What is a Thermocouple?

A thermocouple is a temperature sensing element that operates on the principle that when two dissimilar metals are junctioned and the junction is heated, they produce a low voltage (millivoltage) which is proportional to the temperature. They have a predictable and repeatable relationship between temperature and voltage. They are used widely in higher temperature applications because they can withstand greater temperatures than resistance temperature detectors (RTDs), and are less expensive in most cases.

The practical life of a thermocouple is limited by the environment it is exposed to and also by aging. The thermoelectric coefficients of the wires in a thermocouple change with time and the measurement voltage accordingly drops. As they age within a process, their conductors can also lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. One of the disadvantages of using thermocouples is the loss of accuracy when cycled. For example, when the temperature is raised and lowered significantly over a period of time, this will cause the thermocouple to “drift”, thus creating errors in the reading.