The deterioration of platinum rhodium thermocouples and the service life of platinum rhodium thermocouples

The deterioration of platinum rhodium thermocouples and the service life of thermocouples

Degradation of Platinum Rhodium Thermocouples

The service life of platinum rhodium thermocouples is closely related to long-term operation. The deterioration of thermocouples refers to the phenomenon of aging and deterioration after use. 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, whose position or shape will also change, and it will also react with reducing or oxidizing gases in the surrounding environment. Along with the above changes, the thermoelectric potential of the thermocouple will also undergo extremely sensitive changes. Therefore, the degradation of thermocouples is inevitable.

The service life of platinum rhodium thermocouple

The degradation of platinum rhodium 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 during which the thermocouple deteriorates beyond the allowable error, or even breaks and cannot be used.

(1) The lifespan of prefabricated thermocouples

In our country's standards, only the stability of thermocouples is required. The change in thermoelectric potential before and after 200 hours of use at a certain temperature is specified. However, no regulations on the service life have been found yet. 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 type B, R, and S thermocouples, it is 2000h, while for type K, E, J, and T thermocouples, it is 10000h.

In practical use, prefabricated 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 determination 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 sleeve protection of armored thermocouples and their isolation from the external environment, the material of the sleeve has a significant impact on the lifespan of armored thermocouples. Therefore, thermocouple wires and metal sleeves must be selected according to their intended use. After the material is selected, its lifespan increases with the increase of the diameter of the armored thermocouple. Compared with prefabricated thermocouples, armored thermocouples have many advantages, but their service life is often lower.

summary

Platinum rhodium thermocouple is a commonly used temperature sensor in scientific research and production. Although its structure is simple, it can still produce significant measurement errors if not taken seriously during use. The author discusses in detail the main reasons for errors in temperature measurement points, insertion depth, response time, thermal radiation, and thermal impedance of thermocouples, in response to common problems that may arise during use. The author also points out precautions for the use of thermocouples such as heterogeneity, usage atmosphere, insulation resistance, selective oxidation of K-type thermocouples, K-state, and shunt errors of armored thermocouples. It is helpful in improving measurement accuracy and extending the service life of thermocouples.