There are twenty or thirty kinds of humidity measurement in principle. However, humidity measurement has always been one of the most famous problems in the field of Metrology in the world. A seemingly simple value involves a rather complex physical-chemical theoretical analysis and calculation. Initial participants may neglect many factors that must be paid attention to in humidity measurement, thus affecting the rational use of sensors.
Common humidity measurement methods are: dynamic method (double pressure method, double temperature method, shunt method), static method (saturated salt method, sulfuric acid method), dew point method, wet-dry bulb method and electronic sensor method.
(1)The two-pressure method and the two-temperature method are based on the thermodynamic balance principle of P, V and T. The balance time is long. The split-flow method is based on the accurate mixing of absolute humidity and absolute dry air. Due to the use of modern measurement and control means, these devices can be made quite precise, but because of the complexity, high cost and time-consuming operation, they are mainly used as standard measurement, and their measurement accuracy can reach (+2% RH).
(2)Saturated salt method in static method is the most common method in humidity measurement, which is simple and feasible. However, the saturated salt method requires a strict balance between liquid and gas phases, and requires a higher stability of environmental temperature. It takes a long time to balance and a longer time to use at low humidity. Especially when the difference between indoor humidity and in-bottle humidity is large, every opening requires a balance of 6-8 hours.
(3)Dew point method is a direct result of thermodynamics to measure the temperature of wet air when it reaches saturation. It has high accuracy and wide measuring range. The accuracy of the precise dew point meter for measurement can reach (+0.2 C) or even higher. But the cold mirror dew point meter based on modern optoelectronic principle is expensive and often used in conjunction with standard humidity generator.
(4)Dry-wet bulb method, which was the humidity test invented in the 18th century. It has a long history and is most commonly used. The wet-dry sphere method is an indirect method. It uses the wet-dry sphere equation to calculate the humidity value. This equation is conditional: the wind speed near the wet sphere must be more than 2.5m/s. The common dry-wet bulb thermometer simplifies this condition, so its accuracy is only 5-7% RH, and the dry-wet bulb is not a static method. Don't simply think that as long as the measurement accuracy of two thermometers is improved, it is equal to the measurement accuracy of the hygrometer.
(5)Electronic humidity sensor method
Electronic sensor humidity test meter
instrument products and humidity test belong to the rising industry in the 1990s. In recent years, great progress has been made in the field of humidity sensor research and development at home and abroad. Humidity sensor is developing rapidly from simple humidity sensor to integrated, intelligent and multi-parameter detection, which creates favorable conditions for developing a new generation of humidity measurement and control system, and also improves the humidity measurement technology to a new level.