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How To Use Thermocouple Protection Tube Correctly
- Apr 04, 2018 -

The correct use of thermocouples not only can accurately obtain the temperature value, ensure the product is qualified, but also can save the material consumption of the thermocouple, saving both money and product quality. Improper installation, errors in thermal conductivity and time lag, etc., are major errors in the use of thermocouples.

1. Improper installation error introduced

For example, the location and insertion depth of the thermocouple 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, the depth of insertion should be at least 8 to 10 times the diameter of the protection tube; The gap between the protective sleeve of the thermocouple and the wall is not filled with heat insulation material, so that the heat in the furnace overflows or the cold air invades. Therefore, the gap between the thermocouple protection tube and the wall hole of the thermocouple is blocked with heat insulating material such as fireproof mud or asbestos rope to avoid cold and heat. Convection of air affects the accuracy of temperature measurement; the thermocouple cold junction is too close to the furnace to make the temperature exceed 100°C; thermocouples should be installed as far as possible to avoid strong magnetic fields and strong electric fields, so thermocouples and power cables should not be installed on the thermocouples. Within the same conduit to avoid introducing interference caused by errors; thermocouple can not be installed in the area where the measured medium rarely flows. When using a thermocouple to measure the temperature of the gas inside the tube, the thermocouple must be installed against the flow velocity, and fully in contact with the gas .

2. Insulation variation and introduced error

If the thermocouple is insulated, the protection tube and the wire drawing board will have too much dirt or salt residue, which will lead to poor insulation between the thermocouple pole and the furnace wall, which will be more serious at high temperatures. This will not only cause the loss of thermoelectric power but also introduce interference. The resulting error can sometimes reach Baidu.

3, the error introduced by thermal inertia

Because of the thermal inertia of the thermocouple, the indicator value of the instrument lags behind the change in the measured temperature, and this effect is particularly pronounced in fast measurements. Therefore, thermocouples with thinner thermocouples and smaller diameters should be used as much as possible. When the temperature measurement environment permits, the protection tube can even be removed. Due to the measurement hysteresis, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the fluctuation of the furnace temperature. The larger the measurement lag, the smaller the amplitude of the fluctuation of the thermocouple and the greater the difference from the actual furnace temperature. When using thermocouples with large time constants for temperature measurement or temperature control, although the temperature displayed by the instrument fluctuates 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 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 method is to minimize the size of the hot end. . In use, a protective sleeve with a thin wall and a small internal diameter is generally used with good thermal conductivity. 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 promptly corrected and replaced.

4, thermal resistance error

At high temperatures, such as a layer of coal ash on the protective tube, dust attached to it, the thermal resistance increases, hinder the conduction of heat, then 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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