Thermocouples work by measuring the voltage difference that arises between two dissimilar conductors when these conductors are exposed to a temperature differential. The produced voltage is measured by an internal circuit and translated into a temperature.
Thermocouples are advantageous in their affordability, interchangeability and utilization of industry standardized connectors. Depending on the type of materials used in the probe and the circuit configuration, thermocouples can measure a wide range of temperatures. Thermocouples also require no external power source, making them more convenient than other types of digital thermometers. The low accuracy and responsivity of thermocouples limits their usage to industrial applications that do not require high controllability.
A junction between any two dissimilar conductors results in a potential between these conductors. The magnitude of this voltage difference is roughly linear with temperature. Special alloys are chosen for thermocouple probes to ensure that the increase in voltage that accompanies an increase in temperature is repeatable and predictable. Different probes, containing different alloys, are used for different temperature ranges. In addition to response linearity, the resistance of the probe materials to high-temperature corrosion is also important. The materials must also be nonreactive with one another at elevated temperatures. The formation of intermetallic compounds or alloys between the probe materials at high temperature interferes with measurement.