Thermocouples are widely used in industrial temperature measurement because of their simple structure, easy manufacturing, convenient measurement, etc., but in order to get the correct measurement results, we must understand the characteristics of thermocouples and use them correctly, otherwise they will bring great errors in measurement. The following is an explanation of the factors that affect the instability of thermocouple measurement from several aspects.
1 Effect of unstable thermoelectric properties of thermocouples
1.1 Effects of contamination and stress
At a to > 0 °C, the thermal electromotive force of the thermocouple reduces the EAB(tn,to) and thus reduces the value of the measuring instrument. Therefore, when the reference temperature is not equal to 0°C, it has a very important impact on the accuracy of the measured temperature.
In engineering measurements, the reference temperature is mostly at room temperature or in the fluctuating temperature zone, in which case the actual temperature is measured. Measures such as correction or compensation must be taken.
There are several ways to make up the method: thermoelectric potential correction method, temperature correction method, instrument adjustment starting point method, compensation wire method, and reference end temperature compensator. can achieve corrected measurement results.
When a thermocouple is inserted into the medium to be measured (such as an air stream), it absorbs heat from the medium to increase its own temperature, and at the same time it dissipates heat to the place where the temperature is low by thermal radiation and heat conduction. Due to the heat transfer effect of the thermocouple, the thermocouple cannot reach the temperature that it should achieve if it absorbs heat. After a certain amount of time, dynamic equilibrium is reached when the heat lost outward from the measuring end is equal to the heat absorbed from the gas stream, at which point the thermocouple reaches a stable indication. However, this value does not represent the true temperature of the air flow, because the heat lost by the environment at the measuring end is compensated by the heating of the air flow, that is, the stronger the heat transfer between the measuring end and the environment, the greater the deviation of the temperature from the temperature of the air flow. In this case, if the indicated temperature T of the thermocouple is still used to represent the gas flow temperature T, it is bound to introduce a (T-T-couple) error, i.e., a tensile error.