1 Mirror dew point meter
Gas with different moisture content will condense on the mirror surface at different temperatures. The photoelectric detection technology is used to detect the exposed layer and measure the temperature at the time of condensation.
2 Chilled mirror dew point meter
directly displays the dew point. The methods of mirror refrigeration include: semiconductor refrigeration, liquid nitrogen refrigeration and high-pressure air refrigeration. The mirror-type dew point meter adopts a direct measurement method. This type of dew point meter can be used as a standard dew point meter under the premise of ensuring accurate dew detection, high efficiency of mirror cooling and precise measurement of dew temperature. At present, the highest accuracy in the world can reach ±0.1℃ (dew point temperature), and the general accuracy can reach within ±0.5℃.
3 Electric sensor type dew point meter
Hydrophilic or hydrophobic materials are used as the medium to form a capacitor or resistor. After the gas with moisture flows through, the dielectric constant or conductivity changes accordingly, and the current capacitance or resistance value can be measured. The moisture content of the gas at that time. This type of sensor designed on the basis of the dew point unit system constitutes an electric sensor type dew point analyzer. At present, the highest accuracy in the world can reach ±1.0°C (dew point temperature), and the general accuracy can reach within ±3°C.
4 Electrolysis dew point meter
Using the characteristics of phosphorus pentoxide and other materials that absorb moisture and decompose into polar molecules, thereby accumulating charges on the electrode, design an electrolysis micro-moisture meter based on the absolute moisture content unit system. At present, the highest accuracy in the world can reach ±1.0°C (dew point temperature), and the general accuracy can reach within ±3°C.
5 Crystal oscillator dew point meter
Using the characteristic that the oscillation frequency changes after the crystal is wetted, the crystal oscillator type dew point meter can be designed. This is a relatively new technology, and it is still at an immature stage. There are related products abroad, but the accuracy is poor and the cost is high.
6 Infrared Dew Point Meter
The infrared spectrometer can be designed by using the moisture in the gas to absorb the infrared spectrum. At present, it is difficult for this instrument to measure the low dew point, mainly because the peak detection rate of the infrared detector cannot reach the level of the absorption of trace water, and the content of other components in the gas interferes with the absorption of the infrared spectrum. But this is a very new technology, which is of great significance to the non-contact online monitoring of the moisture content of the ambient gas.
7 Semiconductor sensor dew point meter
Every water molecule has its natural vibration frequency. When it enters the gap of the semiconductor crystal lattice, it resonates with the crystal lattice excited by the charge, and its resonance frequency is proportional to the number of moles of water. The resonance of water molecules can release free electrons from the semiconductor junction, thereby increasing the conductivity of the crystal lattice and reducing the impedance. The semiconductor dew point meter designed with this feature can measure the trace moisture of -100 ℃ dew point.
The definition of dew point is relatively easy to understand. When the gas is cooled by isostatic pressure, the ability of the gas to hold water is weakened, and the temperature at which the moisture in the gas changes from the gas phase to the liquid phase and condenses from the gas is defined as the dew point temperature. There are many methods for measuring dew point in gas, such as electrical parameter method (including aluminum oxide humidity sensor, polymer organic film humidity sensor), P2O5 complete absorption electrolysis, and cold mirror method (measurement based on the physical definition of dew point temperature) , Optics, quartz oscillation and other methods. Each method has its own advantages and disadvantages. The electrical signal (impedance value) of a dew point meter (also called a humidity meter) with alumina as a humidity sensor has a certain relationship with the partial pressure of water vapor in the gas, that is to say, it has a direct correspondence with the dew point of the gas. The indicated units are calculated based on the result of the dew point. The dew point meter using polymer organic film as humidity sensor has a certain relationship between its electrical parameters (capacitance value) and the relative humidity of the gas. Other humidity units, such as the dew point value, are calculated based on the signals of relative humidity and temperature. The instrument of the cold mirror method has a cold mirror sensor, which is composed of a mirror, a semiconductor refrigeration system, a temperature measurement system, an optical measurement system, and a signal control system. When the mirror surface temperature is higher than the dew point, the mirror surface is smooth and the reflected light is strong. When the refrigerator works, the mirror surface temperature drops. When the mirror surface temperature is lower than the dew point temperature, condensate is formed on the mirror surface, and the reflected light is at this time At this time, the signal of the photodetector changes, and then feedback controls the working current of the refrigerator, so that a certain thickness of condensate is maintained on the mirror surface. The temperature at this time is measured by the four-wire platinum resistor buried under the mirror surface. Is the dew point temperature. The advantages of this type of instrument are high measurement accuracy, good stability, and can be used as a standard instrument. Of course, units with sufficient funds can also use this type of instrument as a working measuring instrument. So the above question, the head of the dew point meter is not easy to answer in general.
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Vaisala dew point Meter manual