What is a thermocouple sensor?

A thermocouple is a type of sensor that measures temperatures. The sensor requires two dissimilar metal wires joined together at one end and connected to an instrument that is capable of accepting a thermocouple input and measure the reading. Thermocouples were developed because of the discovery of the Seebeck effect, by Thomas J. Seebeck in 1821. Depending on the configuration, thermocouple cable sensors can deliver accurate measurements over a wide range of temperatures.

How does a thermocouple work?

When two wires composed of dissimilar metals are joined at both ends in an electrical circuit, one of the ends will be heated whilst the other remains cold. As the junction of the paired conductors has both heated and cool metals, a variance in voltage that is approximately proportional to the difference between the temperatures of the two junctions. This voltage is called the thermoelectric voltage and is correlated directly back to the temperature.

A third sensor can be a part of the measuring factor for industrial practices which allows compensation for ambient temperatures (known as cold junction compensation). This is due to laboratory conditions being normally set a 0°C.

Types of thermocouples

Thermocouples are produced in various groupings of metals or calibration variants. The most common types are E, J, K, N and T, but there are additional high temperature thermocouples which include C, R and S types.

The metals for the pairs for each type are:

• Type E - Positive: NiCr (Nickel Chromium) / Negative: Constantan (Copper-Nickel)
• Type J - Positive:  Fe (Iron) / Negative: Constantan (Copper-Nickel)
• Type K - Positive: NiCr (Chromel) / Negative: NiAl (Alumel)
• Type N – Positive: NicroSil (Nickel-Chromiun-Silicon) / Negative: NiSil (Nickel-Silicon)
• Type T - Positive: Cu (Copper) / Negative: Constantan (Copper-Nickel)

The K Type thermocouples are commonly specified due their general-purpose usage, cost and temperature range.

The temperature range and environmental performance will depend on the Type used. Additionally, the cable sized will also make a difference. Cables of all types can be collectively or individually and collectively screened, and are available with armoured tapes.

Thermocouples and RTD sensors are often compared as they perform in a similar fashion, but the two are designed differently so selection will depend on the individual application.