Causes of Shielded Thermocouple Measurement Errors

1. Contamination:

Contamination will affect the Seebeck coefficient of the thermocouple. The thermocouple wire material is often polluted by the surrounding atmosphere or impurities in the shield tube, and the additional potentials generated by different degrees of contamination are also different. This additional potential will alter the original scaling characteristics, which is a factor that makes the thermocouple indication unstable. . For example, platinum-rhodium 10-platinum thermocouple, when the ceramic tube used contains iron impurities, and the platinum-rhodium wire is contaminated with iron, it will affect its thermoelectric characteristics; when used in a high temperature reducing atmosphere containing silicon, due to the silicon, it is reduced to free silicon and combined with a platinum-rhodium wire to form a platinum-silicon compound, making the wire even brittle. Insulated tubesPorcelain ts used in the verification of standard thermocouples are requiredred to be cleaned with aqua regia, fired at high temperature, and the positive and negative electrode puncture polarity is specified. If the positive and negative poles of the thermal electrode are misplaced in the commonly used tube, the platinum in the rhodium platinum hole will seep into the platinum pole and change the thermoelectric characteristics of the standard thermocouple. All of the above conditions will affect the stability of the thermocouple.

2. Thermal electrodes volatilize at high temperatures:

Most thermocouple wire materials are alloys. Since the vapor pressure of each component material is different, the degree of volatilization is also different. , after being used at high temperature for a period of time, the composition ratio of the alloy will change, resulting in msignificant change in the thermoelectric potential.

3. Redox:

The instability of many thermocouples is caused by oxidation of the cable. Copper-constantan, iron-constantan, nickel-chromium-nickel, and other thermocouples can all undergo oxidation reactions. If the thermal electrode is uniformly oxidized, the impact may be less; if it has preferential oxidation, its impact is very serious. In low partial pressure of oxygen (i.e. in the case of lack of oxygen), the chromium of the nickel-chromium electrode will undergo preferential oxidation and change the composition of the even wire.

Fourth, Embrittlement:

Embrittlement is a common factor in thermocouple disposal. Thermal electrodes in thermocouples are caused by factors such as contamination, grain growth, redox reactions and long-term recrystallization at high temperatures,which lead to the embrittlement of the thermal electrodes. Thermodes are used in atomic reactors where they are bombarded by neutrons, where some elements mutate into other elements, changing the composition of the thermode. For example, the rhodium of the platinum-rhodium shielded thermocouple will change to palladium, and a small amount of platinum will change first to gold and then to mercury, which will change the thermoelectric characteristics and reduce the thermoelectric potential. And the base metal thermocouples, some have no obvious transmutation to neutron radiation, such as iron, nickel chromium, nickel aluminum (silicon). However, the transmutation of copper under radiation causes a great change in composition. Therefore, in the case of neutron radiation, it is more appropriate to use a nickel-chromium-nickel-aluminum thermocouple, and a nickel-chromium-silicon-nickel-silicon thermocouple (type N) is preferable.

Five, by external force:

Internal stress formed in the thermocouple wire by severe bending or other type of work hardening may produce physical inhomogeneity, operate with caution Most of the resulting inhomogeneity can be avoided. Proper annealing of the assembled thermocouples can also reduce this inhomogeneity to some extent.

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