What are the reasons for the unqualified use of thermocouples?

Thermocouples can directly measure temperature and convert the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium. Thermocouples are often used as temperature measuring components in furnace temperature testers, so the correct use of thermocouples is very important, as it affects the changes in temperature curves. The phenomenon of unqualified thermocouples during use is mainly caused by the influence of heterogeneous thermocouple wires, shunt errors of armored thermocouples, and improper use of thermocouples;
1. The influence of thermocouple wire heterogeneity

① The thermocouple material itself is heterogeneous. When thermocouples are calibrated in the metrology room, they should be inserted into the thermocouple calibration furnace at a depth of 300mm according to the regulations. Therefore, the calibration results of each thermocouple can only reflect or mainly reflect the thermoelectric behavior of a 300mm long thermocouple wire from the measuring end. However, when the length of the thermocouple is long and most of the thermocouple wires are in the high temperature zone during use, if the thermocouple wires are heterogeneous and in an environment with temperature gradients, they will generate thermoelectric potential locally. The electromotive force is called parasitic potential, and the error caused by parasitic potential is called homogenization error.

② The heterogeneity generated by the use of thermocouple wires. For newly manufactured thermocouples, even if the heterogeneous performance meets the requirements, repeated processing and bending can cause processing distortion and loss of homogeneity. In addition, thermocouples that are exposed to high temperatures for a long time during use can also cause changes in thermoelectric potential due to wire degradation. When the locally degraded part is located in a location with a temperature gradient, parasitic potentials will also be generated and added to the total thermoelectric potential, resulting in measurement errors.

2. The shunt error of armored thermocouples

When measuring furnace temperature with armored thermocouples, if the temperature in the middle of the thermocouple exceeds 800 ℃, it will cause a decrease in insulation resistance and abnormal readings of the thermocouple, which is called shunt error. Due to the insulation material of the armored thermocouple being powdered magnesium oxide, its insulation resistance decreases by an order of magnitude for every 100 ℃ increase in temperature at high temperatures. When the temperature in the middle part is high, there will inevitably be leakage current, resulting in shunt errors in the output potential of the thermocouple.

3. Measurement errors caused by improper use

A thermocouple that has been verified as qualified, if used incorrectly, can also cause significant measurement errors.

① The insertion depth of the thermocouple is insufficient, resulting in low temperature measurement due to thermal conductivity loss;

② Improper handling of the reference temperature, such as flame spraying from the furnace causing the reference temperature to be too high, can also lead to measurement errors.