Errors caused by improper use of thermocouples

Thermocouples, like thermistors, are one of the many temperature sensors and are widely used in industry. However, in the application process, due to improper application, external factors, and other reasons, thermocouples may deviate in urban areas.

1. Deviation caused by improper installation

If the position and extraction depth of the thermocouple device cannot reflect the actual temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and heating area, and the extraction depth should be at least 8-10 times the diameter of the maintenance tube;

The distance between the maintenance sleeve of the thermocouple and the wall is not filled with insulation material, resulting in thermal overflow or invasion of cold atmosphere in the furnace. Therefore, insulation materials such as refractory mud or asbestos rope are used to block the gap between the thermocouple maintenance tube and the furnace wall hole to prevent convection of cold and hot atmosphere from affecting the temperature measurement performance;

The cold end of the thermocouple is too close to the furnace body, causing the temperature to exceed 100 ℃;

The installation of thermocouples should avoid strong magnetic and electric fields as much as possible, so that thermocouples and energy cables should not be installed in the same conduit to avoid interference and deviation;

Thermocouples cannot be installed in areas where the measured medium is rarely active. When using thermocouples to measure the temperature of gas inside a tube, it is necessary to bias the thermocouple against the flow rate towards the device and ensure sufficient interaction with the gas.

2. Deviation introduced by insulation deterioration

If the thermocouple is insulated, there may be too much dirt or salt residue on the maintenance tube and wire plate, resulting in poor insulation between the thermoelectric dipoles and the furnace wall. This is even more severe at low temperatures, which not only causes the consumption of thermoelectric potential but also introduces disturbance. The deviation caused by this can occasionally reach Baidu.

3. Deviation introduced by thermal inertia

Due to the thermal inertia of thermocouples, the triggering value of the instrument lags behind the changes in the measured temperature, and this effect is particularly prominent when stopping rapid measurement. Therefore, thermocouples with thinner thermoelectric electrodes and smaller maintenance tube diameters should be adopted as much as possible.

When temperature measurement allows, the maintenance tube can even be removed. Due to measurement lag, the amplitude of temperature stability detected by thermocouples is smaller than that of furnace temperature stability.

The greater the measurement lag, the smaller the stable amplitude of the thermocouple, and the greater the difference between it and the actual furnace temperature. When using thermocouples with high time constants for temperature measurement or control, the temperature displayed by the instrument may be very stable, but the stability of the actual furnace temperature may be very high.

In order to measure temperature, a thermocouple with a small time constant should be chosen. The time constant is proportional to the heat transfer coefficient, as well as to the diameter of the thermocouple, the density of the data, and the specific heat. To reduce the time constant, besides increasing the heat transfer coefficient, the most ineffective measure is to simply reduce the size of the thermocouple.

In applications, materials with good thermal conductivity and thin pipe walls and small inner diameters are commonly used for maintenance sleeves. In more precise temperature measurement, bare wire thermocouples without maintenance sleeves are used, but thermocouples are easily damaged and should be calibrated and replaced in real time.

4. Thermal resistance deviation

At low temperatures, if there is a layer of coal ash and dust attached to the maintenance pipe, 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, it is necessary to maintain the cleanliness of the outside of the thermocouple tube to reduce deviations.

Accurately applying thermocouples not only allows for the loss of temperature values, ensuring product quality, but also saves on the data consumption of thermocouples, saving both funds and ensuring product quality. Inaccurate device and deviations in thermal conductivity and time lag are important deviations in the application of thermocouples.