When installing and using the thermocouple temperature sensor, you should pay attention to the following matters to ensure the measurement results:
1. Error caused by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the real temperature of the furnace.In other words, the thermocouple should not be installed too close to the door and heating place, the insertion depth should be at least 8-10 times of the diameter of the protective pipe;The gap between the protective sleeve and the wall of the thermocouple is not filled with adiabatic substances,so that the heat in the furnace overflows or the cold air invades.Therefore, the gap between the thermocouple protective pipe and the hole of the furnace wall should be blocked by adiabatic substances such as refractory mud or asbestos rope to avoid the convection of hot and cold air, which affects the accuracy of temperature measurement;The cold end of the thermocouple is too close to the furnace to make the temperature exceed 100 ℃; Thermocouples should be installed as far as possible to avoid strong magnetic fields and strong electric fields. Therefore, thermocouples and power cables should not be installed in the same conduit to avoid errors caused by interference; The thermocouple cannot be installed in the area where the measured medium rarely flows.When the temperature of the gas in the pipe is measured by a thermocouple, the thermocouple must be installed against the flow velocity direction and fully in contact with the gas.
2. Error caused by insulation deterioration
For example,the thermocouple is insulated, too much dirt or salt residue of the protective pipe and the drawing board may lead to poor insulation between the thermocouple and the furnace wall,and it is more serious at high temperature,which not only causes the loss of the thermoelectric potential but also introduces interference.
3. Error caused by thermal inertia
Because of the thermal inertia of the thermocouple,the indication value of instrument is behind the change of the measured temperature, which is especially prominent in the rapid measurement.Therefore, thermocouples with thinner thermal electrodes and smaller diameters of protective pipes should be used as much as possible.When the temperature measurement environment permits, the protective pipe can even be removed.Because of the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation.The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference from the actual furnace temperature.When the temperature is measured or controlled by a thermocouple with a large time constant, although the temperature displayed by the meter fluctuates little, the fluctuation of the actual furnace temperature may be large.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, if the time constant is to be reduced, the most effective way is to reduce the size of the hot end as much as possible, in addition to increasing the heat transfer coefficient.In use, it is usually a material with good thermal conductivity, a protective pipe with a thin tube wall and a small inner diameter. 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 corrected and replaced in time.
4. Thermal resistance error
At high temperatures, if there is a layer of coal ash on the protective pipe, dust attached to it, the thermal resistance increases, hindering the conduction of heat, at this time the temperature indication is lower than the true value of the measured temperature.Therefore, the external cleaning of the protective pipe of thermocouple should be kept to reduce the error.