Selective oxidation
For the Ni-Cr alloy containing Fe, if the oxygen partial pressure is lower than the specific value, the Cr with high affinity with O2 will undergo selective oxidation, which is the unique grain boundary oxidation of Ni-Cr alloy. When you look at the oxide layer on the surface under a microscope, you can see the green precipitates, a phenomenon known as "Green Erosion." This low thermoelectric potential caused by reduced Cr content has become a limiting factor for the long-term application of k-type thermocouple in the heat treatment industry.
If the gas used is pure and does not contain oxygen in the system, the working life of the thermocouple can be extended. However, if there is an oxide layer on the outer surface of the thermocouple wire, sufficient oxygen can be provided for selective oxidation of Cr. Therefore, clean and polished wires should be used in non-oxidizing atmosphere. At the same time, the use of inert gases with little oxygen or low oxygen partial pressure should be prevented as far as possible. When the length of the maintenance tube is relatively large, the oxygen remaining in the tube can still provide conditions for selective oxidation of Cr due to poor air circulation.
Strategies for selective oxidation
In order to avoid or slow down the deterioration caused by selective oxidation of the k-type thermocouple, corresponding measures should be taken on the construction of thermocouple in addition to improving the material.
*The metal with more affinity to oxygen than Cr was selected as the getter and sealed in the maintenance tube to avoid selective oxidation of Cr. It is also possible to increase the oxygen content by increasing the diameter of the maintenance tube or by blowing air.
*Materialize the assembled thermocouple.
The effect of applying environment
The maximum application temperature of thermocouple stability to the same sensor, such as k-type thermocouple, also varies with diameter due to different application temperature and environment, and the K type thermocouple with the same diameter also varies greatly in stability due to their different structures. When selecting a thermocouple, the following points must be considered for application conditions.
*Common temperature and maximum operating temperature;
*Applied environment such as oxidation restoration
*Resistance to vibration.
The effect of insulation resistance
At high temperature, the insulation resistance of the thermocouple decreases rapidly with the rise of temperature, so there will be leakage current. The current will flow into the instrument through the insulation that has fallen through the insulation resistance, making the instrument's indication unstable or producing measurement error, and possibly breaking the mark of the recording instrument.
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