The impact of thermocouple usage environment

For alloys containing Fe Ni, Cr with high affinity for O2 is selectively oxidized when the oxygen partial pressure is below a certain value, because Ni>; The unique grain boundary oxidation of Cr alloy. By observing the outer surface oxide layer under a microscope, green precipitates can be seen, which is generally referred to as“ Green Etching” The hyperbolic cosine value. Especially when the temperature is within the range of 1050 ℃ at 800 ℃, if the system contains reducing gases such as CO and H2, the positive electrode of the K-type thermocouple is more prone to selective oxidation. The decrease in thermoelectric potential caused by the decrease in Cr content has become a limiting factor for the long-term use of K-type thermocouples in the heat treatment industry.

If the gas is pure and the system does not contain oxygen, the lifespan of the thermocouple can be extended. However, if there is an oxide layer on the surface of the thermocouple core wire, it can provide oxygen for selective oxidation of Cr. Therefore, when used in a non oxidizing environment, please use clean and ground even numbered wires. Meanwhile, please try to avoid using in inert gases with trace amounts of oxygen and air with low oxygen partial pressure. When the length and diameter of the protective tube are large (i.e., the protective tube is thin), it becomes oxygen deficient due to poor air circulation, and a small amount of residual oxygen can still provide conditions for the selective oxidation of Cr.

(2) Countermeasures for selective oxidation

In order to prevent or mitigate the degradation caused by oxidation in the selection of K-type thermocouples, in addition to improving the material, measures should also be taken in the structure of the thermocouples

① Choose a metal with stronger affinity for oxygen than Cr as the absorber, seal it in a protective tube to prevent selective oxidation of Cr, increase the diameter of the protective tube, and also increase the oxygen content through jet injection.

② Prefabricated thermocouple materialization. The author has developed a thermocouple for solid carbon infiltration furnaces, which is a combination thermocouple with a sealed structure that can prevent selective oxidation of Cr. After years of use by more than ten enterprises including Wazi Group, FAW, FAW, Ibsen Industrial Furnace, Heavy Duty, countersunk, Qianjiang Motorcycle, etc., this solution has been proven to be effective. The lifespan exceeds 12 months and the user is satisfied.

(3) The impact of usage environment

The stability of thermocouples varies depending on the operating temperature and environment. The maximum operating temperature of sensors of the same type, such as K-type thermocouples, also varies according to their diameter. K-type thermocouples with the same diameter also have significant differences in stability depending on their structure. When choosing a thermocouple, the following points must be considered regarding the usage conditions

① Common temperature and highest usage temperature

② Oxidation reduction and other usage environments

③ Vibration resistance performance

In assembled thermocouples, the influence of atmosphere first depends on the material of the protective tube and the structure of the thermocouple, so it is necessary to be familiar with the physical and chemical properties of various protective tube materials. For example, in the powder metallurgy industry, molybdenum tubes are commonly used as thermocouple protection tubes. The use effect is very good in an H2 atmosphere at 1600 ℃. But molybdenum tubes will corrode in a short period of time due to oxidation in an oxidizing atmosphere. Secondly, choose the appropriate thermocouple according to the usage environment. Choose a platinum rhodium thermocouple in an oxidizing environment above 1300 ℃, and a tungsten rhenium thermocouple under reducing and vacuum conditions.

Regarding the K-type thermocouple, it is suitable for working in atmospheres such as air and O2. However, when used in H2, its surface is reduced to H2, which has no effect in the short term. When exposed to H2 for a long time, even numbered wire grains grow and break while accelerating the reduction process Under reducing atmospheres such as CO and gas, its degradation is significantly accelerated and extremely deteriorated.

In sheath thermocouples, the atomic radius of hydrogen is small, making it easy to enter the interior through the jacket and accelerate degradation, resulting in a significant decrease in thermoelectric potential.

(4) The influence of insulation resistance

At high temperatures, the insulation resistance of thermocouples decreases sharply with increasing temperature, resulting in leakage current. This current flows through the insulation material with decreased insulation resistance to the instrument, causing unstable readings or measurement errors. The instrument may be tampered with