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Dew point is a temperature value, why use it to express humidity? This is because when the water vapor in the air has reached saturation, the temperature is the same as the dew point; When the water vapor is not saturated, the air temperature must be higher than the dew point temperature. Therefore, the difference between dew point and air temperature can indicate the saturation degree of water vapor distance in the air. At 100% relative humidity, the ambient temperature is the dew point. The smaller the dew point is than the temperature of the surrounding environment, the smaller the possibility of dew condensation is, which means that the drier the air is, and the dew point is not affected by temperature, but by pressure.
principle
The dew point meter is a special instrument that can measure the dew point temperature, so that a mirror surface is cooled in the sample wet air until the moment when ice crystals appear on the mirror surface, the average temperature of the mirror surface is measured, that is, the dew point temperature. It has high humidity measurement accuracy, but needs a mirror with high finish, a temperature control system with high accuracy, and an optical detection system for ice crystals with high sensitivity. The pipe that sucks sample air must be kept clean during use, otherwise the impurities in the pipe will absorb or release water, causing measurement error.
Relevant background of dew point meter
In order to obtain high-quality products or equipment for normal operation, many industries, such as petrochemical, electric power, electronics, aerospace, metallurgy, textiles, have increasingly high requirements for humidity measurement. Therefore, humidity measurement has gradually become a new technical field. In 1986, China formally established the humidity and moisture professional committee, and carried out many academic exchanges, Some metrological verification regulations for humidity have also been gradually established.
classification
1. Mirror dew point meter
Gases with different moisture content will condense on the mirror surface at different temperatures. Photoelectric detection technology is adopted to detect the exposed layer and measure the temperature when dewing, so as to directly display the dew point. The mirror cooling methods include semiconductor cooling, liquid nitrogen cooling and high pressure air cooling. The mirror dew point meter adopts a direct measurement method. On the premise of ensuring accurate dew detection, high efficiency of mirror cooling and precise measurement of condensation temperature, the dew point meter can be used as a standard dew point meter. At present, the highest accuracy in the world is ± 0.1 ℃ (dew point temperature), and the general accuracy can be within ± 0.5 ℃.
2. Electric sensor dew point meter
Hydrophilic or hydrophobic materials are used as the medium to form capacitance or resistance. After the gas containing water flows through, the dielectric constant or conductivity changes accordingly. The current capacitance or resistance value can be measured to know the current gas moisture content. This kind of sensor, which is designed on the basis of dew point unit system, constitutes an electrical sensor dew point analyzer. At present, the highest accuracy in the world is ± 1.0 ℃ (dew point temperature), and the general accuracy can be within ± 3 ℃.
3. Dielectric dew point meter
Based on the characteristics of phosphorus pentoxide and other materials, which can absorb moisture and decompose into polar molecules, so as to accumulate charges on the electrode, an electrolytic micro moisture meter based on the absolute moisture content unit system was designed. At present, the highest accuracy in the world is ± 1.0 ℃ (dew point temperature), and the general accuracy can be within ± 3 ℃.
4. Crystal oscillation dew point meter
The crystal oscillation dew point meter can be designed by taking advantage of the change of oscillation frequency after crystal wetting. This is a relatively new technology, which is still in its immature stage. There are related products abroad, but the precision is poor and the cost is high.
5. Infrared dew point meter
The infrared dew point meter can be designed by taking advantage of the absorption characteristics of water in the gas to the infrared spectrum. 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 level of micro water absorption, and the content of other components in the gas interferes with the infrared spectrum absorption. However, this is a very new technology, which is of great significance for the non-contact online monitoring of environmental gas moisture content.
6. Semiconductor sensor dew point meter
Each water molecule has its natural vibration frequency. When it enters the gap of the semiconductor lattice, it will resonate with the lattice excited by the charge, and its resonance frequency is proportional to the number of moles of water. The resonance of water molecules can make the semiconductor junction emit free electrons, thus increasing the conductivity of the lattice and reducing the impedance. The semiconductor dew point meter designed by using this characteristic can measure the trace moisture at - 100 ℃ dew point.
purpose
It is widely used in industrial fields and industrial sites with special explosion-proof requirements.
Medium gas, as a process medium, participates in process reaction, or serves as a protective gas, or as a standard gas, is widely used in the corresponding production process in modern industry.
