What is a thermocouple? How do they work?

A thermocouple is a device for measuring temperature. It comprises two dissimilar metallic wires joined together to form a junction. When the junction is heated or cooled, a small voltage is generated in the electrical circuit of the thermocouple which can be measured, and this corresponds to temperature.

In theory, any two metals can be used to make a thermocouple but in practise, there are a fixed number of types that are commonly used. They have been developed to give improved linearity and accuracy and comprise specially developed alloys.

Thermocouples can be made to suit almost any application. They can be made to be robust, fast responding and to measure a very wide temperature range.

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Thermocouple use

What does a thermocouple do?

A thermocouple is simply a sensor that is used for measuring temperature. This design of sensor consists of two dissimilar metal wires which are joined together at one end, connected to an instrument that is capable of accepting a thermocouple input and measure the reading. Thermocouples can provide temperature measurements over a wide spectrum of temperatures dependant on which thermocouple type you use.

A front and back diagram to show what a thermocouple is and explain how it measures temperatures using two metallic wires.

Thermocouple diagram

How it works

How does a thermocouple work?

Thermocouples have been developed as a result of the discovery of the Seebeck effect, by Thomas J. Seebeck in 1821.

Two dissimilar metals are joined together at both ends in an electrical circuit. One “junction” is the measuring junction or “hot end”. The other is the reference junction or “cold end”. A sensitive voltmeter is connected into one of the conductors.

Under laboratory conditions the reference junction would be held at a known temperature, usually 0°C but in normal industrial practise the junction is left at ambient temperature and an external sensor is used to compensate for this variation (known as cold junction compensation, usually a thermistor bead is used to measure the ambient temperature).

Quite simply as the temperature rises or falls at the measuring junction a voltage is generated within the circuit which correlates directly to temperature and can easily be converted by reference to the appropriate tables.

Thermocouple probe

What is a thermocouple probe?

What is meant by a thermocouple? Now we have an understanding of how a thermocouple works and what thermocouples are, one of the popular questions is what is a thermocouple probe?

A thermocouple probe is the design of the sensor that it is made in. Whether the sensor is a thermocouple type K, J, T, N all of these thermocouple types can be manufactured in the same size tube, housing or construction. To get an idea of the typical design available check out some of our thermocouples sensor range.

What does a thermocouple look like? Thermocouples look different due to the constructions they are manufactured in. Although a thermocouple itself is two bare wires joined together, these two wires can be put inside various constructions to protect them so they last longer.

Type K Thermocouple

What is a type K thermocouple?

A popular question asked is what is a type K thermocouple?

A type K Thermocouple is made up of two dissimilar metals which are Nickel-Chromium / Nickel-Alumel. Type K is the most popular type of thermocouple because it is inexpensive, accurate, reliable (depending on the design used for your application) and covers a wide temperature range.

Type K thermocouples can be found in a broad range of applications due to its expansive temperature range capabilities. The maximum continuous temperature is up to around 1100 Deg C.

Thermocouples with a plug or cable can be identified by their colour coding. In this case type K is green. If you have a cable the green leg is positive and the white leg is negative.

A Process Parameters Type K thermocouple made from Nickel-Chromium or Nickel-Alumel with green casing.

Type K Thermocouple

Type J Thermocouple

What is a type J thermocouple?

So what is a type J thermocouple? Type J Thermocouples are also very common. It has a smaller temperature range than a type K thermocouples with a range of 0 to 600 Deg C. The Type J is made up of two dissimilar metals which are Iron / Copper-Nickel (Also known as Constantin). Cost-wise they are very similar to type K.

One of the most popular applications you find type J being used is in the plastics industry.

Thermocouples with a plug or cable can be identified by their colour coding. In this case type J is Black. If you have a cable, the black leg is positive and the white leg is negative.

A Process Parameters type J thermocouple made from Iron / Copper-Nickel (aka Constantin) with black casing.

Type J Thermocouple

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