Thread Content
In order to obtain high-quality products and ensure the proper operation of equipment, many industries such as petrochemicals, power generation, electronics, aerospace, metallurgy, and textiles have increasing demands for humidity measurement. As a result, humidity measurement has gradually become an emerging field of technology. In 1986, China established a specialized committee for humidity and moisture, organized numerous academic conferences, and gradually developed various standards for the measurement and calibration of humidity. According to relevant regulations, humidity is defined as the amount of water vapor in a gas, with common units including grams per liter, PPM, mmHg, dew point, and relative humidity. *It is common to use a dew point of -20°C as a threshold to classify the measured gases into high-humidity gases and low-humidity gases (i.e., those with trace amounts of water); this section focuses on the measurement of low-humidity gases. Mirror-type dew point meter: Gases with different moisture contents will form dew on a mirror at various temperatures. Using photoelectric detection technology, the dew layer is detected and the temperature at the time of dew formation is measured, thereby directly displaying the dew point. Methods of mirror cooling include semiconductor cooling, liquid nitrogen cooling, and high-pressure air cooling. The mirror-type dew point meter employs a direct measurement method; by ensuring accurate dew detection, high-efficiency cooling of the mirror, and precise measurement of the dew point temperature, this type of dew point meter can be used as a standard dew point meter. Internationally, the highest precision reaches ±0.1°C (dew point temperature), while the typical precision is within ±0.5°C. Electrical sensor-type dew point meters use hydrophilic or hydrophobic materials as media to form capacitors or resistors; when a gas containing moisture passes through, the dielectric constant or conductivity changes accordingly. By measuring the capacitance value or resistance value at that moment, it is possible to determine the moisture content in the gas. Sensors designed on the basis of the dew point unit system constitute electric sensor-type dew point analyzers. Internationally, the highest precision reaches ±1.0°C (dew point temperature), while the typical precision is within ±3°C. The electrolytic dew point meter takes advantage of the property that materials such as phosphorus pentoxide absorb moisture and then decompose into polar molecules, thereby accumulating charge on the electrodes; this principle is utilized to develop an electrolytic micro-humidity meter based on an absolute humidity measurement system. A crystal oscillation dew point meter can be designed by utilizing the property that the oscillation frequency changes when a crystal becomes wet. This is a relatively new technology that is still not very mature. There are related products abroad, but their precision is poor and their cost is high. Infrared dew point meters can be designed by utilizing the property of water vapor in gases to absorb infrared light. It is difficult for this instrument to detect low dew points, mainly because the peak detection sensitivity of the infrared detector is not yet sufficient to detect trace amounts of water absorption, and the presence of other components in the gas interferes with the absorption in the infrared spectrum. But this is a very new technology, and it holds great significance for the non-contact online monitoring of the moisture content in ambient gases. Semiconductor sensors: Each water molecule has its own natural vibration frequency. When it enters the gaps in the semiconductor lattice, it resonates with the lattice that is charged and excited; this resonance frequency is proportional to the number of moles of water. The resonance of water molecules enables semiconductor junctions to emit free electrons, thereby increasing the electrical conductivity of the lattice and reducing its impedance. Semiconductor dew point meters designed using this feature can detect trace amounts of water at a dew point of -100°C.