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Four precautions for the installation and use of temperature and humidity transmitters

by:Rika Sensors     2021-11-13
Four precautions for the installation and use of temperature and humidity transmitters
The temperature and humidity transmitter is a monitoring instrument that can continuously measure temperature and humidity, and can sense the temperature and convert it into a usable output signal. In daily monitoring work, it is also one of the most prone to failure of the instrument.

Temperature transmitter is the core part of temperature measuring instrument, and there are many varieties. According to the measurement method, it can be divided into two categories: contact type and non-contact type. According to the characteristics of transmitter materials and electronic components, it can be divided into two types: thermal resistance and thermocouple.

So what should be paid attention to when using different types of temperature and humidity transmitters?

1. Errors caused by improper installation.

For example, 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. The depth of insertion should be at least 8-10 times the diameter of the protective tube; the gap between the protective tube of the thermocouple and the wall is not filled with insulation material, which causes heat overflow or cold air intrusion in the furnace, so between the thermocouple protective tube and the hole in the furnace wall The gap should be blocked with refractory mud or asbestos rope 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 as much as possible And strong electric field, so thermocouple and power cable should not be installed in the same conduit to avoid introducing interference and causing errors; thermocouple cannot be installed in an area where the measured medium seldom flows. When using thermocouple to measure the gas temperature in the tube, The thermocouple must be installed against the direction of the flow rate and fully in contact with the gas.

2. Errors introduced by the deterioration of insulation

For example, if the thermocouple is insulated, too much dirt or salt slag on the protection tube and pull-out board causes poor insulation between the thermocouple poles and the furnace wall, which is even worse at high temperatures. To be serious, this 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

Because of 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 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 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, 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 measured temperature. The true value is low. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce errors.

The installation location of the temperature and humidity transmitter should be representative. Avoid installing it in dead ends of temperature, strong magnetic field and beside the furnace door, or too close to heating objects. The junction box of the temperature and humidity transmitter must not touch the container wall of the measured medium. The temperature at the junction box of the temperature and humidity transmitter should not exceed 100℃

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