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The main error of thermocouple in use

by:Sure     2021-08-08
1. Errors 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 the protection tube 8 to 10 times the diameter; the gap between the thermocouple's protective tube and the wall is not filled with insulating material, which causes heat overflow or cold air intrusion in the furnace, so thermocouple 1, improper installation introduces errors such as thermocouple installation position and insertion depth Can not reflect the true temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and the heating place, and the depth of insertion should be at least 8-10 times the diameter of the protective tube; between the protective tube of the thermocouple and the wall No insulation material is filled in the interval to cause heat overflow or cold air intrusion in the furnace, so the gap between the thermocouple protection tube and the furnace wall hole should be blocked with refractory mud or asbestos rope to prevent the convection of cold and hot 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 the thermocouple and power cable should not be installed in the same conduit to avoid introducing interference and causing errors ; The thermocouple can not be installed in the area where the measured medium seldom flows. When using the thermocouple to measure the gas temperature in the tube, the thermocouple must be installed against the direction of the flow rate and fully contacted with the gas. 2. Errors introduced by the deterioration of insulation, such as the insulation of thermocouples, excessive dirt or salt slag on the protection tube and pull-out plate, resulting in poor insulation between the thermocouple poles and the furnace wall, which is more serious at high temperatures, which will not only cause thermoelectricity The loss of potential and also introduces 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 meter lags behind the change of the measured temperature, this effect is particularly prominent during rapid measurement. 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 from 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, the material with good thermal conductivity is usually used, and the tube wall is thin and the inner diameter is small. In more precise temperature measurement, a bare-wire thermocouple without protective sleeve is used, but the thermocouple is 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 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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