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Home> Industry Information> 4 reasons affecting the thermocouple

4 reasons affecting the thermocouple

April 24, 2019

(1) Selective oxidation

For Fe-containing Ni-Cr alloys, if the partial pressure of oxygen is below a certain value, selective oxidation occurs with Cr, which has the same affinity as O2, and is a grain boundary oxidation characteristic of Ni-Cr alloys. Such as the microscope to observe the outer surface of the oxide layer, you can see the green precipitate, a phenomenon commonly known as "green eclipse." Especially when the temperature in the range of 800 ℃ -1050 ℃, the system contains CO, H2 and other reducing gases, K-type thermocouple positive, more prone to selective oxidation. This low level of Cr caused by the thermoelectric power is low, has become the K-type thermocouple in the heat treatment industry long-term use of the limiting factor.

If the gas used is very pure, and the system does not contain oxygen, you can extend the life of the thermocouple. However, if the surface of the thermocouple wire has an oxide layer, sufficient oxygen can still be provided for the selective oxidation of Cr. Therefore, when used in a non-oxidizing atmosphere, clean, polished wire should be used. At the same time, should be avoided as far as possible with a small amount of oxygen in the inert gas or oxygen partial pressure is low air use. When the length of the protective tube is relatively large (that is, the protective tube is very thin), the remaining small amount of oxygen remains available for the selective oxidation of Cr due to the poor circulation of air and the formation of anoxic conditions.

(2) Countermeasures for selective oxidation

In order to prevent or slow down the K-type thermocouple due to selective oxidation caused by deterioration, in addition to material to be improved, but also the thermocouple structure should take appropriate measures:

① select the oxygen affinity stronger than Cr metal as a getter, sealed in a protective tube to prevent the selective oxidation of Cr, you can also increase the diameter of the protection tube or inflatable method to increase the oxygen content.

② assembly-type thermocouples entity. The patented product developed by the author, a solid type carburizing thermocouple, has been developed as a fabricated thermocouple with a sealed structure to prevent the selective oxidation of Cr. The Wazhou Group, FAW, FAW, Ipsen industrial furnaces , Heavy, Shen tooth, such as Qianjiang Motorcycle use more than a dozen years of evidence that this program is valid. Service life of 12 months or more, the user is satisfied

(3) the impact of the use of the atmosphere

Thermocouple stability, due to the use of temperature, the atmosphere varies, the same sensor, such as K-type maximum temperature also varies with diameter, K-type thermocouple of the same diameter due to structural differences, its stability There are also big differences. In the choice of thermocouple, you must consider the conditions for the use of the following points:


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