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How to use thermocouple correctly

by:Sure     2021-08-10
The correct use of thermocouples can not only accurately obtain the temperature value and ensure that the product is qualified, but also save the material consumption of the thermocouple, which saves money and guarantees product quality. Incorrect installation, thermal conductivity and time lag errors are the main errors in the use of thermocouples. 1. The errors that may be caused by improper installation, such as the installation position and insertion depth of the thermocouple cannot be reversed. The correct use of the thermocouple can not only accurately obtain the temperature value, ensure that the product is qualified, but also save the material consumption of the thermocouple, which saves money and can Ensure product quality. Incorrect installation, thermal conductivity and time lag errors are the main errors in the use of thermocouples. 1. The errors that may be caused by improper installation, such as the installation position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and heating, and the insertion depth should be at least for protection 8-10 times the diameter of the tube; the space between the thermocouple protection tube and the wall is not filled with insulation material, which causes heat overflow or cold air intrusion in the furnace, so the gap between the thermocouple protection tube and the furnace wall hole should be refractory mud or asbestos Insulating materials such as ropes are blocked to prevent the convection of hot and cold air from affecting the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body to cause the temperature to exceed 100℃; the installation of the thermocouple should avoid strong magnetic fields and strong electric fields as much as possible, so it should not be Install the thermocouple and the power cable in the same conduit to avoid introducing interference and causing errors; the thermocouple cannot be installed in an area where the measured medium seldom flows. When using a thermocouple to measure the gas temperature in the tube, the thermocouple must be opposed to the flow rate It is installed in the same direction and fully in contact with gas. 2. The error introduced by the deterioration of the insulation, such as the insulation of the thermocouple, the excessive dirt or salt slag on the protection tube and the pull-out plate, causes poor insulation between the thermocouple poles and the furnace wall, which is more serious at high temperatures, which will not only cause thermoelectricity Potential loss and also introduce interference, the error caused by this can sometimes reach Baidu. 3. The error introduced by thermal inertia. Due to the thermal inertia of the thermocouple, the indicator value of the instrument lags behind the change of the measured temperature. This effect is particularly prominent when performing rapid measurements. Therefore, thermocouples with thinner thermoelectrodes and smaller protective tube diameters should be used as much as possible. The protective tube can even be removed when the temperature measurement environment permits. 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 greater the measurement lag, the smaller the amplitude of thermocouple fluctuations, and the greater the difference with the actual furnace temperature. When a thermocouple with a large time constant is used for temperature measurement or temperature control, although the temperature displayed by the meter fluctuates very little, the actual furnace temperature may fluctuate greatly. 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 directly proportional to the diameter of the thermocouple hot end, the density of the material, and the specific heat. If you want 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, materials with good thermal conductivity are usually used, and protective sleeves with thin tube walls and small inner diameters are used. In more precise temperature measurement, bare-wire thermocouples without protective sleeves are used, but the thermocouples are easily damaged and should be corrected and replaced in time. 4. Thermal resistance error At high temperature, if there is a layer of coal ash on the protection tube and dust is attached to it, the thermal resistance will increase and hinder the heat conduction. At this time, the temperature indication 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.
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