Dew point meter, dew point moisture meter
Dew point meter, dew point moisture meter
    A physical property analyzer that measures the dew point of a gas. The dew point can be simply understood as the temperature at which the water vapor content in the gas reaches saturation, which is one of the ways to express the absolute humidity of the gas. Therefore, the dew point meter is a special kind of hygrometer. Let a mirror surface cool in the sample's moist air, until the moment the mirror surface reveals a dew (or ice crystal), measure the average temperature of the mirror, which is the temperature of the dew (frost) point.
1 Introduction
In winter, we will see a common phenomenon. Due to the low outdoor temperature, the hot and humid indoor air will condense on the window glass, blurring the window glass. If we carefully observe and study it, if the dehumidifier is turned on indoors and the humidity in the room is gradually removed, then even though the outdoor temperature is the same, we will find that the dew on the window glass will slowly disappear and the window glass Showing the essence of transparent and clean. If the outdoor temperature drops at this time, then when the temperature drops to a certain level, although the dehumidifier has made the indoor air very dry, there is still a fuzzy layer of frost 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 studies have found that this relationship is a one-to-one correspondence, that is, each condensation temperature (we call it the dew point temperature) corresponds to the environment. A moisture content of the atmosphere. The dew point can be simply understood as the temperature at which the water vapor content in the gas reaches a saturated state, which is one of the ways to express the absolute humidity of the gas; therefore, the dew point temperature is a unit system for measuring the moisture content of the gas. A dew point analyzer is an instrument that measures the absolute moisture content in a gas based on this unit system.
In summary, the object measured by the dew point meter cannot be separated from the gas.
Dew point meter In order to get high-quality products or equipment to run normally, many industries such as petrochemical, electric power, electronics, aerospace, metallurgy, textile, etc., require more and more humidity measurement. Therefore, humidity measurement has gradually become a new phenomenon. In the field of technology, in the year of 86 China officially established the humidity and moisture professional committee and carried out several academic exchange meetings. Some measurement verification procedures for humidity were gradually established. According to the relevant regulations, humidity is defined as the content of water vapor in the gas. Commonly used units are: g/l, PPM, mmHg, dew point and relative humidity. It is customary to divide the measured gas into high-humidity gas and low-humidity gas (ie, trace amounts of water) at the dew point of 20° C. This section focuses on the measurement of low-humidity gas.
2 humidity measurement principle
Mirror type
Gases with different moisture contents will dew on the mirror surface at different temperatures. Photoelectric detection technology was used to detect the exposed layer and measure the temperature of condensation.
Cold mirror dew point meter
Dew point is displayed directly. Mirror cooling methods include: semiconductor refrigeration, liquid nitrogen refrigeration, and high-pressure air refrigeration. The direct dew point meter uses a direct measurement method. The dew point meter can be used as a standard dew point meter under the premise of ensuring accurate inspection, high efficiency of mirror cooling, and accurate measurement of condensation temperature. At present, the highest precision in the world is ±0.1°C (dew point temperature), and the general accuracy can reach within ±0.5°C.
Hydrophilic or hydrophobic materials are used as the medium to form capacitors or resistors. After the gas containing water has flowed, the dielectric constant or conductivity changes accordingly. When the capacitance value or resistance value is measured at the time, it can be known. Gas water content at the time. This type of sensor designed on a dew point system constitutes an electrical sensor dew point analyzer. At present, the highest precision in the world reaches ±1.0°C (dew point temperature), and the general accuracy can reach within ±3°C.
Electrolytic dew point meter
After using phosphorus pentaoxide and other materials to decompose into polar molecules and accumulating charge on the electrodes, an electrolysis micro moisture analyzer based on absolute moisture content system was designed.
Crystal oscillator type dew point meter
By using the characteristics of crystal oscillator wet frequency changes, you can design crystal oscillation dew point meter. This is a relatively new technology and it is still not at a very mature stage. There are related products abroad, but the accuracy is poor and the cost is high.
Infrared dew point meter
An infrared dew point meter can be designed using the infrared absorption of water in the gas. The instrument is difficult to measure low dew point, mainly the infrared detector's peak detection rate can not reach the magnitude of trace water absorption, as well as other components of the gas content of the infrared spectrum absorption interference. However, this is a very new technology and it is of great significance for non-contact online monitoring of the moisture content of environmental gases.
