A thermocouple, also known as a thermoelectrical thermometer, is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the Seebeck effect, and this voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature sensors. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self-powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve. Thermocouples are widely used in science and industry. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety devices for gas-powered appliances.
Principle of operation In 1821, the German physicist Thomas Johann Seebeck discovered that a magnetic needle held near a circuit made up of two dissimilar metals got deflected when one of the dissimilar metal junctions was heated. At the time, Seebeck referred to this consequence as thermo-magnetism. The magnetic field he observed was later shown to be due to thermo-electric current. In practical use, the voltage generated at a single junction of two different types of wire is what is of interest as this can be used to measure temperature at very high and low temperatures. The magnitude of the voltage depends on the types of wire being used. Generally, the voltage is in the microvolt range and care must be taken to obtain a usable measurement. Although very little current flows, power can be generated by a single thermocouple junction. Power generation using multiple thermocouples, as in a thermopile, is common.
The standard configuration of a thermocouple is shown in the figure. The dissimilar conductors contact at the measuring (aka hot) junction and at the reference (aka cold) junction. The thermocouple is connected to the electrical system at its reference junction. The figure shows the measuring junction on the left, the reference junction in the middle and represents the rest of the electrical system as a voltage meter on the right. The temperature Tsense is obtained via a characteristic function E(T) for the type of thermocouple which requires inputs: measured voltage V and reference junction temperature Tref. The solution to the equation E(Tsense) = V + E(Tref) yields Tsense. Sometimes these details are hidden inside a device that packages the reference junction block (with Tref thermometer), voltmeter, and equation solver.
Seebeck effect
The Seebeck effect refers to the development of an electromotive force across two points of an electrically conducting material when there is a temperature difference between those two points. Under open-circuit conditions where there is no internal current flow, the gradient of voltage ( ∇ V {\displaystyle \scriptstyle {\boldsymbol {\nabla }}V} ) is directly proportional to the gradient in temperature ( ∇ T {\displaystyle \scriptstyle {\boldsymbol {\nabla }}T} ):
∇ V = − S ( T ) ∇ T , {\displaystyle {\boldsymbol {\nabla }}V=-S(T){\boldsymbol {\nabla }}T,}
where S ( T ) {\displaystyle S(T)} is a temperature-dependent material property known as the Seebeck coefficient. The standard measurement configuration shown in the figure shows four temperature regions and thus four voltage contributions:
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