Thermocouple Temperature Sensors

A thermocouple is a temperature sensor formed by joining two wires of different metals or alloys at one end. When that junction is at a different temperature from the wires' free ends, a small voltage appears across the open ends. This phenomenon, the Seebeck effect, depends on the temperature difference and on the particular pair of materials used, and it is the basis for measuring temperature with a thermocouple.

Because the output depends on a temperature difference rather than an absolute value, practical instruments compensate for the temperature of the connection point, a step known as cold-junction compensation. Standard thermocouple types are designated by letters such as K, J, T, E, N, R, S and B, each combining specific metals or alloys suited to a particular temperature range and environment. Type K, made of nickel-based alloys, is among the most common general-purpose choices and spans a wide range, while platinum-based types R, S and B are used for very high temperatures.

Thermocouples are valued for their wide measuring range, ruggedness, small size and low cost, and they respond quickly because the junction has little mass. Their voltage output is small and somewhat nonlinear, so it is usually converted to temperature using standard reference tables or polynomial approximations. A closely related device is the thermopile, which connects many thermocouple junctions in series so their individual voltages add. Thermopiles produce a larger, more easily measured signal and are widely used in non-contact infrared thermometers and radiation detectors.

Frequently asked questions

What is the Seebeck effect?

It is the generation of a voltage in a circuit of two dissimilar conductors when their junctions are held at different temperatures. The voltage is proportional to the temperature difference and varies with the materials.

How does a thermopile differ from a thermocouple?

A thermopile is several thermocouple junctions wired in series so their voltages add, giving a larger output. This makes it useful for detecting small amounts of heat, as in infrared sensors.

Why do thermocouples need cold-junction compensation?

The output reflects the temperature difference between the measuring junction and the connection point. Knowing the connection-point temperature lets the instrument calculate the actual measured temperature.