The correct use of thermocouples can not only accurately obtain the temperature value and ensure that the product is qualified, but also save the data consumption of thermocouples, which can not only save money but also ensure product quality. Errors such as incorrect installation, thermal conductivity and time lag are the first errors in the application of thermocouples.
1. Thermal resistance error
At high temperatures, if there is a layer of coal ash on the maintenance pipe and dust adheres to it, the thermal resistance is added to prevent the conduction of heat, and the temperature indication is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple maintenance tube should be kept clean to minimize errors.
2. Errors introduced due to deterioration of insulation
If the thermocouple is insulated, the maintenance pipe and the cable plate are too dirty or salt, resulting in 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 disturbance, and the resulting errors can sometimes reach hundreds of degrees.
3. Errors in the introduction of thermal inertia
This effect is particularly outstanding when performing rapid measurements, as the thermal inertness of the thermocouple causes the apparent value to lag behind the change in the measured temperature. Therefore, thermocouples with thinner thermoelectrodes and smaller diameter maintenance tubes should be used as much as possible. If the temperature measurement environment permits, the maintenance tube can even be removed. Because of the measurement hysteresis, 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 shaking 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 on the surface fluctuates slightly, the fluctuation of the actual furnace temperature may be large. In order to accurately measure the 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 data and the specific heat. In application, materials with good thermal conductivity and maintenance sleeves with thin walls and small inner diameters are usually selected. In more precise temperature measurement, bare wire thermocouples without maintenance sleeves are used, but the thermocouples are easy to be damaged and should be calibrated and replaced in time.
4. Errors caused by improper introduction of devices
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 8-10 times the diameter of the maintenance tube; The distance between the thermocouple maintenance sleeve and the wall is not filled with thermal insulation substances, resulting in heat overflow or cold air intrusion in the furnace, so the gap between the thermocouple maintenance tube and the furnace wall hole should be blocked by thermal insulation substances such as refractory mud or asbestos rope to avoid the convection of cold and hot air and affect the accuracy of temperature measurement; The cold end of the thermocouple is too close to the furnace body to exceed 100°C; The device of the thermocouple should avoid strong magnetic fields and strong electric fields as much as possible, so the thermocouple and the power cable should not be installed in the same conduit to avoid the introduction of disturbance and error; When measuring the temperature of the gas in the tube with a thermocouple, the thermocouple must be installed in the direction of the flow velocity and be in full contact with the gas.
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