Thermocouple degradation and service life

The service life of the thermocouple and its degradation, degradation of thermocouple, thermocouple after use, namely, the phenomenon of the aging metamorphism. Made of metal or alloy, thermocouple, its internal grain to grow at high temperature. Alloy containing a small amount of impurities at the same time, its position or shape will change, and, on the surrounding environment of the oxidation reduction or gas will react. With these changes, the thermocouple thermoelectric emf will also be extremely sensitive to change. So the thermocouple degradation phenomenon is inevitable.

the service life of the thermocouple

the deteriorating is a quantitative change process of the thermocouple, it is difficult for its quantitative, will vary depending on the type of thermocouple, diameter, temperature, atmosphere, the varies of time. The service life of the thermocouple is refers to the thermocouple degradation to exceed the permitted error, even break time should not be used.

( 1) Prefabricated thermocouple life

is only for the stability of the thermocouple in our standard requirements. At a certain temperature is to be fixed after 200 h, thermoelectric emfs changes before and after use. But, have not found concerning the service life. The requirements of Japan on the service life of the thermocouple, according to Japan's JIS ( C - 1602 - 1995). The thermocouple continuous use time specified in the standard. For B, R, S type thermocouple for 2000 h, K, E, J, T thermocouple for 10000 h.

in the actual use, prefabricated thermocouple protection tube, there are usually only under special circumstances are the bare wire. In most cases, therefore, to protect the life of the tube determines the life of thermocouple. The actual service life of judgment of thermocouple, must be through long-term data collection, accumulation of actual service condition, may give more accurate results [ 5] 。

( 2) Armoured thermocouple life

because of armoured thermocouple casing protection and isolated from the external environment, therefore the life of the tube material of armoured thermocouple shadow

ring is very big, must choose according to use thermocouple wire and metal casing. When the material is selected, its life and increases with the increase of diameter of armoured thermocouple. Armoured thermocouple compared with prefabricated thermocouple, although have many advantages, but life often below fabricated thermocouple.

5。 Conclusion

thermocouple is the most commonly used temperature sensor of scientific research, production, although the structure is simple, however, do not pay attention to is still in use will produce larger error of measurement. The author in view of the problems in use, the choice of temperature measurement points are discussed in detail, the thermocouple insertion depth, response time, thermal radiation and thermal impedance, the main reasons for causing errors and points out that the thermocouple heterogeneity, using atmosphere, insulation resistance, the K type thermocouple selective oxidation, K status and armoured thermocouple shunt error of matters needing attention in use, etc. To improve the measurement precision and prolong the service life of the thermocouple has certain help.