The dew point meter is an instrument that directly measures the dew point temperature. 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. 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. It has high humidity measurement precision, but requires a high-quality mirror surface, high-accuracy temperature control system, and a highly sensitive optical detection system for dew drops (ice crystals). Use must be kept in the sample air suction pipe clean, otherwise the impurities in the pipe will absorb or release moisture causing measurement error.
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Gravimetry is a classic measurement method. Let the sample flow through a certain desiccant, the moisture content of which is absorbed by the desiccant, and accurately weigh the moisture content absorbed by the desiccant. 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 especially 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 for measuring humidity in the market purely by using this method.
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.
The electrolysis method is that the moisture absorbed by the desiccant is electrolyzed by the electrolytic cell into hydrogen gas and oxygen gas. 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.
The vibration frequency method is to replace the desiccant in the gravimetric method with a hygroscopic quartz crystal. According to the characteristics that the crystal absorbs water with different masses when the vibration frequency is different, the sample gas and the standard dry air flow pass through the crystal, resulting in different Vibration frequency difference â–³ f1 and â–³ f2, calculate the difference between the two frequencies can be obtained sample gas humidity. This method has the same advantages as the electrolytic method, and it does not need to be dried before use. The typical representative instrument is 560B from American AMETEK Company.
The cold mirror method is also a classic measurement method. Let the sample flow through the condensation mirror in the dew point cold mirror chamber, and through equal pressure cooling, make the sample gas reach the state of saturation condensation (drop of condensation on the condensation mirror). 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 manufacturers of this method are the British MICHELL company, the United States General Eastern company and Switzerland's MBW company.
The RC method is a continuous improvement of the humidity measurement method. Using a high-purity aluminum rod, the surface is oxidized to form 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. Therefore, 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. Use 3 to 6 months for calibration. 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 products are British MICHELL's EASYDEW series, using ceramic-based aluminum oxide capacitors and C2TX microprocessors.
Humidity Measurement Method MICHELL's Optidew Vision Cold Mirror Dew Point Meter Mirror Dew Point Meter The gas with different moisture content will condense on the mirror surface at different temperatures. Photoelectric detection technology was used to detect the dew layer and measure the temperature of the dew condensation. The dew point was directly displayed. 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.
The electric sensor type dew point meter uses a hydrophilic material or a hydrophobic material as a medium to form a capacitance or resistance. After the gas containing water has flowed, the dielectric constant or the conductivity changes accordingly, and the capacitance value or the resistance at that time is measured. The value will know the gas moisture content at that 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.
Electrolysis dew point meter uses phosphorus pentaoxide and other materials to decompose into polar molecules after absorbing moisture, thereby accumulating charge characteristics on the electrode, and designing an electrolysis micro moisture meter based on the absolute moisture content unit system. At present, the highest precision in the world reaches ±1.0°C (dew point temperature), and the general accuracy can reach within ±3°C.
The crystal oscillation type dew point meter utilizes the characteristic that the oscillation frequency changes after the crystal is wetted, and the crystal oscillator type dew point meter can be designed. 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 dewpoint instrument can use infrared moisture absorption characteristics of the water in the gas, you can design infrared dewpoint instrument. At present, it is difficult for the instrument to detect low dew point, mainly because the peak detection rate of the infrared detector cannot reach the magnitude of trace water absorption, and the interference of other components in the gas to the infrared spectrum absorption. 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 dew point meter each water molecule has its natural vibration frequency, when it enters the gap of the semiconductor lattice, it will generate resonance with the lattice excited by the charge, and its resonant frequency is proportional to the molar number of water. The resonance energy of the water molecule causes the semiconductor junction to release free electrons, thereby increasing the conductivity of the lattice and reducing the impedance. A semiconductor dew point meter designed with this characteristic can measure trace moisture at -100°C dew point. Several trace water measurement methods and dew point meter selection.
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Gravimetry is a classic measurement method. Let the sample flow through a certain desiccant, the moisture content of which is absorbed by the desiccant, and accurately weigh the moisture content absorbed by the desiccant. 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 especially 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 for measuring humidity in the market purely by using this method.
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.
The electrolysis method is that the moisture absorbed by the desiccant is electrolyzed by the electrolytic cell into hydrogen gas and oxygen gas. 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.
The vibration frequency method is to replace the desiccant in the gravimetric method with a hygroscopic quartz crystal. According to the characteristics that the crystal absorbs water with different masses when the vibration frequency is different, the sample gas and the standard dry air flow pass through the crystal, resulting in different Vibration frequency difference â–³ f1 and â–³ f2, calculate the difference between the two frequencies can be obtained sample gas humidity. This method has the same advantages as the electrolytic method, and it does not need to be dried before use. The typical representative instrument is 560B from American AMETEK Company.
The cold mirror method is also a classic measurement method. Let the sample flow through the condensation mirror in the dew point cold mirror chamber, and through equal pressure cooling, make the sample gas reach the state of saturation condensation (drop of condensation on the condensation mirror). 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 manufacturers of this method are the British MICHELL company, the United States General Eastern company and Switzerland's MBW company.
The RC method is a continuous improvement of the humidity measurement method. Using a high-purity aluminum rod, the surface is oxidized to form 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. Therefore, 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. Use 3 to 6 months for calibration. 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 products are British MICHELL's EASYDEW series, using ceramic-based aluminum oxide capacitors and C2TX microprocessors.
Humidity Measurement Method MICHELL's Optidew Vision Cold Mirror Dew Point Meter Mirror Dew Point Meter The gas with different moisture content will condense on the mirror surface at different temperatures. Photoelectric detection technology was used to detect the dew layer and measure the temperature of the dew condensation. The dew point was directly displayed. 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.
The electric sensor type dew point meter uses a hydrophilic material or a hydrophobic material as a medium to form a capacitance or resistance. After the gas containing water has flowed, the dielectric constant or the conductivity changes accordingly, and the capacitance value or the resistance at that time is measured. The value will know the gas moisture content at that 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.
Electrolysis dew point meter uses phosphorus pentaoxide and other materials to decompose into polar molecules after absorbing moisture, thereby accumulating charge characteristics on the electrode, and designing an electrolysis micro moisture meter based on the absolute moisture content unit system. At present, the highest precision in the world reaches ±1.0°C (dew point temperature), and the general accuracy can reach within ±3°C.
The crystal oscillation type dew point meter utilizes the characteristic that the oscillation frequency changes after the crystal is wetted, and the crystal oscillator type dew point meter can be designed. 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 dewpoint instrument can use infrared moisture absorption characteristics of the water in the gas, you can design infrared dewpoint instrument. At present, it is difficult for the instrument to detect low dew point, mainly because the peak detection rate of the infrared detector cannot reach the magnitude of trace water absorption, and the interference of other components in the gas to the infrared spectrum absorption. 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 dew point meter each water molecule has its natural vibration frequency, when it enters the gap of the semiconductor lattice, it will generate resonance with the lattice excited by the charge, and its resonant frequency is proportional to the molar number of water. The resonance energy of the water molecule causes the semiconductor junction to release free electrons, thereby increasing the conductivity of the lattice and reducing the impedance. A semiconductor dew point meter designed with this characteristic can measure trace moisture at -100°C dew point. Several trace water measurement methods and dew point meter selection.
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