When installing and using the temperature sensor, the following matters should be noted to ensure the best measurement effect:
1. Errors introduced by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and the heating place, and the insertion depth should be at least 8 to 10 times the diameter of the protective tube; The gap between the thermocouple's protective sleeve and the wall is not filled with heat-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 heat-resistant material such as refractory mud or asbestos rope to avoid cold and heat Air convection affects the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body so that the temperature exceeds 100 ℃; the installation of the thermocouple should avoid strong magnetic fields and strong electric fields as much as possible, so the thermocouple and power cable should not be installed in In the same duct to avoid errors caused by interference; thermocouples cannot be installed in the area where the measured medium rarely flows. When measuring the temperature of the gas in the pipe with the thermocouple, the thermocouple must be installed against the direction of the flow rate and fully in contact with the gas .
2. Errors caused by poor insulation
If the thermocouple is insulated, too much dirt or salt slag on the protection tube and the cable board causes poor insulation between the thermocouple and the furnace wall, which is more serious at high temperatures. This will not only cause the loss of thermoelectric potential but also introduce interference. The resulting errors can sometimes reach Baidu.
3. Error introduced by thermal inertia
Due to the thermal inertia of the thermocouple, the indicator value of the instrument lags behind the change of the measured temperature. This effect is particularly prominent when performing rapid measurement. Therefore, thermocouples with thinner hot electrodes and smaller protective tube diameters should be used as much as possible. When the temperature measurement environment permits, the protection 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 larger the measurement hysteresis, the smaller the amplitude of the thermocouple fluctuation and the greater the difference from the actual furnace temperature. When measuring or controlling the temperature with a thermocouple with a large time constant, 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 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 method is to minimize the size of the hot end . In use, usually a material with good thermal conductivity, a thin protective tube wall and a small inner diameter. In the more precise temperature measurement, the bare wire thermocouple without protective sleeve is used, but the thermocouple is easily damaged, and it should be corrected and replaced in time.
4. Thermal resistance error
At high temperatures, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance increases and hinders 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 errors.
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