Thermocouple degradation and service life

The service life of thermocouples is related to their deterioration, which refers to the phenomenon of aging and deterioration of thermocouples after use. Thermocouples made of metals or alloys have internal grains that gradually grow at high temperatures. At the same time, the alloy contains a small amount of impurities, and its position or shape will also change, and it will react with reducing or oxidizing gases in the surrounding environment. Along with the above changes, the thermoelectric electromotive force of the thermocouple will also be extremely sensitive to changes. Therefore, the degradation of thermocouples is inevitable.
The service life of thermocouples
The degradation of thermocouples is a quantitative process that is difficult to quantify and will vary with the type, diameter, operating temperature, atmosphere, and time of the thermocouple. The service life of a thermocouple refers to the time when the deterioration of the thermocouple exceeds the allowable error, and even when the wire is broken and cannot be used.
(1) The lifespan of assembled thermocouples
The stability of thermocouples is only required in Chinese standards. The change in thermoelectric electromotive force before and after use is specified at a certain temperature for 200 hours. However, no regulations have been found regarding the service life. The requirements for the service life of thermocouples in Japan are based on the continuous use time of thermocouples specified in the Japanese JIS (C-1602-1995) standard. For B, R, and S type thermocouples, it is 2000h, while for K, E, J, and T type thermocouples, it is 10000h.
In practical use, assembled thermocouples usually have protective tubes and are only used with bare wires in special circumstances. Therefore, in most cases, the lifespan of the protective tube determines the lifespan of the thermocouple. The assessment of the actual service life of thermocouples must be based on long-term collection and accumulation of data under actual usage conditions in order to provide more accurate results [5].
(2) The lifespan of armored thermocouples
Due to the insulation of armored thermocouples from the external environment through sleeve protection, the sleeve material has an impact on the lifespan of armored thermocouples
The noise is loud, and thermocouple wires and metal sleeves must be selected according to the purpose. When the material is selected, its lifespan increases with the increase of the diameter of the armored thermocouple. Compared with assembled thermocouples, armored thermocouples have many advantages, but their service life is often lower than that of assembled thermocouples.
5. Conclusion
Thermocouples are the most commonly used temperature sensors in scientific research and production. Although their structure is simple, careless use can still result in significant measurement errors. The author discusses in detail the main reasons for errors in the selection of temperature measurement points, insertion depth, response time, thermal radiation, and thermal impedance of thermocouples in response to common issues that may arise during use. The author also points out the precautions to be taken in the use of thermocouples, such as heterogeneity, atmosphere, insulation resistance, selective oxidation of K-type thermocouples, K-state, and shunt errors in armored thermocouples. It is helpful for improving measurement accuracy and extending the service life of thermocouples.