low room temperature called “cold zone or reference temperature.”.
Now two ends of this metal pair are placed in two different temperature zone.A net thermoelectric voltage is generated according to the temperature difference between two ends.This voltage is measured in the open pair placed in cold zone or reference zone.
Facts of thermocouple:
Two metal wire as a pair measures temperature in terms of voltage. Hot zone is the temperature to be measured ,two metal joint together. Cold zone established in the PLC section or in the panel,voltage measured in the open wires.