As a kind of process environment, ambient gas is widely used in the relevant process environment of civil industry and military industry.
In order to obtain high-quality products or normal operation of equipment, many industries such as petrochemical, electric power, electronics, aerospace, metallurgy, textile and so on require higher and higher humidity measurement.
Precautions
1. Effect of mirror surface pollution on dew point measurement
In dew point measurement, mirror pollution is a prominent problem, and its influence mainly displays in two aspects; One is Raoult effect, the other is to change the background radiation level of the mirror. The Raoult effect is caused by water-soluble substances. If the measured gas carries this substance (usually soluble salts), the mirror will dewater in advance, which will cause a positive deviation in the measurement results. If the pollutant is insoluble particles, such as dust, it will increase the background scattering level, so that the photoelectric dew point meter will have zero drift. In addition, it is self-evident that the vapor of some easily condensed substances (such as organic substances) with boiling points lower than water will interfere with the measurement of dew point. Therefore, no matter what type of dew point meter is, the mirror surface should be prevented from being polluted. Generally speaking, the impact of gas analysis pollution in industrial processes is relatively serious. However, even in the measurement of pure gas, the contamination of the mirror will accumulate with time.
2. Selection of measurement conditions
In the design of dew point instrument, various factors that directly affect the heat and mass exchange in the dewing process should be emphatically considered. This principle is also applicable to the selection of operating conditions of dew point instrument with low degree of automation. Here we mainly discuss the cooling rate of the mirror surface and the flow rate of the sample gas.
A. The temperature of the measured gas is usually room temperature. Therefore, when air flows through the dew point chamber, it will inevitably affect the heat and mass transfer process of the system. When other conditions are fixed, increasing the flow rate will benefit the mass transfer between the airflow and the mirror. Especially when measuring the low frost point, the flow rate should be increased appropriately to speed up the formation of outcrops, but the flow rate should not be too large, otherwise it will cause overheating problems. This is especially obvious for thermoelectric refrigeration dew point meter with small refrigeration power. If the flow rate is too high, the pressure in the dew point chamber will decrease, and the change of the flow rate will affect the heat balance of the system. Therefore, it is necessary to select an appropriate flow rate in dew point measurement. The selection of flow rate should depend on the refrigeration method and the structure of the dew point chamber. The general flow rate range is 0.4~0.7L/min. In order to reduce the influence of heat transfer, pre cooling treatment can be considered before the measured gas enters the dew point chamber.
B. In the dew point measurement, the control of mirror cooling speed is an important problem. For the automatic photoelectric dew point meter, it is determined by design, while for the dew point meter with hand controlled cooling capacity, it is a problem in operation. Because there is a process of heat conduction between the cooling point of the cold source, the temperature measuring point and the mirror surface, and there is a certain temperature gradient. Therefore, thermal inertia will affect the process and speed of condensation (frost) and bring errors to the measurement results. This situation also varies with the temperature measuring elements used. For example, due to the structural relationship, the temperature gradient between the measuring point of the platinum resistance temperature sensor and the mirror surface is relatively large, and the heat conduction speed is relatively slow, so that the temperature measurement and condensation cannot be carried out synchronously. Moreover, the thickness of the exposed layer cannot be controlled. This will produce a negative error for visual inspection.
C. Another problem is that too fast cooling may cause "supercooling". We know that under certain conditions, when the water vapor reaches the saturation state, the liquid phase still does not appear, or when the water is below zero, it still does not freeze. This phenomenon is called supersaturation or "supercooling". For the condensation (or frost) process, this phenomenon is often caused by the fact that the measured gas and mirror surface are very clean, or even lack of enough condensation cores. Suomi found in the experiment that if a highly polished mirror surface and its cleanliness meet the chemical requirements, the dew formation temperature is several degrees lower than the real dew point temperature. The supercooling phenomenon is transient, and the total time is related to the dew point or frost point temperature. This phenomenon can be observed through a microscope. One solution is to repeat the operation of heating and cooling the mirror until this phenomenon is eliminated. Another solution is to directly use the steam pressure data of supercooled water. And this is exactly consistent with the definition of relative humidity when the meteorological system is below zero.
Introduction to types and principles of dew point