Reasons and elimination methods for inaccurate thermocouple temperature measurement errors

Installing measuring components such as thermal resistors and thermocouples on industrial control sites can introduce errors due to improper installation or environmental influences, resulting in inaccurate temperature measurement.

Installation diagram for eliminating errors in thermocouple temperature measurement

Improper installation error
For example, the position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace,

1. Thermocouples should not be installed too close to doors or heating areas, as the cold end of the thermocouple is too close to the furnace body, causing the temperature to exceed 100 ℃;

2. The insertion depth should be at least 8-10 times the diameter of the protective tube;

3. 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;

4. 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;

5. 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.

Error in insulation deterioration
If the thermocouple is insulated, excessive dirt or salt residue on the protective tube and wire plate can cause poor insulation between the thermoelectric dipoles and the furnace wall, which is even more severe at high temperatures. This not only causes loss of thermoelectric potential but also introduces interference, resulting in errors that can unintentionally reach up to Baidu.

Error introduced by thermal inertia
Due to the thermal inertia of thermocouples, the indicated value of the instrument lags behind the changes in the measured temperature, which is particularly prominent during rapid measurements. Therefore, thermocouples with thinner thermoelectric electrodes and smaller protective tube diameters should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to measurement lag, the amplitude of temperature fluctuations detected by thermocouples is smaller than that of furnace temperature fluctuations. The larger 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 for temperature measurement or control, although the temperature reflected by the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to accurately measure temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the thermocouple's hot end, material density, and specific heat. 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 as much as possible. In use, materials with good thermal conductivity and protective sleeves with thin pipe walls and small inner diameters are usually used. In more precise temperature measurements, bare wire thermocouples without protective sleeves are used, but thermocouples are prone to damage and should be calibrated and replaced in a timely manner.

Thermal resistance error
When measuring high temperatures, if there is a layer of coal ash and dust attached to the thermocouple protection tube, the thermal resistance increases, hindering the conduction of heat. At this time, the temperature reading is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce errors.

Incorrect use of compensating wire error
The thermocouple extension wire must use the corresponding thermocouple compensation wire. If different types of compensation wires are used incorrectly or copper wires are used, it may result in inaccurate and biased temperature measurement.