If installed incorrectly, errors such as thermal conductivity and time lag can occur, which are the main errors in the use of thermocouples.
1. Errors introduced by improper installation
The thermocouple should not be installed too close to the door and the heating place, the insertion depth should be at least 8~10 times the diameter of the protective tube, and the installation position and insertion depth should not reflect the real temperature of the furnace. Therefore, the gap between the thermocouple protection tube and the furnace wall should be blocked with thermal insulation materials such as refractory mud or asbestos rope to avoid the convection of cold and hot air and affect the accuracy of temperature measurement. The installation of thermocouples should avoid strong magnetic fields and strong electric fields as much as possible, and thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors. Thermocouples cannot be installed in areas where the medium to be measured rarely flows, and when measuring the temperature of the gas in the tube with thermocouples, the thermocouples must be installed in the direction of the flow velocity and in full contact with the gas.
2. Errors introduced by deterioration of insulation
For example, excessive dirt or salt slag of the thermocouple protection tube and cable plate leads to poor insulation between the thermoelectric dipole and the furnace wall, which is more serious at high temperatures, which will not only cause the loss of thermoelectric potential but also introduce interference, and the resulting error can sometimes reach hundreds of degrees.
3. Errors introduced by thermal inertia
This effect is particularly noticeable when making fast measurements due to the thermal inertness of the thermocouple, which causes the indicated value of the meter to lag behind the change in the temperature being measured. Therefore, thermocouples with thinner thermodes and smaller diameter protective tubes 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 that of the furnace temperature fluctuation. The greater the measurement hysteresis, the smaller the amplitude of the thermocouple's fluctuations and the greater the difference from the actual furnace temperature. When using a thermocouple with a large time constant to measure or control the temperature, although the temperature displayed by the meter fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to accurately measure temperature, thermocouples with small time constants should be selected. The time constant is inversely proportional to the heat transfer coefficient, and is directly proportional to the diameter of the thermocouple hot end, the density of the material and the specific heat. In use, materials with good thermal conductivity, thin walls and small inner diameter are usually used. In more precise temperature measurements, bare wire thermocouples without protective sleeves are used, but the thermocouples are easily damaged and should be calibrated 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 adheres to it, the thermal resistance increases, hindering the conduction of heat, and 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 errors.