Thermocouples

General

Thermocouples are sensors for measuring temperature. The main components of thermocouples are a thermocouple, which generates the so-called thermoelectric voltage, and a connection part, such as a plug. Thermocouples are used in many areas of temperature measurement. They are characterized by a high degree of design flexibility and a fast response.

Overview of thermocouples

The following thermocouples are defined in ISO EN 60584-1:

Various combinations of metals/alloys have proven to be advantageous because they have an almost linear voltage curve with temperature.

It is important to know that the thermoelectric voltage does not occur at the peak. The voltage occurs between the two wires. Most of the voltage occurs in the area of the greatest temperature difference, for example in the transition from the furnace chamber to the insulation.

Thermoelectric voltage

The thermocouple consists of two wires made of different alloys. Each of these wires develops a certain voltage as soon as one end is warmer than the other. If you now connect both wires, the difference in the respective voltage in the wire can be measured. This difference is called thermoelectric voltage.

How exactly does thermoelectric voltage arise?

There are free electrons in every electrical conductor. They can move in the conductor and thus enable an electric current to flow, for example.

In physical terms, these free electrons take up a certain amount of space, which increases with the temperature.

In a uniformly heated copper conductor, this space is approximately the same size. This is shown schematically in Figure 1.

Now heat one end of the conductor.

This increases the space for the free electrons to move in this area. Due to the effect that like charges repel each other, the more heated electrons "push" the rest towards the cold end.

As a result, there is a shortage of electrons at the heated end and a surplus at the cold end. This creates a voltage.

Important to know: The level of the voltage is mainly influenced by the chemical composition of the wire and the temperature difference!

However, no signal can be generated with the single voltage of the wire. Therefore, a second wire is required. Both wires should be connected at the point with the highest temperature.

The voltage difference between the two wires serves as a signal and is defined as thermoelectric voltage.