Many companies areapplythermocoupleDuring the process, the impact of the environment on thermocouples is often ignored. Below are corresponding solutions for the impact of the environment on thermocouples. Enterprises can create their own solutions when choosing thermocouples, hoping to assist companies that use thermocouples.

1) Selective oxidation

Regarding Ni Cr alloys containing Fe, if the oxygen partial pressure is lower than a specific value, Cr with high affinity for O2 will undergo selective oxidation, which is the grain boundary oxidation of Ni Cr alloys. If the surface oxide layer is examined under a microscope, green precipitates can be seen, which is commonly referred to as "green corrosion". Especially when the temperature is within the range of 800 ℃ to 1050 ℃ and the system contains restorative gases such as CO and H2, the positive electrode of K-type thermocouples is more prone to selective oxidation. The low 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 used is very pure and the system does not contain oxygen, it can extend the service life of the thermocouple. However, if there is an oxide layer on the surface of the thermocouple wire, sufficient oxygen can still be provided for the selective oxidation of Cr. Therefore, when used in a non oxidizing atmosphere, clean and polished thermocouple wires should be used. At the same time, it should be avoided as much as possible to use it in inert gases with trace amounts of oxygen or air with low oxygen partial pressure. When the length to diameter ratio of the maintenance tube is large (i.e. the maintenance tube is very thin), due to poor air circulation, it forms an oxygen deficient state, and the remaining small amount of oxygen can still provide conditions for the selective oxidation of Cr.

2) Countermeasures for selective oxidation

To avoid or mitigateType K thermocoupleDue to selective oxidation causing deterioration, in addition to improving the material, corresponding measures should also be taken in the construction of thermocouples:

① Choose a metal with a stronger affinity for oxygen than Cr as the absorber and seal it inside the maintenance tube to prevent selective oxidation of Cr. Alternatively, increase the diameter of the maintenance tube or blow air to increase the oxygen content.

② Prefabricated thermocouple materialization. The product developed by the author - solid carburizing furnace thermocouple, which is an assembled thermocouple with a sealed structure, can avoid selective oxidation of Cr. It has been proven effective by more than ten enterprises such as Wazhou Group, FAW, FAW, FAW, Epson Industrial Furnace, Shenyang Heavy Duty, Shenyang Gear, Qianjiang Motorcycle, etc. after years of use. The service life is over 12 months, and users are very satisfied. 3) The stability of thermocouples is affected by the use of atmosphere. Due to different operating temperatures and atmospheres, the maximum operating temperature of the same sensor, such as K-type thermocouples, also varies with different diameters. K-type thermocouples with the same diameter also have significant differences in stability due to their different constructions. When choosing a thermocouple, it is necessary to consider the following points based on the application conditions: ① Common temperature and maximum operating temperature

② Atmosphere for oxidation recovery and other applications

③ Regarding the anti vibration performance of prefabricated thermocouples, the influence of the atmosphere first depends on the material of the maintenance tube and the structure of the thermocouple. Therefore, it is necessary to be familiar with and control the physical and chemical properties of various maintenance tube materials. For example, in the powder metallurgy industry, molybdenum tubes are commonly used as thermocouple maintenance tubes. The application effect is good in an H2 atmosphere at 1600 ℃. However, the molybdenum tube corrodes quickly due to oxidation in an oxidizing atmosphere. Secondly, suitable thermocouples should be selected based on the operating atmosphere, and in an oxidizing atmosphere above 1300 ℃, appropriate thermocouples should be chosenplatinum-rhodium thermocoupleIt is better to use tungsten rhenium thermocouples under restorative and vacuum conditions. Regarding K-type thermocouples, they are suitable for working in atmospheres such as air and O2. However, when used in H2, their appearance is restored by H2, which has no effect in a short period of time. If exposed to H2 for a long time, while accelerating the restoration, the thermocouple wire will experience grain growth and breakage; In a restorative atmosphere such as CO or gas, its degradation will significantly accelerate and exceed the standard. Regarding armored thermocouples, the atomic radius of hydrogen is very small, making it easy to penetrate through the outer shell and enter its interior. This will also accelerate degradation, resulting in a significant decrease in thermoelectric potential.

4) The influence of insulation resistance

　　thermocoupleUsing insulation materials, the insulation resistance rapidly decreases with the increase of temperature at high temperatures, resulting in leakage current. This current flows into the instrument through the insulation resistance that has dropped before, causing unstable instrument readings or measurement errors, and may also cause the recording instrument to randomly mark points.