Thermocouple instability

The source of thermocouple instability factors: 1. Dirt and dirt affect the Seebeck coefficient of the thermocouple. The single-wire material of the thermocouple is often polluted by impurities in the ambient atmosphere and protective tube. The additional potential generated by a certain degree of pollution is also different. The additional potential The change in the original indexing characteristics is one of the reasons why the displayed value of the thermocouple becomes unstable. For example, the ceramic tube used in the platinum-rhodium 10-platinum thermocouple contains iron impurities, and when the platinum-rhodium wire is contaminated with iron, it will affect the thermoelectric characteristics. When used in a high-temperature reducing atmosphere containing silicon, silicon is reduced to free silicon, so it is synthesized with platinum-rhodium wire to form a platinum-silicon compound, and the coupled wire becomes brittle. The insulating tubes used in the calibration of the reference thermocouple are required to be cleaned with aqua regia, fired at high temperature, and the perforation polarities of the positive and negative electrodes are specified. If the positive and negative of the thermode are wrong with the usual tube, the platinum in the platinum-rhodium hole will penetrate into the platinum electrode, changing the thermoelectric characteristics of the standard thermocouple. These conditions can affect the stability of the thermocouple. 2. Most of the coupler materials of high temperature volatile thermode thermocouples are alloy materials. Due to the different vapor pressures of each component material, the degree of volatilization is also different. After a certain period of use at high temperature, the proportion of alloy components will change, and the thermoelectric potential will change. Significant changes occurred. 3. Redox The instability of many thermocouples is caused by the oxidation of the filaments. Oxidation reactions occur in thermocouples such as copper-constantan, iron-constantan, and nickel-chromium-nickel-silicon. The uniform oxidation of the hot electrode may have little effect. If there is a preference for oxidation, the effects are severe. Under low oxygen partial pressure (ie, in the absence of oxygen), the chromium in the nickel-chromium electrode undergoes preferential oxidation, which changes the combined composition of the coupling. 4. Embrittlement Embrittlement is a common factor in the scrapping of thermocouples. The hot electrode of the thermocouple is the cause of the embrittlement of the hot electrode due to fouling, grain growth, the occurrence of redox reactions, and recrystallization under long-term high temperature. Thermoelectrodes are used in reactors and are bombarded with neutrons, some of which change into other elements, changing the thermode's composition. The rhodium of the sheathed platinum-rhodium thermocouple becomes palladium, and a small amount of platinum first becomes gold and then becomes mercury, the thermoelectric characteristics change, and the thermoelectric potential becomes smaller. The change of neutron radiation of low-cost metal thermocouples to iron, nickel-chromium, nickel-aluminum (silicon), etc. is not clear. But copper caused a large compositional change under radiation. Therefore, in the case of neutron irradiation, a nickel-chromium-nickel-aluminum thermocouple is preferably used, and a nickel-chromium-nickel silicon (n-type) thermocouple is more preferable. 5. Vigorous bending and any other type of work hardening by external forces can create physical inhomogeneities in the internal stresses that develop on the thermocouple wire harness, and careful operation can avoid most of the resulting inhomogeneities. Assembled thermocouples can reduce this inhomogeneity to some extent, even with proper annealing.

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