How do thermocouples work?

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Thermocouples belong to a large group of thermometers. Simplified described, a thermocouple is made of two simple wires. They are mainly used for industrial measurements in large quantities. They are most often used at high temperatures.

But how can you measure temperatures with two wires?

eXacal Präzisions-Thermoelement

eXacal precision thermocouple

 

Principle of the Seebeck Effect

Thermocouples are based on the effect described by Seebeck 1821 that a small current circulates when two different wires are connected together and there is a temperature difference at their contact points.

The connected wires are called thermocouple. This effect can be explained by a schematic diagram of an electrical circuit:

The right example shows a neutral wire. The temperature T1 on the left side corresponds to the temperature T2 on the right side. The red small circles are supposed to represent atoms between which free electrons move. These electrons are shown as green dots in the example. The wire is in a stable condition. Because of the same temperature the free electrons are equally distributed in the conductor.

In the example on the left, heat is applied to the left side (T1) of the conductor +Q. The temperature at point T1 is now higher than at point T2. The supplied energy on the left side of the wire increases the speed of the free electrons. Due to the increase in speed, the free electrons oscillate more strongly and are displaced to the right end of the wire. At the point T2, where the temperature is lower, a charge concentration is created. This centre of charge is always on the cold side of the wire.

Although this effect is temperature-dependent, it does not yet enable reproducible temperature measurement. However, if two different metals, in which the electrons behave differently, are brought together, a charge difference can occur.

The example of a type N thermocouple (NiCrSi / NiSi) can be imagined as follows:

Here is important:

  • the wires must be connected
  • there must be a temperature difference between T1 and T2

In the example, heat +Q is supplied at measuring point T1. The temperature T2 is now lower than T1.
Since the two conductors are not made of the same material, different charge concentrations arise. The resulting thermoelectric voltage can be measured with a voltmeter V.

This voltage depends on the temperature and is well reproducible. The construction shown is suitable for measuring temperatures and corresponds to a simple thermocouple.