The cause of the measurement error caused by the thermocouple

Although the structure of the thermocouple is simple, various problems still occur in use, which may cause large measurement errors. In order to improve measurement accuracy, reduce measurement error, and extend the life of thermocouples, users should not only have the operational skills of the instrument, but also have knowledge of physics, chemistry and materials. There are many kinds of measuring materials for thermocouples. Different materials have different measurement ranges. The selection of products requires the user to provide the highest measured temperature and common measurement temperature, as well as the requirements for measurement accuracy to determine the thermocouple used.

In general, there are three main reasons for the measurement error of thermocouples.

First, the error introduced by improper installation

Installation of the instrument is also an important part. We must pay more attention to it. The installation is not only affecting the measurement accuracy, but also reducing the service life. For example, the position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace. In other words, the thermoelectric Occasionally should not be placed too close to the door and heating, the depth of insertion should be at least 8 to 10 times the diameter of the protection tube; the gap between the protective sleeve and the wall of the thermocouple is not filled with thermal substances, causing heat or cold air inside the furnace. Intrusion, so the gap between the thermocouple protection tube and the hole of the furnace wall is blocked by insulating materials such as refractory mud or asbestos rope to avoid the convection of hot and cold air, which affects the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body to make the temperature exceed 100. °C; thermocouple installation should avoid strong magnetic field and strong electric field as much as possible, so thermocouple and power Cable should not be installed in the same conduit to avoid introducing interference and error; thermocouple can not be installed in the measured medium with little flow In the area, when the temperature of the gas in the tube is measured by a thermocouple, the thermocouple must be installed against the flow direction and sufficiently in contact with the gas.

Second, the error introduced by the insulation is poor

This reason is a problem with the quality of thermocouple products. Therefore, we require that the quality of the products should be kept in the production process, and we must do a good job in the raw materials, such as thermocouple insulation, protection of the pipe and wire plate dirt or salt slag, resulting in poor insulation between the thermocouple and the furnace wall. It is more serious at high temperatures, which not only causes the loss of the thermoelectric potential but also introduces interference, and the resulting error can sometimes reach Baidu. Choose a good material to make a good product.

Third, the error introduced by thermal inertia

This situation is relatively complicated and is related to many factors on the spot. Therefore, we must fully consider various factors when selecting products. Because the thermal inertia of the thermocouple makes the indication value of the instrument lag behind the measured temperature, this effect is especially prominent when making rapid measurements. Therefore, thermocouples with thinner hot electrodes and smaller diameters of protective tubes should be used as much as possible. When the temperature measurement environment permits, the protection tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The larger the measurement hysteresis, the smaller the amplitude of the thermocouple fluctuation and the greater the difference from the actual furnace temperature. When the temperature is measured or controlled by a thermocouple with a large time constant, although the temperature displayed by the meter fluctuates little, the fluctuation of the actual furnace temperature may be large. In order to accurately measure the temperature, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and is proportional to the diameter of the hot end of the thermocouple, the density of the material, and the 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. . In use, it is usually a material with good thermal conductivity, a protective tube with a thin tube wall and a small inner diameter. In the more precise temperature measurement, a bare wire thermocouple without a protective sleeve is used, but the thermocouple is easily damaged and should be corrected and replaced in time. Understand every aspect of the site, understand the measurement requirements, and choose the right instrument is the joint responsibility of the manufacturer and the instrument technician.