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Using environment impact on the thermocouple

[2011-11-22]

1.      Using environment
Thermocouple stability, the use of temperature, atmosphere, different different, the same kind of sensors, such as K-type thermocouple temperature is also due to the maximum diameter varies, the same diameter K-type thermocouple is also due to the structure of different, its stability is also very different. In the choice of thermocouple, for conditions of use must consider the following points:
(1) commonly used temperature and maximum temperature
(2) oxidation-reduction atmosphere such as the use
(3) anti-vibration performance
For the thermocouple assembly is concerned, the impact of the atmosphere, depends primarily on the thermocouple protection tube material and structure, therefore, familiar with and master a variety of pipe materials to protect the physical and chemical properties is necessary. For example: In the powder metallurgy industry, commonly used as a thermocouple protection tube molybdenum tube. The H2 atmosphere at 1600 ℃, the use of better. However, molybdenum tube in oxidizing atmosphere, the very short time due to oxidation and erosion. Secondly, the atmosphere should be based on use, select the appropriate thermocouple, at 1300 ℃ above oxidizing atmosphere, choose platinum rhodium thermocouple, in reducing, vacuum tungsten rhenium thermocouples better.
For K-type thermocouple, suitable for air, O2 and other atmosphere, but when used in the H2, H2 reduction of its surface is, no short-term impact, if prolonged exposure to H2, the reduction in speed at the same time, grain growth will occur even while the broken wire; in the CO or other reducing gas atmosphere, the degradation will significantly speed up the ultra-poor.
For sheathed thermocouples, the hydrogen atom radius is small, easy to access through its internal coat, the same will also accelerate deterioration, resulting in a significant reduction in the value of thermoelectric power.
2.      The impact of insulation resistance
Thermocouples with insulating material, at high temperatures, the insulation resistance and abrupt decrease with increasing temperature, therefore, the leakage currents, the current through the insulation resistance has fallen into the instrument of logistics, the gauge indicates unstable or produce measurement error may also occur recorder Cookin point phenomenon.