If the gas used is pure and the system contains no oxygen, the service life of the thermocouple can be extended. However, if there is an oxide layer on the surface of the thermocouple wire, it can still provide enough oxygen for the selective oxidation of Cr. Therefore, when used in a non-oxidizing atmosphere, a clean, polished wire should be used. At the same time, the use of inert gases with trace amounts of oxygen or low oxygen partial pressure should be avoided as much as possible. When the length of the protective tube is relatively large (ie, the protective tube is very thin), due to poor air circulation, an oxygen-deficient state is formed, and the residual small amount of oxygen can still provide conditions for the selective oxidation of Cr.
Countermeasures In order to prevent or reduce the deterioration caused by the selective oxidation of K type thermocouples, besides improving the materials, measures should be taken on the thermocouple structure:
1 Select a metal with a stronger oxygen affinity than Cr as a getter, enclosed in a protective tube to prevent the selective oxidation of Cr, or increase the oxygen content by increasing the diameter of the protective tube or by blowing gas.
2 fabricated thermocouples. The service life is more than 12 months. (2) The use of the atmosphere affects the stability of the thermocouple. Due to the different operating temperatures and atmospheres, the maximum operating temperature for the same type of sensor, such as the K-type thermocouple, varies with the diameter. The K-type thermocouples with the same diameter also have very different stability due to their different structures. When selecting a thermocouple, the following points must be considered for the conditions of use:
1 Commonly used temperature and maximum use temperature 2 Oxygen reduction and other use of the atmosphere 3 Anti-vibration performance For the assembled thermocouple, the influence of the atmosphere first depends on the material of the protection tube and the structure of the thermocouple. Therefore, it is familiar with and masters various protection tube materials. Physical and chemical properties are necessary. For example: In the powder metallurgy industry, molybdenum tubes are commonly used as thermocouple protection tubes. In the 1600 °C H2 atmosphere, the use of better. However, the molybdenum tube is corroded by oxidation in a short time in an oxidizing atmosphere. Second, according to the use of atmosphere, choose a suitable thermocouple, in the oxidizing atmosphere above 1300 °C, choose platinum rhodium thermocouple, in the reducing, vacuum conditions using a tungsten-tungsten thermocouple better.
For type K thermocouples, it is suitable for working in atmospheres such as air, O2, etc., but when used in H2, the surface is reduced by H2 and has no effect for a short time. If it is exposed to H2 for a long time, it will be accelerated at the same time. The filament grows and breaks due to grain growth. In a reducing atmosphere such as CO or gas, the deterioration will be significantly accelerated and it will be worse.
For armored thermocouples, the atomic radius of hydrogen is so small that it easily enters the interior through the jacket, and it also deteriorates rapidly, resulting in a significant reduction in the thermoelectric value.
(3) Influence of insulation resistance Insulators for thermocouples, at high temperatures, their insulation resistance decreases rapidly with increasing temperature. Therefore, there will be a leakage current that flows into the instrument through the insulation whose insulation resistance has dropped so that The instrument indicates instability or measurement error, and it may also happen that the recorder is chaotic.
(1) Selective Oxidation For Fe-containing Ni-Cr alloys, if the oxygen partial pressure is lower than a certain value, then Cr, which has a high affinity with O2, will undergo selective oxidation, which is a characteristic grain boundary oxidation of the Ni-Cr alloy. If the outer surface oxide layer is observed with a microscope, green precipitates can be seen. This phenomenon is often referred to as "green etching." Especially when the temperature is in the range of 800°C to 1050°C, and the system contains CO, H2, and other reducing gases, the positive electrode of the K-type thermocouple is more prone to selective oxidation. This low thermoelectric potential due to reduced Cr content has become a limiting factor for the long-term use of K-type thermocouples in the heat treatment industry.