Humidity measurement in constant temperature and humidity test and environmental test test is divided into two or thirty kinds in principle, but humidity measurement is always one of the famous problems in the field of world metrology. A seemingly simple measure of value, involving quite complex physics-chemical theoretical analysis and calculations, may involve many factors that must be noted in humidity measurement, thus affecting the rational use of the sensor.
Common methods of measuring humidity include: dynamic method (double pressure method, double temperature method, split method), static method (saturated salt method, sulfuric acid method), dew point method, dry and wet ball method and electronic sensor method.
1 Double pressure method and double temperature method are based on the thermodynamic P, V, T balance principle, and the equilibrium time is long. The split method is based on the precise mixing of absolute moisture and absolute dry air. Due to the adoption of modern measurement and control methods, these devices can be made quite sophisticated, but because of the complexity and cost of the equipment, the operation is time-consuming and labor-intensive, mainly used as a standard measurement, and its measurement accuracy can reach ±2% RH or more.
2 The saturated salt method in the static method is the most common method in humidity measurement, which is simple and easy. However, the saturated salt method requires strict balance between liquid and gas phases, and requires high stability to ambient temperature. It is required to wait for a long time to balance, and the low humidity point requires longer. Especially when the indoor humidity and the humidity inside the bottle are large, it needs to be balanced for 6~8 hours each time.
3 Dew point method is to measure the temperature when the humid air reaches saturation, which is a direct result of thermodynamics, high accuracy and wide measurement range. The accuracy of the precision dew point meter for measurement can reach ±0.2 °C or even higher. However, the cold mirror dew point meter using the modern light-electric principle is expensive and often used in conjunction with a standard humidity generator.
4 dry wet ball method, this is the method of measuring humidity invented in the 18th century. It has a long history and is most commonly used. The dry-wet ball method is an indirect method that converts the humidity value using the dry-wet ball equation, which is conditional: the wind speed near the wet ball must be above 2.5 m/s. The common dry and wet bulb thermometer simplifies this condition, so its accuracy is only 5~7% RH. The dry and wet bulbs are not static. Do not simply think that as long as the measurement accuracy of the two thermometers is increased, the humidity is increased. The measurement accuracy of the meter.
5 electronic humidity sensor method
Electronic humidity sensor products and humidity measurement belong to the industry that emerged in the 1990s. In recent years, domestic and foreign companies have made great progress in the research and development of humidity sensors. Humidity sensors are rapidly evolving from simple humidity sensors to integrated, intelligent and multi-parameter detection, creating favorable conditions for the development of a new generation of humidity measurement and control systems, and raising humidity measurement technology to a new level.
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