Analyzing the Causes of Thermocouple Measurement Errors

　　The correct use of thermocouples can not only accurately obtain the temperature value and ensure the product quality, but also save the material consumption of thermocouples, saving money and ensuring product quality. If the installation is incorrect, errors such as thermal conductivity and time lag will occur, which are the main errors in the use of thermocouples.

　　1. Errors introduced by improper installation

　　Thermocouples should not be installed too close to doors and heating areas. The insertion depth should be at least 8 to 10 times the diameter of the protection tube. The installation position and insertion depth cannot reflect the actual temperature of the furnace. The gap between the thermocouple protection sleeve and the wall is not filled with insulating material, which causes heat overflow or cold air intrusion in the furnace. Therefore, the gap between the thermocouple protection tube and the furnace wall hole should be blocked with insulating materials such as refractory mud or asbestos rope to prevent convection of hot and cold air from affecting the accuracy of temperature measurement. The installation of thermocouples should avoid strong magnetic fields and strong electric fields as much as possible. Thermocouples and power cables should not be installed in the same conduit to avoid interference and errors. Thermocouples cannot be installed in areas where the measured medium rarely flows. When using thermocouples to measure the gas temperature in the tube, the thermocouple must be installed against the flow direction and fully in contact with the gas.

　　2. Errors introduced by insulation deterioration

　　If there is too much dirt or salt residue on the thermocouple protection tube and the wire drawing plate, the insulation between the thermocouple poles and the furnace wall will be poor, which is more serious at high temperatures. This will not only cause the loss of thermoelectric potential but also introduce interference. The error caused by this can sometimes reach hundreds of degrees.

　　3. Errors 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. This effect is particularly prominent when performing rapid measurements. Therefore, thermocouples with thinner thermocouples and smaller protective tube diameters should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference with the actual furnace temperature. When using a thermocouple with a large time constant to measure or control temperature, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to accurately measure the temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and is 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, thin walls and small inner diameter protective sleeves are usually used. In more precise temperature measurements, bare wire thermocouples without protective sheaths are used, but thermocouples are easily damaged and should be calibrated and replaced in a timely manner.

　　4. Thermal resistance error

　　At high temperatures, if there is a layer of soot on the protection tube, dust will adhere to it, and the thermal resistance will increase, hindering the conduction of heat. 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.