Thermocouples are the most commonly used temperature sensors. Thermocouples are self-powered, (require no excitation current), can operate over a wide and high temperature range, and have fast response times.
Two dissimilar metal wires are joined together forming an electrical junction. The temperature difference of the two dissimilar conductors produces a temperature dependent voltage as a result of the thermo electric effect, (Seebeck effect). The voltage is measured and used to calculate the temperature.
There are different hot junction types: insulated hot junction (SO), grounded hot junction (SP), one hot Junction for 2 TC insulated from the sheath (SOA), hot junctions insulated from each other and from the sheath (SOB).
Thermocouples have a built in electronic cold junction compensation to correct the missing thermoelectric voltage due to the fact that the thermocouple cold end at the instrument is not at 0°C.
There are several types of thermocouples made from different materials for different temperature ranges and different sensitivities. The most commonly used is the K type. Here a list of the common types of thermocouples: