The correct use of thermocouples can not only accurately obtain the temperature value and ensure that the products are qualified, but also save the material consumption of thermocouples, save money and ensure the product quality. Incorrect installation, thermal conductivity and time lag are the main errors of thermocouples in use.
1. Error caused by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the real temperature of the furnace. In other words, the thermocouple should not be installed too close to the door and heating, and the insertion depth should be at least 8 ~ 10 times the diameter of the protective tube; The space between the protective sleeve and the wall of the thermocouple is not filled with insulating materials, resulting in heat overflow or cold air intrusion in the furnace. Therefore, the gap between the protective sleeve of the thermocouple and the hole of the furnace wall shall be blocked with insulating materials such as refractory mud or asbestos rope, so as to prevent the convection of cold and hot air from affecting the accuracy of temperature measurement; The cold end of the thermocouple is too close to the furnace body to make the temperature exceed 100 ℃; The installation of thermocouples should avoid strong magnetic field and strong electric field as far as possible, so thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors; The thermocouple cannot be installed in the area where the measured medium rarely flows. When the thermocouple is used to measure the gas temperature in the pipe, the thermocouple must be installed against the flow direction and fully in contact with the gas.
2. Error caused by insulation deterioration
For example, if the thermocouple is insulated, there is too much dirt or salt slag on the protective tube and pull plate, resulting in poor insulation between the thermoelectric dipole and the furnace wall, which is more serious at high temperature, which will not only cause the loss of thermal potential, but also introduce interference.
3. Error introduced by thermal inertia
Due to the thermal inertia of the thermocouple, the indicated value of the instrument lags behind the change of the measured temperature, which is particularly prominent in the rapid measurement. Therefore, thermocouples with thin Thermoelectrode and small diameter of protective tube should be used as far as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation. The greater the measurement lag, the smaller the amplitude of thermocouple fluctuation and the greater the difference from the actual furnace temperature. When thermocouple with large time constant is used for temperature measurement or control, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to measure the temperature accurately, the thermocouple with small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and is directly proportional to the diameter of the hot end of the thermocouple, the density of the material and the specific heat. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end. In use, materials with good thermal conductivity and protective sleeves with thin pipe wall and small inner diameter are usually used. In the more precise temperature measurement, the bare wire thermocouple without protective sleeve is used, but the thermocouple is easy to be damaged and should be corrected and replaced in time.
4. Thermal resistance error
At high temperature, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance increases and hinders heat conduction. At this time, the temperature indication is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce the error.
