Humidity measurement technology has a long history. With the development of electronic technology, modern measurement technology has also developed rapidly. There are as many as two or thirty kinds of humidity measurement in principle. There are absolute humidity, relative humidity, dew point, the ratio of moisture to dry gas (weight or volume) and so on.
Common humidity measurement methods
dynamic method (double pressure method, double temperature method, shunt method), static method (saturated salt method, sulfuric acid method), dew point method, dry and wet bulb method and various electronic sensor methods.
The dual pressure method and dual temperature method
The dual pressure method and dual temperature method are based on the thermodynamic P, V, T balance principle, and the balance time is long. The split method is based on the precise mixing of absolute humidity and absolute dry air. Due to the use of modern measurement and control methods, these devices can be quite precise, but due to complex, expensive, time-consuming and labor-intensive operation, they are mainly used for standard measurement, and their measurement accuracy can reach ±2%RH – ±1.5%RH.
The saturated salt method in the static method
The saturated salt method in the static method is the most common method in humidity measurement, which is simple and easy to implement. However, the saturated salt method has strict requirements on the balance of liquid and gas phases, and has higher requirements on the stability of ambient temperature. It takes a long time to equilibrate, and low humidity points require longer. Especially when the difference between the indoor humidity and the humidity in the bottle is large, it needs to be balanced for 6 to 8 hours each time it is opened.
The dew point method
The dew point method is to measure the temperature when the humid air reaches saturation, which is the direct result of thermodynamics, with high accuracy and wide measurement range. The accuracy of the precision dew point meter for measurement can reach ±0.2℃ or even higher. However, the chilled mirror dew point meter using the modern optical-electrical principle is expensive and is often used in conjunction with a standard humidity generator.
Dry and wet bulb method
Dry and wet bulb method, which is a method of measuring humidity invented in the 18th century. It has a long history and is the most commonly used. The wet and dry bulb method is an indirect method, which uses the wet and dry bulb equation to convert the humidity value, and this equation is conditional: that is, the wind speed near the wet bulb must reach more than 2.5m/s. The common wet and dry bulb thermometer simplifies this condition, so its accuracy is only 5~7%RH, which is obviously lower than that of electronic humidity sensor. Obviously, the wet and dry bulb is not a static method. Don’t simply think that improving the measurement accuracy of the two thermometers is equivalent to improving the measurement accuracy of the hygrometer.
