The correct use of thermocouples can not only accurately get the temperature value, to ensure that the product is qualified,and also can save the material consumption of thermocouples, both save money and can ensure product quality. Incorrect installation, thermal conductivity and time lag errors, they are the main errors in the use of thermocouples.

1. Error introduced by improper installation

If the installation position and insertion depth of the thermocouple can not reflect the true temperature of the furnace, in other words, Thermocouple should not be placed too close to the door and heated, The depth of insertion shall be at least 8 to 10 times the diameter of the protective tube; The interval between the protective casing of the thermocouple and the wall is not filled with heat overflow or cold air intrusion in the furnace, Therefore, the gap between the thermocouple protection pipe and the furnace wall hole should be blocked by refractory mud or asbestos rope to avoid the convection of the cold and hot air and affect the accuracy of temperature measurement; The thermocouple cooling end is too close to the furnace body so that the temperature exceeds 100℃; Thermocouple should be installed to avoid a strong magnetic field and a strong electric field as far as possible, Therefore, the thermocouple and the power cable should not be installed in the same catheter to avoid introducing interference caused by error; Thermocouples cannot 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 rate direction, And is also in full contact with the gas.

2. The error introduced by the poor insulation

If the thermocouple insulation, the protection pipe and wire board dirt or too much salt residue causes the poor insulation between the thermoelectric dipole and the furnace wall, which is more serious at high temperature, which will not only cause the loss of thermal potential but also introduce interference, the resulting error can sometimes reach Baidu.

3. The errors introduced by the thermal inertia

 Because the thermal inertia of the thermocouple lags the indicated value of the meter behind the change of the measured temperature,

This effect is particularly prominent when making rapid measurements. Therefore, thermocouples with relatively thin thermal electrode and smaller diameter of the protection pipe should be used as far as possible. When the temperature measurement environment permits, the protection pipe can even be removed. Due to the presence of measurement hysteresis, the amplitude of the temperature fluctuations detected with thermocouples is smaller than the furnace temperature fluctuations. The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuations, and the greater the difference from the actual furnace temperature. When the thermocouple with a large time constant is used for temperature measurement or temperature control, although the temperature fluctuation shown by the instrument is very small, but the actual furnace temperature may fluctuate very large. To measure the temperature accurately, the thermocouples 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 thermal end of the thermocouple, the density of the material and the specific heat. If the time constant, besides increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end. In use, usually use good thermal conductivity materials, thin pipe wall, small inner diameter protective casing. In the more precise temperature measurement, the bare wire thermocouple without protected casing is used, but the thermocouple is easy to damage, and should be corrected and replaced in time.

4. Thermal resistance error

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