The measurement methods of dew point meters (humidity instruments) can be described as diverse, and their performance and price are also very different. This requires us to be cautious when selecting instruments. Not only should we consider performance and price, we should also consider the occasion where the instrument is used and what is measured. The type and corrosiveness of the gas. The general principles are as follows:
(1) 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 chilled mirror method instrument can be used.
(3) On-site inspection: If the measurement accuracy is high, the chilled mirror method instrument (ibid.) can be used.
(4) Continuous online monitoring: If the accuracy requirement is not too high, the resistance-capacitance method instrument can be used.
(5) Natural gas explosion-proof measurement: In the petrochemical and natural gas industries, we all require explosion-proof treatment, so we need a specific intrinsically safe explosion-proof dew point meter.
(6) Dew point measurement in the gas air separation industry: We know that the air separation industry generally requires a very low moisture content and a dew point below -70°C (1ppm level). In principle, cold mirror dew point meter, electrolysis dew point meter, and film dew point The instrument can not measure the gas below -80℃, so it is more suitable to choose the dew point instrument based on the principle of capacitance method.
What is dew point temperature
In winter, we will see a common 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, there will still be a fuzzy dew layer on the window glass. This phenomenon shows that the condensation temperature on the glass is related to the moisture content of the atmosphere in which the glass is located. Further research has 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 value for the moisture content of the atmosphere. Dew point can be simply understood as the temperature at which the water vapor content in the gas reaches the saturation state, and is one of the ways to express the humidity of the gas; it can be seen that the dew point temperature is a unit system for measuring the moisture content of the gas. The dew point analyzer is an instrument that measures the moisture content in a gas based on this unit system.
Dew point meter classification
1. Mirror type dew point meter
Gases with different moisture content will condense on the mirror surface at different temperatures. Using photoelectric detection technology, the exposed layer is detected and the temperature at the time of condensation is measured, and the dew point is directly displayed. 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, mirror cooling GX rate and precise measurement of the condensation temperature. At present, the high precision of Z in the world can reach ±0.1℃ (dew point temperature), and the general precision can reach within ±0.5℃.
2. 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 high precision of Z in the world can reach ±1.0℃ (dew point temperature), and the general precision can reach within ±3℃.
3. Dielectric method 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, an electrolysis micro-moisture meter based on the moisture content unit system is designed. At present, the high precision of Z in the world can reach ±1.0℃ (dew point temperature), and the general precision can reach within ±3℃.
4. Crystal oscillator dew point meter
Using the characteristic that the oscillation frequency changes after the crystal is wetted, the crystal oscillation 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.
5. 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.
6. Semiconductor sensor dew point meter
Each 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.
Dew point sensor calibration procedure
Temperature sensor cleaning
Compressed air dew point sensor
Compressed air dew point calculator
