How Thermocouples Work

A thermocouple is a junction of two dissimilar materials which when joined at both ends and with an indicating meter connected in one wire, will cause an electromotive force (emf) to register if the temperature of the two ends are different.


By placing one end at the measuring point called the hot junction and the other at a different point normally a colder temperature and called the cold junction the difference emf generated can be measured. The emf is proportional to the difference in temperature between the two junctions. Traditionally the cold junction was maintained at 0°C by placing it in crushed ice.

Nowadays the cold junction is measured electronically and then automatically compensated for and the emf can then be related to the temperature of the hot or measuring junction. Various combinations of material can be used with predictable output signals depending upon the application, these materials are documented under published standards for example:
British-BS.EN, German-DIN, American-ANSI, European-IEC.

Thermocouples are generally less accurate than Platinum Resistance but are more robust, lower cost and can operate over a wider temperature range. They are available to suit a wide range of process temperatures and environments as shown below and can have an isolated or grounded junction i.e. the thermocouple is separate from, or joined to, the outer sheath. Grounded thermocouples offer faster response but can cause problems in the process due to earth loops etc.