The thermocouples that are qualified by analysis and verification are not qualified in use. This phenomenon is unknown and has not aroused people's attention. The unqualified phenomenon in the application of the thermocouple that caused the verification is mainly due to the influence of the inhomogeneity of the thermocouple wire, the shunt error of the armored thermocouple and the improper use of the thermocouple. The electrician learning network editor explains the mystery in this article.
The influence of the inhomogeneity of thermocouple wire ①The material of thermocouple is inhomogeneous. When the thermocouple is checked in the measuring room, according to the requirements of the regulations, the depth of insertion into the thermocouple verification furnace is 300mm. Therefore, the verification result of each thermocouple can only show or mainly show the 300nm long couple wire from the measurement end. The thermoelectric behavior. However, when the length of the thermocouple is long, most of the wires are in the high temperature area during use. If the thermocouple wire is inhomogeneous and is in a place with a temperature gradient, then part of it will generate thermoelectromotive force. This electromotive force is called parasitic potential, and the error caused by the parasitic potential is called homogeneous error.
The inhomogeneity of thermocouple wire after use. Regarding the newly-made thermocouple, even if the heterogeneous performance meets the requirements, the repeated processing and bending will cause the thermocouple to produce processing distortion, and it will lose its homogeneity. Moreover, the thermocouple will lose its homogeneity when it is used for a long time under high temperature. The deterioration of the thermal electromotive force caused the change. When part of the deterioration is locally in a place with a temperature gradient, it will also produce a parasitic electric potential superimposed on the total thermoelectromotive force and present a measurement error.