Correct installation and use of anti-corrosion thermocouples

1. Errors introduced by improper installation:

For example, the installation position and insertion depth of the thermocouple cannot reflect the actual temperature of the furnace. 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 to 10 times the diameter of the protection tube; the gap between the thermocouple protection tube and the wall is not filled with insulating material, which causes heat overflow or cold air intrusion in the furnace. Therefore, the gap between the thermocouple protection tube and the furnace wall hole should be blocked with insulating materials such as refractory mud or asbestos rope to prevent hot and cold air convection and affect the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body so that the temperature exceeds 100℃; the installation 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 interference and errors; the thermocouple cannot be installed in the area where the measured medium rarely flows. When using a thermocouple to measure the gas temperature in the tube, the thermocouple must be installed against the flow direction and fully in contact with the gas.

2. Errors introduced by insulation degradation:

If the thermocouple is insulated, the protection tube and the wire drawing board are too dirty or have too much salt residue, resulting in poor insulation between the thermocouple poles and the furnace wall. This is more serious at high temperatures, which will not only cause the loss of thermoelectric potential but also introduce interference. The errors caused by this can sometimes reach hundreds of degrees.

3. Errors introduced by thermal inertia:

Due to the thermal inertia of the thermocouple, the indicated value of the instrument lags behind the change of the measured temperature. This effect is particularly prominent when performing rapid measurements. Therefore, thermocouples with thinner thermocouples and smaller protection tube 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 lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference with the actual furnace temperature. When using a thermocouple with a large time constant to measure or control temperature, although the temperature displayed by the instrument fluctuates very little, 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. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the effective way is to minimize the size of the hot end. In use, materials with good thermal conductivity, thin walls and small inner diameter protective sleeves are usually used. In more precise temperature measurements, bare wire thermocouples without protective sleeves are used, but thermocouples are easily damaged and should be calibrated and replaced in time.

4. Thermal resistance error:

At high temperatures, if there is a layer of coal ash on the protective tube and dust is 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 outside of the thermocouple protective tube should be kept clean to reduce the error.