Analysis of the Use and Installation of Thermocouple Sensors

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

Improper installation of the error such as the location of the thermocouple and the insertion depth can not reflect the real 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 the diameter of the protection tube 8 to 10 times; the gap between the protective sleeve of the thermocouple and the wall is not filled with heat insulating material, so that the heat in the furnace overflows or the cold air intrudes. Therefore, the space between the thermocouple protection tube and the wall hole is insulated by fireproof mud or asbestos rope, etc. The material is blocked to avoid the convection of hot and cold air and affect the accuracy of temperature measurement; the thermocouple cold end is too close to the furnace body 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 should not be The power cable is installed in the same conduit to avoid introducing interference to cause errors; the thermocouple cannot be installed in the area where the measured medium rarely flows. When measuring the gas temperature in the tube with a thermocouple, the thermocouple must be installed against the flow direction. And fully in contact with the gas.

The error introduced by the thermal inertia is due to the thermo-inertia of the thermocouple, which causes the indicator value of the instrument to lag behind the change in the measured temperature. This effect is particularly pronounced when performing rapid 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 a thermocouple with a large time constant is used 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.

Thermal resistance error at high temperatures, such as a layer of coal ash on the protective tube, dust attached to the above, the increase in thermal resistance, hinder the conduction of heat, metal tube float flowmeter, turbine flowmeter, vortex flowmeter, when the temperature value Lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce errors.

Insulation is poor and the introduced error is such as thermocouple insulation. Excessive dirt or salt residue of the protection tube and wire-pull board causes poor insulation between the thermocouple pole and the furnace wall, which is more serious at high temperatures, which not only causes thermoelectric potential. Loss and will also introduce interference, resulting in errors sometimes up to Baidu.