How to effectively control the error in thermocouple measurement?

How to reduce the measurement error caused by the use of thermocouples? First of all, in order to solve the error, we need to understand the cause of the error in order to effectively solve the problem! Let’s look at a few reasons for the error.

First, make sure that the thermocouple is installed correctly. If it is not installed properly, an error will occur. The following are four points of thermocouple installation.
1. The insertion depth should be at least 8 times the diameter of the protective tube; the space between the protective tube and the thermocouple wall is not filled with insulating material, which will cause heat overflow in the furnace or cold air intrusion, and make the thermocouple protective tube and the furnace wall hole The gap is blocked by insulating materials such as refractory mud or cotton rope to avoid convection of hot and cold air, which affects the accuracy of temperature measurement.
2. The cold end of the thermocouple is too close to the furnace body, and the temperature of the measuring part is too high;
3. The installation of thermocouple should try to avoid strong magnetic field and strong electric field, so thermocouple and power cable should not be installed on the same pipe to avoid errors caused by interference.
4.Thermocouples cannot be installed in areas where the measured medium seldom flows. When using a thermocouple to measure the gas temperature in the tube, the thermocouple must be installed in the reverse speed direction and be in full contact with the gas.

Secondly, when using a thermocouple, the insulation change of the thermocouple is also one of the reasons for the error:
1. Excessive dirt and salt slag between the thermocouple electrode and the furnace wall will cause poor insulation between the thermocouple electrode and the furnace wall, which will not only cause loss of thermoelectric potential, but also interference, and sometimes the error can even reach hundreds of degrees Celsius.
2. Error caused by thermal resistance of thermocouple:
The presence of dust or coal ash on the thermocouple protection tube increases thermal resistance and hinders heat conduction, and the temperature indication value is lower than the true value of the measured temperature. Therefore, keep the thermocouple protection tube clean.
3. Errors caused by the inertia of thermocouples:
The inertia of the thermocouple makes the indicating value of the instrument lag behind the change of the measured temperature, so thermocouples with extremely small temperature differences and small protective tube diameters should be used as much as possible. Due to the hysteresis, the temperature fluctuation range detected by the thermocouple is smaller than the furnace temperature fluctuation range. Therefore, in order to accurately measure temperature, materials with good thermal conductivity should be selected, and protective sleeves with thin walls and small inner diameters should be selected. In high-precision temperature measurement, bare-wire thermocouples without protective sleeves are often used.

In short, the measurement error of the thermocouple can be reduced in four aspects: one step is to check whether the thermocouple is installed correctly, the second step is to check whether the insulation of the thermocouple is changed, the third step is to check whether the thermocouple protection tube is clean, and the fourth step is thermoelectric Error caused by even inertia!

Post time: Dec-17-2020