Possible factors leading to inaccurate temperature measurement of K-type thermocouple

As a typical temperature instrument, K-type thermocouple has been widely used in industrial production measurement due to its unique performance. However, in the process of using thermocouples in industrial enterprises, K-type thermocouples often have inaccurate temperature measurement. In order to avoid errors as much as possible, we must know what are the factors that cause inaccurate temperature measurement of thermocouples? Let's explain this issue to you.

1. Errors introduced by improper installation

Thermocouples should not be installed too close to the door and heating. The insertion depth should be at least 8 to 10 times the diameter of the protection tube. The installation position and insertion depth cannot reflect the actual temperature of the furnace. The gap between the thermocouple protection sleeve and the wall is not filled with insulating materials, resulting in 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 installation of thermocouples should avoid strong magnetic fields and strong electric fields as much as possible. Thermocouples and power cables should not be installed in the same conduit to avoid interference and errors. Thermocouples cannot be installed in areas where the measured medium rarely flows. When using thermocouples to measure the temperature of the gas in the tube, the thermocouple must be installed against the flow direction and fully in contact with the gas.

2. Errors introduced by insulation deterioration

For example, if there is too much dirt or salt residue on the thermocouple protection tube and the wire drawing plate, the insulation between the thermocouple poles and the furnace wall will 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 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 from 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 tube 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.

Through the above three points on the analysis of factors leading to K-type thermocouple errors, I hope it will be helpful to everyone's work and study.