Effective methods to reduce measurement errors in thermocouples

How can we reduce the measurement errors generated during the use of thermocouples? Firstly, to solve the problem of errors, we need to understand the reasons behind them in order to effectively address the issue! Below we will analyze several reasons for errors:
Firstly, let's confirm if the thermocouple installation is correct. Improper installation can result in errors. Here are four points summarized for the installation of thermocouples:

1. The insertion depth should be at least 8-10 times the diameter of the protective tube; The gap between the protective sleeve of the thermocouple and the wall is not filled with insulation material, which may cause hot overflow or cold air to enter the furnace. Therefore, the gap between the thermocouple protective tube and the furnace wall hole should be blocked with insulation materials such as refractory mud or asbestos rope to prevent the convection of cold and hot air from affecting the accuracy of temperature measurement;

2. The cold end of the thermocouple is too close to the furnace body, causing the temperature in the measured area to be too high;

3. The installation of thermocouples should avoid strong magnetic and electric fields as much as possible, so thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors;

4. Thermocouples cannot be installed in areas where the measured medium flows very little. When measuring the gas temperature inside a tube with a thermocouple, it must be installed against the flow velocity direction and fully in contact with the gas.

Secondly, in the use of thermocouples, the deterioration of thermocouple insulation is also one of the reasons for errors:

Excessive dirt and salt residue on the protective tube and wire plate can lead to poor insulation between thermocouple electrodes and furnace walls, especially at high temperatures. This not only causes loss of thermoelectric potential but also interference, and the resulting errors can sometimes reach hundreds of degrees Celsius.

Furthermore, errors caused by the thermal resistance of thermocouples:

If there is dust or coal ash on the thermocouple protection tube, it will cause an increase in thermal resistance and hinder the conduction of heat. At this time, the temperature reading will be lower than the true value of the measured temperature. So it is necessary to keep the thermocouple protection tube clean.

Later, the error was caused by the thermal inertia of the thermocouple

The inertia of thermocouples causes the indicated value of the instrument to lag behind the changes in the measured temperature, so thermocouples with thinner thermoelectric electrodes and smaller protective tube diameters should be used as much as possible. Due to hysteresis, the amplitude of temperature fluctuations detected by thermocouples is smaller than that of furnace temperature fluctuations. So in order to accurately measure temperature, it is necessary to choose materials with good thermal conductivity, such as protective sleeves with thin pipe walls and small inner diameters. In temperature measurement with high precision requirements, bare wire thermocouples without protective sleeves are often used.
In summary, we can reduce the measurement error of thermocouples by checking four aspects: first, check whether the thermocouple is installed correctly; second, check whether the insulation of the thermocouple is worn or deteriorated; third, check whether the thermocouple protection tube is clean; fourth, check whether the error is caused by the thermal inertia of the thermocouple!