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Thermocouples

General

Thermocouples are active contact sensors that find wide application in industrial measuring and control technology because of their small structural shape and a very large temperature range from -200°C to +1700°C (or 2300°C). They are especially useful in the construction of systems, machines and devices. They are also used in molten salt and metal treatment. In many high temperature applications they are a cost saving alternative to radiation meters.

Application

To maintain high measurement accuracy of thermocouples over long periods of time, it is necessary to select them carefully for their intended use. Two factors are very important: maximum temperature and the  medium in which they have to function.

NiCr-Ni thermocouple (type K) is, for example, is highly resistant to oxidizing agents, but if it is heated to over 850°C then its thermal electric properties change in such a way that even at low temperatures (< 100°C) there are residual deviations. 

Thermal pairs of types J and L, which are made from  Fe-CuNi, are threatened by oxidizing agents at above 550°C, but are highly resistant to reducing agents. The temperature limit is about 700°C. The same is valid for Cu-CuNi thermocouples which, however, are only suitable for maximum temperatures in the range 400 - 600°C.

For very high temperatures ranging up to 1700°C, only thermocouples with platinum or its alloys are suitable (types S, R and B). All platinum rhodium - platinum thermocouples are sensitive to sulphuric and phosphoric gases.

A general problem with unprotected thermocouples is that at temperatures above 1000°C, foreign materials and metal vapours can diffuse into them and cause a change in their thermal electric properties.

For these reasons, protective procedures must be taken dependent on the atmosphere, on the temperature range and the thermocouple material. For example, the thermocouples could be enclosed in a gas-tight ceramic or metallic protective tube or sheaths (also named thermowell).

Functional Principle

The temperature to be measured is transformed by a thermocouple into an electrical potential without the support of any auxiliary voltage source.

Combining two metals with thermal energy as different as possible into a thermocouple, a potential arises between the free ends, which depends on the difference between the thermal energies of the two metals.

The norms DIN 43 710 and IEC 60584 specify the most important and frequently used combinations of materials with their temperature - potential properties.

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