At high temperature, if there is a layer of coal ash on the protection tube and dust is attached to it, the thermal resistance increases, hindering the conduction of heat. At this time, the indicated temperature is lower than the true value of the measured temperature. Therefore, the thermocouple protection tube should be kept clean to reduce errors.

 

 

If thermocouple insulation, protection tube and cable board dirt or salt residue too much between thermodipole and furnace wall insulation, more serious at high temperature, this will not only cause the loss of thermoelectric potential and will also introduce interference, resulting in errors sometimes up to 100 degrees.

 

 

The thermal inertia of the thermocouple makes the indication of the instrument lag behind the change of the measured temperature. Therefore, the thermocouple with thin hot electrode and small diameter of protective tube should be adopted as far as possible. When the temperature environment permits, you can even remove the protection tube. The amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation because of measurement lag. The larger the measurement delay, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference between the actual furnace temperature. When the thermocouple with large time constant is used to measure or control the temperature, the temperature displayed by the instrument fluctuates 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 chosen. The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the thermal end of the thermocouple, the density of the material and the specific heat. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to reduce the size of the thermal end as far as possible. In use, the protective casing with good thermal conductivity, thin wall and small inner diameter is usually used. In more precise temperature measurement, the naked wire thermocouple without protective sleeve is used, but the thermocouple is easy to be damaged, so it should be corrected and replaced in time.

 

 

Such as thermocouple installation location and insert depth can not reflect the true temperature of the furnace, in other words, thermocouple should not be installed too close to the door and heating place, insert depth should be at least 8 ~ 10 times of the diameter of the protection tube; The gap between the protection sleeve of thermocouple and the wall is not filled with adiabatic material, which leads to the overflow of heat or the invasion of cold air in the furnace. Therefore, the gap between the thermocouple protection tube and the hole in the furnace wall should be blocked with refractory or asbestos insulation material to prevent the convection of cold and hot air from affecting the accuracy of temperature measurement. The cold end of thermocouple is too close to the furnace body to make the temperature exceed 100℃; The installation of thermocouple should avoid the strong magnetic field and strong electric field as far as possible, so the thermocouple and the power cable line should not be installed in the same catheter in order to avoid interference caused errors; The thermocouple should not be installed in an area where the measured medium rarely flows. When measuring the gas temperature in the tube with a thermocouple, the thermocouple must be installed against the flow direction and in full contact with the gas.

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