1. Errors caused by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace, etc.
In other words, the thermocouple should not be installed too close to the door and the heating place, and the depth of insertion should be at least 8-10 times the diameter of the protective tube; the space between the protective tube of the thermocouple and the wall is not filled with insulating material, which causes the furnace Internal heat overflow or cold air intrusion, so the gap between the thermocouple protection tube and the furnace wall hole should be blocked with refractory mud or asbestos rope 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 The temperature of the thermocouple should exceed 100℃; the installation of thermocouple should avoid strong magnetic field and strong electric field as much as possible. Therefore, thermocouple and power cable should not be installed in the same conduit to avoid introducing interference and causing errors; thermocouple cannot be installed in the measured In areas where the medium rarely flows, when using a thermocouple to measure the temperature of the gas in the tube, the thermocouple must be installed against the direction of the flow rate and fully in contact with the gas.
2. Errors introduced by insulation deterioration
If the thermocouple is insulated, too much dirt or salt slag on the protection tube and pull-out plate will cause poor insulation between the thermocouple poles 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. The error caused by this can sometimes reach Baidu.
3. Errors introduced by thermal inertia
Due to the thermal inertia of the thermocouple, the indicator value of the meter lags behind the change of the measured temperature,
This effect is particularly prominent when performing rapid measurements. Therefore, thermocouples with thinner thermoelectrodes and smaller protective tube diameters should be used as much as possible. The protection tube can even be removed when the temperature measurement environment permits. 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 thermocouple fluctuations and the greater the difference from the actual furnace temperature. When using a thermocouple with a large time constant to measure or control temperature, although the temperature displayed by the meter 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 directly proportional to the diameter of the thermocouple hot end, the density of the material, and the specific heat. 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 are usually used, and protective sleeves with thin tube walls and small inner diameters are used. In more precise temperature measurement, bare-wire thermocouples without protective sleeves are used, but thermocouples are easily 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 will increase and hinder the 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 errors.