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What is a dew point meter?

Overview of Dew Point Analyzer
In winter, we all see such a phenomenon. Due to the low outdoor temperature, the humid and hot indoor air will condense on the window glass, making the window glass blurred. If we observe and study carefully, if we turn on the dehumidifier indoors to gradually remove the humidity in the room, even though the outdoor temperature is still the same, we will find that the dew on the window glass will gradually disappear, and the window glass will be heavy again. Reveal the essence of transparency and cleanliness. If the outdoor temperature drops at this time, when the temperature drops to a certain level, although the dehumidifier has made the indoor air very dry, a fuzzy dew layer will still appear on the window glass. This phenomenon shows that the condensation temperature on the glass is related to the water content of the atmosphere in which the glass is located. Further research has found that this relationship is a one-to-one relationship, that is, each condensation temperature (we call it the dew point temperature) corresponds to the environment A value for the moisture content of the atmosphere. It can be seen that the dew point temperature is a unit system for measuring the moisture content of a gas. The dew point analyzer is based on this unit system and measures the absolute moisture content in the gas.
In summary, the object measured by the dew point meter is inseparable from gas, and the corresponding gas is nothing more than three uses: power gas, medium gas and ambient gas
As a power source, power gas is supplied to pneumatic instruments and pneumatic equipment, and is widely used in industrial fields and industrial sites with special explosion-proof requirements.
As a process medium, or participate in the process reaction, or as a protective gas, or as a standard gas, the medium gas is widely used in the corresponding production process in modern industry
As a process environment, ambient gas is widely used in related process environments of civil industry and military industry.
The dew point meter is used in the occasions where the moisture content of the above three types of gases needs to be monitored
The above is a general introduction to the measurement principle of the instrument. In addition, with suitable instruments, accurate and reliable measurement results cannot be guaranteed. The dew point measurement result is affected by many factors, such as the material of the sampling pipe, the sealing of the valve joint, and the valve installation position. For example, before the measuring instrument is installed, it will be vented to the atmosphere after it comes out of the measuring instrument. At this time, it is normal pressure measurement. If installed after the measuring instrument, the gas is advanced to the measuring instrument, and then discharged after the valve adjusts the flow, this is pressure measurement. At the same time, attention should be paid to the purification treatment of the gas before entering the measuring instrument. If there are particulate pollutants in the gas, a filter that removes particulate pollutants should be used, and if liquid pollutants need to be removed, a coalescing filter should be used. .

All in all, because the moisture content in the gas has a wide range, from tens of thousands of ppm to the ppb level, and the partial pressure of water vapor cannot be measured directly like the total pressure of the gas, nor can it be measured non-contact like the temperature, so it is more difficult Because it needs to be traced to temperature or pressure, or other basic quantities, its accuracy is difficult to achieve the mk level of temperature measurement. In addition, the atmosphere contains a lot of moisture, which will affect the measurement results, so it is difficult to measure the dew point (humidity).
In the design of the dew point instrument, various factors that directly affect the heat and mass exchange of the condensation process should be considered. This principle is also applicable to the selection of the operating conditions of the dew point instrument with a low degree of automation. Here we mainly discuss the problems of mirror cooling speed and sample gas flow rate.
(1) The temperature of the measured gas is usually room temperature. Therefore, when the airflow passes through the dew point chamber, the heat and mass transfer process of the system must be affected. When other conditions are fixed, increasing the flow rate will facilitate the mass transfer between the airflow and the mirror. Especially when measuring low frost points, the flow rate should be appropriately increased to speed up the formation of the exposed layer, but the flow rate should not be too large, otherwise it will cause overheating problems. This is especially obvious for thermoelectric refrigeration dew point meters with relatively small refrigeration power. If the flow rate is too large, the pressure in the dew point chamber will decrease, and the change of the flow rate will affect the heat balance of the system. Therefore, it is necessary to select an appropriate flow rate in the dew point measurement, and the choice of flow rate should depend on the refrigeration method and the structure of the dew point chamber. The general flow rate range is 0.4~0.7 -1. In order to reduce the influence of heat transfer, pre-cooling can be considered before the measured gas enters the dew point chamber.
(2) In the dew point measurement, the control of the cooling speed of the mirror is an important issue. For the automatic photoelectric dew point meter, it is determined by the design, while for the dew point meter with hand-controlled cooling capacity, it is a problem in operation. Because the heat conduction between the cooling point, the temperature measurement point and the mirror surface of the cold source has a process and there is a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of condensation (frost) and bring errors to the measurement results.


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