Effect of K thermocouple wire heterogeneity

1) When the thermocouple is verified in the measuring chamber, the depth of the thermocouple inserted into the verification furnace is only 300mm according to the regulation requirements.So each thermocouple calibration results, specifically only or mainly reflected from the measuring end started to 300 mm long silk thermal behavior, however, when the length of the thermocouple is longer than most of wire at high temperature, if the thermocouple wire is homogeneous, then according to the rule of homogeneous circuit, has nothing to do with the length of the measurement results.However, thermocouple wire is not homogeneous, especially low metal thermocouple wire is not homogeneous, and in the situation with a temperature gradient, then its local thermoelectric emf will be generated, this electromotive force is called parasitic potential.Errors caused by parasitic potential are called heterogenous errors.In the existing precious metal, cheap metal thermocouple verification regulations, the thermocouple of the uneven quality has not made provisions, only in the thermocouple wire material standard, the thermocouple wire has certain requirements.The non-uniform thermoelectric electromotive force (EMF) was calculated by the first - and - last test for the low - metal thermocouple.Regular thermocouple wire production plant, according to the national standard requirements, the production of non-uniform thermoelectric emf meet the requirements of products.

2) thermocouple wire after using the heterogeneity of thermocouple for new system, even the uneven thermoelectric emfs can meet the requirements, however, repeated machining, bending the thermocouple produce distortion, will also lose homogeneity, and the use of thermal couple in long-term under high temperature will also change caused by accidentally silk degradation of thermoelectric emfs, for example: insert the thermocouple in industrial furnace, degradation will occur along accidentally wire length direction, and increased with the temperature, the degradation is enhanced, as part of the degradation in the temperature gradient, superposition of parasitic electromotive force will be generated in the total thermoelectric DongShiZhong measurement error occurs.In practice, the author found that some thermocouples have been certified by the measurement department of qualified products (mostly low metal thermocouples) to the field use but not qualified.After returning to the measurement department, the verification is still qualified, and the main reason is caused by the uneven even-wire.Thermocouple production technicians have experienced that the failure rate of thermocouple increases with the increase of its length.All are affected by the uneven quality of thermocouple wire.In short, the error caused by the heterogeneity, i.e. parasitic electromotive force, depends on the degree of heterogeneity and the temperature gradient of the thermocouple wire itself, which is extremely difficult to quantify.

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