Semiconductor sensor
Each water molecule has its natural frequency of vibration. When it enters the gaps in the semiconductor lattice, it resonates with the lattice excited by the charge, and its resonant frequency is proportional to the number of moles of water. The resonance energy of the water molecule causes the semiconductor junction to release free electrons, thereby increasing the conductivity of the crystal lattice and reducing the impedance. A semiconductor dew point meter designed with this characteristic can measure trace moisture at -100°C dew point.
Weight method
It is a classic measurement method. Let the measured sample flow through a certain desiccant, the moisture content of which is absorbed by the desiccant, and the moisture content absorbed by the desiccant is precisely weighed. The ratio of the sample volume to the sample volume is the humidity of the sample gas. The advantage of this method is its high precision, with a maximum allowable error of up to 0.1%. The disadvantage is that the specific operation is difficult. In particular, a sufficient amount of absorbed water quality (generally not less than 0.6 g) must be obtained. This is particularly difficult for low-humidity gases. Increasing the sample gas flow results in increased measurement time and errors (measured humidity is not an instantaneous value). Therefore, this method is only suitable for measuring the gas with a dew point of -32°C or more. It can be said that there are fewer instruments on the market that use this method to measure humidity.
From the above analysis, the key to the gravimetric method is how to accurately measure the moisture content absorbed by the desiccant because direct measurement is more difficult, and two methods for indirectly measuring the absorbed water content are derived.
Electrolytic method
That is, the moisture absorbed by the desiccant is electrolyzed into hydrogen gas and oxygen gas through an electrolytic cell. The size of the electrolysis current is proportional to the moisture content, and the humidity of the sample gas can be measured by detecting the current. This method compensates for the shortcomings of the gravimetric method. The measuring range can reach below -80°C, and the accuracy is good and the price is cheap. The disadvantage is that the electrolytic cell gas path needs to be dried for a long time before use, and the corrosiveness and cleanliness of the gas are required. High demand. There are many instruments using this method. Typical are 1-C type micro-water meters from Edgetech, and M303 from DuPont and domestic USI series.
Vibration frequency method
The desiccant in the gravimetric method is replaced with a hygroscopic quartz crystal. According to the characteristics that the crystal absorbs water with different masses at different vibration frequencies, the sample gas and the standard dry air flow pass through the crystal, resulting in different vibration frequency differences. f1 and f2, calculate the difference between the two frequencies to get the sample gas humidity. This method has the same advantages as the electrolytic method, and does not need to be dried before use. The typical representative instrument is British Michell's QMA series, the United States AMETEK's 560B.
Cold Mirror
It is also a classic measurement method. Let the sample flow through the condensation mirror of the dew point cold mirror chamber, through the equal pressure refrigeration, so that the sample gas reaches a state of saturation condensation (droplets on the condenser mirror), and the temperature of the condensation mirror at this time is the dew point of the sample gas. The main advantage of this method is high accuracy, especially after using semiconductor refrigeration and photoelectric detection technology, the uncertainty can even reach 0.1 °C; the disadvantage is that the response speed is slow, especially below the dew point -60 °C, the balance time is even several Hours, and this method has higher requirements on the cleanliness and corrosion of the sample gas, otherwise it will affect the photoelectric detection effect or produce 'false condensation' and cause measurement error. Representative representatives of this method are the British company Michell, General Eastern, and MBW.
Resistive capacitance method
Is a continuous improvement of the humidity measurement method. Using a high-purity aluminum rod, the surface is oxidized to an ultra-thin aluminum oxide film, and a layer of extra-spaced mesh gold film is coated on the surface. A capacitance is formed between the gold film and the aluminum rod, resulting in the water absorption characteristics of the aluminum oxide film. The capacitance value changes with the amount of moisture in the sample gas, and the humidity of the sample gas can be obtained by measuring the capacitance value. The main advantage of this method is that the measurement range can be even lower, even up to -100°C. Another outstanding advantage is that the response speed is very fast, and the response from dry to wet can reach 90% in one minute, so it is mostly used in on-site and rapid measurement occasions; The accuracy is poor, and the uncertainty is mostly ±2 to 3°C. Aging and drift are serious and must be calibrated after 3 to 6 months of use. Representative manufacturers of this method are the British Alpha Humidity Instrument Company, Ireland's PANAMETRICS Company and the United States' XENTAUR Company. However, with the continuous efforts of various manufacturers, this method is gradually being perfected. For example, the stability of the sensor is greatly improved by changing the materials and improving the process, and the compensation linearity of the sensor response curve is made linear to solve the problem of automatic calibration. Representative product is Britain's Michelh's Easidew series, using ceramic-based aluminum oxide capacitors and C2TX microprocessors.
