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Selection of dew point meter

Humidity is defined as the content of water vapor in the gas and is an important environmental parameter. The water vapor in the atmosphere affects the physical, chemical and biological processes in nature. Humidity affects the heat, electricity, light, and transmission properties of the gas. Environmental humidity also affects the moisture content in solid and liquid materials, causing dimensional changes in materials, corrosion of metal materials, and even determining the survival of organisms. Humidity measurement and control are widely used in aerospace, microelectronics, atomic energy, petrochemical, electric power, meteorology, storage and other fields. In view of the importance of humidity measurement, countries are constantly updating their humidity standards.
The basic quantity for measuring gas humidity is the mixing ratio, that is, the ratio of the mass of water vapor to the mass of dry gas, which can be achieved with a gravimetric hygrometer. In a gravimetric hygrometer, the water vapor and carrier gas in the airflow are separated, collected and weighed separately (or indirectly measuring the volume and density of dry gas). However, it is difficult to measure the mixing ratio by the gravimetric method and the operation is cumbersome. Most of the actual measured humidity values ​​use the dew point, although the dew point is not the most basic unit of humidity. If the relationship between mixing ratio and humidity is compared to the relationship between thermodynamic temperature scale and temperature, the relationship between dew point and humidity is similar to the relationship between practical temperature scale (ITS-90) and temperature.
The principle of    dew point hygrometer can be explained by a simple experiment. If you put a smooth metal surface in the air with a relative humidity lower than 100% and let it cool, when the temperature drops to a certain value, the relative humidity near the surface reaches 100%, and there will be exposed on the surface. form. Because the water vapor in the air is saturated at this temperature, the water film attached to the cold surface and the water in the air are in dynamic equilibrium, that is, the number of water molecules leaving and returning to the surface in a unit time is the same. It is the Regnault principle. The principle can be described as: when a certain volume of humid air is uniformly cooled under a constant total pressure, until the water vapor in the air reaches a saturated state, this state is called the dew point; in the cooling process, the partial pressure of both gas and water vapor constant. If the temperature of the air is Ta and the temperature of dew formation is Td, the relative humidity of the humid air can be calculated by the following formula, U=[(saturated water pressure at the dew point temperature Td)/(saturated water at the original temperature Ta Air pressure)]*100% The value of saturated water pressure in the formula can be obtained by looking up the table. Below 0°C, when the water vapor reaches saturation, the water freezes on the mirror surface. The temperature at this time is also called the frost point.

7.1 The selection of dew point meter
The measurement methods of dew point meters (humidity instruments) can be described as various, and their performance and price are also very different. This requires us to be cautious when selecting instruments. Not only should the performance and price be considered, but also the occasion where the instrument is used and the measurement. The type and corrosiveness of the gas. The general principles are as follows:
(1) National Humidity Benchmark: Considering the requirements for high measurement accuracy and ideal sample gas, a chilled mirror dew point meter should generally be used.
(2) Enterprise benchmark or laboratory analysis: If the measurement accuracy is high, the cold mirror method instrument can be used; if the range requirement is lower (dew point below -80°C) and the gas is clean, the electrolytic method instrument can be used.
(3) On-site inspection: If the measurement accuracy is high, the chilled mirror method instrument (same as above) can be used; if the measurement speed is required or the gas pollution is heavy, it is best to use the resistance-capacitance method instrument.
(4) Continuous online monitoring: If the accuracy requirement is not too high, the resistance-capacitance method instrument can be used.
7.2 Problems that should be paid attention to in the measurement of dew point meter
The influence of mirror pollution on dew point measurement: In dew point measurement, mirror pollution is a prominent problem, and its influence is mainly manifested in two aspects; one is the Raoul effect, and the other is to change the level of mirror background radiation. The Raoult effect is caused by water-soluble substances. If this substance (usually a soluble salt) is carried in the measured gas, the mirror surface will condense in advance, which will cause a positive deviation in the measurement result. If the pollutant is water-insoluble particles, such as dust, etc., it will increase the level of background scattering, which will cause the zero-point drift of the photoelectric dew point meter. In addition, some vapors of easily condensable substances (such as organic matter) with a lower boiling point than water will obviously interfere with the measurement of dew point. Therefore, regardless of any type of dew point meter, the mirror surface should be prevented from being polluted. Generally speaking, the impact of industrial process gas analysis pollution is more serious. But even in the measurement of pure gas, the contamination of the mirror surface will accumulate over time.


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