Causes of Thermocouple Errors

Correct use of thermocouples can not only accurately obtain temperature values ​​and ensure product quality, but also save thermocouple material consumption, saving money and ensuring product quality. Improper installation, thermal conductivity and time lag errors are the main errors in the use of 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 materials, 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 from affecting 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 caused by poor insulation of thermocouples
If the thermocouple is short-circuited, the protection tube and the wire board are too dirty or the salt residue is too much, which causes the insulation between the thermocouple poles and the furnace wall to be poor, which is more serious at high temperatures. This will not only cause the loss of thermoelectric potential but also introduce interference. The error caused by this can sometimes reach hundreds of degrees.

3. Errors caused by the thermal inertia of thermocouples
Due to the thermal inertia of thermocouples, 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 most 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. Errors caused by the thermal resistance of the thermocouple itself
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 errors.