3 Measurement Considerations
Effect of mirror contamination on dew point measurement
In the dew point measurement, mirror pollution is a prominent problem, and its impact is mainly manifested in two aspects; one is the Raoul effect, and the second is to change the background radiation level of the mirror. The Raoul effect is caused by water-soluble substances. If the test gas carries this substance (usually soluble salts), the mirror surface will dew condensation in advance, causing a positive deviation in the measurement results. If the contaminants are particles that are insoluble in water, such as dust, the scattering level of the background will increase, and the zero point drift of the photoelectric dew point meter will occur. In addition, vapors of substances that evaporate easily, such as organics, that have a boiling point lower than that of water will, of course, interfere with dew point measurements. Therefore, any type of dew point meter should prevent contamination of the mirror. In general, the impact of industrial process gas analysis pollution is more serious. However, even in the measurement of pure gas, the pollution of the mirror will accumulate over time.
Selection of measurement conditions
In the design of the dew point instrument, various factors that directly affect the heat and mass exchange in the dew condensation process should be considered. This principle also applies to the selection of the dew point instrument operating conditions that are not very automatic. Here mainly discuss the mirror cooling speed and sample gas flow problems.
1. The temperature of the gas to be measured is usually room temperature. Therefore, when the airflow passes through the dew point chamber, it must affect the heat transfer and mass transfer process of the system. When other conditions are fixed, increasing the flow rate will facilitate mass transfer between the airflow and the mirror. Especially when measuring the low frost point, 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 true for thermoelectric refrigeration dewpoint meters with relatively low cooling power. Too much flow rate will also result in dew point chamber pressure drop and the change of flow rate will affect the thermal balance of the system. Therefore, it is necessary to select the appropriate flow rate in the dew point measurement. The choice of flow rate depends on the cooling method and the structure of the dew point chamber. The general flow rate range is 0.4~0.7L. Between min-1. In order to reduce the effect of heat transfer, pre-cooling may be considered before the measured gas enters the dew point chamber.
2. The control of the mirror cooling rate in the dew point measurement is an important issue. The dew point meter for the automatic dew point meter is determined by the design, and the dew point meter for hand control of the cooling amount is an operation problem. Because there is a process of heat conduction between the cooling point, temperature measurement point and mirror surface of the cold source, there is a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of condensation (frost) and introduce errors to the measurement results. This situation differs depending on the temperature measuring element used. For example, due to the structural relationship, the temperature gradient between the measuring point and the mirror surface of the platinum resistance temperature sensing element is relatively large, and the heat conduction speed is also relatively slow, so that the temperature measurement and the condensation occur. Cannot synchronize. Moreover, the thickness of the exposed layer cannot be controlled. This will produce negative errors for visual inspection.
3. Another problem is that cooling too fast may cause "too cold." We know that under certain conditions, when the water vapor reaches saturation, the liquid phase still does not appear, or water still freezes below zero, this phenomenon is called supersaturation or “too cold”. For the condensation (or frost) process, this phenomenon is often caused by the very clean gas and mirror surface, or even the lack of a sufficient number of condensation cores. Suomi found in experiments that if a highly polished mirror is clean and chemically satisfactory, the formation temperature of the dew is a few degrees lower than the true dew point temperature. The phenomenon of overcooling is short-lived, and the total length of time is related to the dew point or frost point temperature. This phenomenon can be observed through a microscope. One of the solutions is to repeat the heating and cooling of the mirror until the phenomenon is eliminated. Another solution is to directly use the vapor pressure data of supercooled water. And this coincides with the definition of relative humidity when the meteorological system is below zero.

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