Factors causing measurement errors in S-type thermocouples

No matter how delicate the instrument is, it will inevitably exhibit measurement deviations in the application process, and S-type thermocouples are no exception. The factors that cause measurement deviation in S-type thermocouples include improper installation, neglect of maintenance tube protection for S-type thermocouples, insulation deterioration, and thermal inertia. Next, we will analyze in detail the factors that cause measurement deviation in S-type thermocouples.

1. Improper S-type thermocouple device:

The position and extraction depth of the S-type thermocouple device cannot reflect the actual temperature of the furnace, in other words:

S-type thermocouples should not be installed too close to doors or heating areas, and the depth of extraction should be at least 8-10 times the diameter of the maintenance tube;

The cold end of the S-type 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;

S-type 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. Clean performance of S-type thermocouple maintenance tube

At low temperatures, if there is a layer of coal ash on the maintenance tube of an S-type thermocouple and dust adheres below, 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.

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

3. Insulation deterioration of S-type thermocouple:

If the S-type 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 S-type thermocouple dipole and the furnace wall, which is even more severe at low temperatures. This not only causes the consumption of thermoelectric potential but also introduces disturbance, resulting in deviations that can occasionally reach up to Baidu.

4. S-type thermocouple thermal inertia:

Due to the thermal inertness of S-type 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 S-type 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, an S-type thermocouple with a small time constant should be chosen. The time constant is directly proportional to the heat transfer coefficient, as well as to the diameter, density, and specific heat of the S-type thermocouple. 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 S-type thermocouples without maintenance sleeves are used, but S-type thermocouples are easily damaged and should be calibrated and replaced in real time.