An RTD is a sensor made of a metal, such as platinum, that exhibits a predictable increase in electrical resistance with increasing temperature. Most RTD sensors use platinum because it has a nearly linear resistance-temperature relationship and resists chemical attack. The electrical resistance of a metal conductor depends on its "resistivity" and on its total volume. For particular RTD applications, these factors help determine the appropriate metals and the lengths and cross sectional areas of the sensors.
A thermocouple is essentially two wires made of different metals, or alloys, connected at two junctions to form a circuit. A thermocouple instrument measures small voltages, called the "Seebeck effect," that develop in response to differences of temperature between the junctions. For every metal pair, the voltage developed is a known function of the temperature difference .
RTDs and thermocouples often can be used for the same application, yet each has advantages and disadvantages that should be considered in relation to the application.