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The application of thermocouple in life

[2012-02-19]

Thermocouple degradation:
 Thermocouple on the life of their deterioration, deterioration of the thermocouple so that by the use of thermocouples, the emergence of the phenomenon of aging deterioration. Constituted by a metal or alloy thermocouple, at high temperatures to gradually grow its internal grain. Also contains a small amount of impurities in the alloy, its location or shape will also change, and the surrounding environment should be restored or oxidizing gas react. With these changes, the thermocouple thermal emf will also be extremely sensitive to change. Therefore, the phenomenon of thermocouple deterioration is inevitable.
Thermocouple life:
Thermocouple degradation is a quantitative process, its very difficult to quantify, will vary with the type of thermocouple diameter atmosphere temperature, time varies. The life of the thermocouple deterioration is developed to exceed the allowable error, and even break time can not be used.
1.    Life of thermocouple assembly
China standard only requires the stability of the thermocouple. Which provides that in a certain temperature by 200h, using the before and after the thermal electromotive force changes. However, life has not been found to have provided. The requirements of the thermocouple life in Japan, is based on the Japanese JIS (C-1602-1995) standard for the continuous use of the thermocouple time. The B.S.R -type thermocouple in terms of the 2000h, K. E .T-type thermocouple J 10000h.
In actual use, there is usually fabricated thermocouple protection tube, and only in exceptional circumstances only use the bare wire. Therefore, in most cases, the decision to protect the life of the thermocouple tube life. The actual life of the thermocouple to determine, to be accumulated through long-term collection ﹑ actual state of the data, be possible to give more accurate results [5].
2.    The life of sheathed thermocouple
As the armored thermocouple protection tube and cut off from the outside world, so the casing material on the life of sheathed thermocouple great influence, must be selected at the thermocouple wire and metal casing. When the material is selected, its life and with the increase in diameter sheathed thermocouple increase. Sheathed thermocouple compared with the thermocouple assembly, although a number of advantages, but life is often less than the assembly-type thermocouple.
R & D, Manufacturing and the thermocouple is the most commonly used temperature sensors, although the structure is simple, but still will not pay attention to the use of a greater measurement error. Authors for use in easy problems, a detailed discussion of temperature measurement point selection, the insertion depth of the thermocouple response time heat radiation and heat resistance, etc. The main reason of error, and that the use of thermocouples is not homogeneous atmosphere insulation resistance, K-type thermocouple selective oxidation K Thermocouple diversion status and error so use precautions. To improve the measurement accuracy of some help to extend the life of the thermocouple.