The correct use of thermocouples not only can accurately obtain the temperature value, ensure the product is qualified, but also can save the material consumption of the thermocouple, saving both money and product quality. Improper installation, errors in thermal conductivity and time lag, etc., are major errors in the use of thermocouples.
Improperly installed errors such as the location of the thermocouple and the depth of insertion do not reflect the true temperature of the furnace. In other words, thermocouples should not be placed too close to the door and heat, and the depth of insertion should be at least the diameter of the protective tube. 8 to 10 times that of the thermocouple protective sleeve and the wall without filling with heat insulating material to cause the heat in the furnace to overflow or cold air to invade, so the gap between the thermocouple protection tube and the furnace wall hole is made of refractory mud or asbestos rope, etc. The insulation material is blocked to avoid the convection of hot and cold air and affect the accuracy of temperature measurement; the thermocouple cold end is too close to the furnace body to make the temperature exceed 100°C; the thermocouple installation should avoid the strong magnetic field and strong electric field as much as possible, so it should not be thermoelectric Even power cables are installed in the same conduit to avoid introducing interference to cause errors; thermocouples cannot be installed in areas where there is little flow of the measured medium. When thermocouples are used to measure the gas temperature in the tubes, the thermocouples must be installed against the flow velocity. , and full contact with the gas.
2 Insulation is poor and the introduced error is insulated by thermocouples. Excessive dirt or salt residue on the protection tube and the wire drawing board results in poor insulation between the thermocouple pole and the furnace wall, which is more serious at high temperatures. This not only causes thermoelectric potential. The loss also introduces interference, and 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 in the measured temperature. This effect is particularly pronounced during fast measurements. Therefore, thermocouples with thinner thermocouples and smaller 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 hysteresis, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the fluctuation of the furnace temperature. The larger the measurement lag, the smaller the amplitude of the fluctuation of the thermocouple and the greater the difference from the actual furnace temperature. When using thermocouples with large time constants for temperature measurement or temperature control, the temperature displayed by the instrument will fluctuate little, but 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. 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, a protective sleeve with a thin wall and a small internal diameter is generally used with good thermal conductivity. In the more precise temperature measurement, a bare wire thermocouple without a protective sleeve is used, but the thermocouple is easily damaged and should be promptly corrected and replaced.
4 When the thermal resistance error is high, for example, if there is a layer of coal ash on the protection tube, and the dust adheres 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.