Thermocouples are the most widely used temperature sensors in industry due to their low cost, simplicity, robustness, size and useable temperature range.
A thermocouple consists of two wires made of dissimilar metals and welded at one end. This junction is called the hot or measuring junction. The other junction is called the cold or reference junction and is connected to the output device (voltmeter, temperature indicator).
These wire pairs or thermoelements can be composed of noble metals - such as platinum or rhodium - or base metals, such as copper, iron or nickel-copper alloy.
An electromotive force (emf) of typically a few millivolts is generated by thermal gradients along the length of the exposed conductors.
The electromotive force E is a function of the temperature gradient as follows:
where S(T, x) is the Seebeck coefficient. For homogeneous thermoelements, S(T,x) = S(T).
The correspondence between emf and temperature is available for the most commonly used thermocouples in reference tables (for thermocouple types R, S, B, J, T, E, K, N).
The standardised thermocouples are identified by a letter. The positive element of the thermocouple is always quoted first. Written standards for these thermocouples give (i) temperature versus emf in a given temperature range, (ii) compensating cables, connectors and associated standard colours and (iii) tolerances.
Medium temperature thermocouples
High temperature thermocouples
Thermocouples for use at high temperatures have been developed, particularly for nuclear and space technology applications, for use to 2000 °C and more. High temperature thermocouples are usually made of noble metals (platinum, rhodium, iridium) and their alloys, and refractory metals with very high melting points - mainly tungsten, rhenium, molybdenum, niobium, tantalum, and their alloys.
Thermocouple advantages and disadvantages