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The correct use of thermocouples can not only accurately obtain the temperature value and ensure that the products are qualified, but also save the material consumption of thermocouples, which not only saves money but also ensures product quality. Incorrect installation, errors such as thermal conductivity and time lag, they are the main errors of thermocouples in use.
1. Errors caused by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the real 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 8 to 10 times the diameter of the protection tube; The space between the protective sleeve of the thermocouple and the wall is not filled with insulating material, which causes the heat to overflow in the furnace or the intrusion of cold air. Therefore, the gap between the protective tube of the thermocouple and the hole of the furnace wall should be blocked with insulating material such as refractory mud or asbestos rope to avoid cold and heat. Air convection affects the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body so that the temperature exceeds 100 °C; 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 pipe to avoid errors caused by the introduction of interference; the thermocouple cannot be installed in the area where the measured medium rarely flows. When the thermocouple is used to measure the gas temperature in the pipe, the thermocouple must be installed against the direction of the flow velocity and fully in contact with the gas. .
2. Errors caused by poor insulation
If the thermocouple is insulated, too much dirt or salt slag on the protection tube and the wire drawing board will lead to poor insulation between the thermocouple and the furnace wall, which is more serious at high temperature, which will not only cause the loss of thermoelectric potential but also introduce interference. The error caused by this can sometimes reach Baidu.
3. Errors introduced by thermal inertia
This effect is especially pronounced when making fast measurements because the thermal inertia of the thermocouple causes the meter's indicated value to lag behind the change in the temperature being measured. Therefore, a thermocouple with a thinner thermal electrode and a smaller diameter of the protection tube should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than that of the furnace temperature fluctuation. The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuations and the larger 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 instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. For accurate temperature measurement, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and 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, materials with good thermal conductivity are usually used, and the tube wall is thin and the inner diameter is small to protect the sleeve. In more precise temperature measurement, bare wire thermocouples without protective sleeves are used, but thermocouples are easily damaged and should be corrected and replaced in time.
4. Thermal resistance error
When the temperature is high, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance will increase and hinder the conduction of heat. At this time, the temperature indication value 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.
1. Errors caused by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the real 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 8 to 10 times the diameter of the protection tube; The space between the protective sleeve of the thermocouple and the wall is not filled with insulating material, which causes the heat to overflow in the furnace or the intrusion of cold air. Therefore, the gap between the protective tube of the thermocouple and the hole of the furnace wall should be blocked with insulating material such as refractory mud or asbestos rope to avoid cold and heat. Air convection affects the accuracy of temperature measurement; the cold end of the thermocouple is too close to the furnace body so that the temperature exceeds 100 °C; 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 pipe to avoid errors caused by the introduction of interference; the thermocouple cannot be installed in the area where the measured medium rarely flows. When the thermocouple is used to measure the gas temperature in the pipe, the thermocouple must be installed against the direction of the flow velocity and fully in contact with the gas. .
2. Errors caused by poor insulation
If the thermocouple is insulated, too much dirt or salt slag on the protection tube and the wire drawing board will lead to poor insulation between the thermocouple and the furnace wall, which is more serious at high temperature, which will not only cause the loss of thermoelectric potential but also introduce interference. The error caused by this can sometimes reach Baidu.
3. Errors introduced by thermal inertia
This effect is especially pronounced when making fast measurements because the thermal inertia of the thermocouple causes the meter's indicated value to lag behind the change in the temperature being measured. Therefore, a thermocouple with a thinner thermal electrode and a smaller diameter of the protection tube should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than that of the furnace temperature fluctuation. The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuations and the larger 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 instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. For accurate temperature measurement, a thermocouple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and 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, materials with good thermal conductivity are usually used, and the tube wall is thin and the inner diameter is small to protect the sleeve. In more precise temperature measurement, bare wire thermocouples without protective sleeves are used, but thermocouples are easily damaged and should be corrected and replaced in time.
4. Thermal resistance error
When the temperature is high, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance will increase and hinder the conduction of heat. At this time, the temperature indication value 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.
