Deterioration of platinum-rhodium thermocouples and the service life of thermocouples The phenomenon of deterioration. Thermocouples made of metals or alloys gradually grow their internal grains at high temperatures. At the same time, the alloy contains a small amount of impurities, its position or shape will also change, and it will also react to reducing or oxidizing gases in the surrounding environment. With the above changes, the thermoelectromotive force of the thermocouple will also change extremely sensitively. Therefore, the deterioration of the thermocouple is inevitable. The service life of platinum-rhodium thermocouples The deterioration of platinum-rhodium thermocouples is a quantitative process, which is difficult to quantify. It will vary with the type, diameter, 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, or even when the wire is broken and cannot be used. (1) The lifespan of the assembled thermocouple In our country's standards, only the stability of the thermocouple is required. That is, it is stipulated that after 200h at a certain temperature, the change of thermoelectromotive force before and after use. However, no regulations have been found for service life. Japan's requirements for the service life of thermocouples are based on the continuous use time of thermocouples specified in the Japanese JIS (C-1602-1995) standard. For B, R, and S thermocouples, it is 2000h, and for K, E, J, and T thermocouples, it is 10000h. In actual use, the assembled thermocouple usually has a protective tube, and it is only used in a bare wire under special circumstances. Therefore, in most cases, the life of the protection tube determines the life of the thermocouple. The judgment of the actual service life of the thermocouple must be through long-term collection and accumulation of data in the actual use state, and it is possible to give a more accurate result [5]. (2) The life of armored thermocouples. Because armored thermocouples are protected by sleeves and isolated from the external environment, the material of the sleeve has a great influence on the life of armored thermocouples. The thermocouple wire and metal sleeve must be selected according to the application. When the material is selected, its life will increase as the diameter of the sheathed thermocouple increases. Compared with fabricated thermocouples, armored thermocouples have many advantages, but their service life is often lower than that of fabricated thermocouples. Summary The platinum-rhodium thermocouple is a very commonly used temperature sensor in scientific research and production. Although the structure is simple, carelessness in use will still produce large measurement errors. The author discussed in detail the selection of temperature measurement points, the insertion depth, response time, thermal radiation and thermal impedance of thermocouples, and pointed out the inhomogeneity of thermocouples, the use atmosphere, etc. Insulation resistance, selective oxidation of K-type thermocouple, K state and shunt error of armored thermocouple, etc. It is helpful to improve the measurement accuracy and extend the service life of the thermocouple.
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