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Humidity measurement technology has a long history. With the development of electronic technology, modern measurement technology has also developed rapidly. Humidity measurement is divided into as many as 20 to 30 in principle. The expression methods of humidity include absolute humidity, relative humidity, dew point, the ratio of moisture to dry gas (weight or volume), etc. But humidity measurement has always been one of the famous problems in the world of metrology. A seemingly simple value, when studied in-depth, involves quite complex physics-chemical theoretical analysis and calculations. The beginner may ignore many factors that must be paid attention to in humidity measurement, thus affecting its reasonable use.

Standard 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.

· Dry and wet bulb method and various electronic sensor methods.

Double pressure method and double temperature method: It is based on the principle of thermodynamics P, V, T balance, the balance time is longer, and the split method is based on the precise mixing of absolute moisture and final dry air. Due to the modern measurement and control methods, this equipment can be made quite precise. Still, because the equipment is complex, expensive, and time-consuming and labor-intensive, it is mainly used for standard measurement, and its measurement accuracy can reach ±2%RH-±1.5%RH.

The saturated salt method in static method: It is the most common method in humidity measurement, simple and easy to implement. However, the saturated salt method has strict requirements on the balance of liquid and gas phases and higher requirements on ambient temperature stability. To use it requires a long time to balance, and the low humidity point requires a longer time. Especially when the difference between the indoor humidity and the humidity in the bottle is significant, it needs to be balanced for 6 to 8 hours each time it is turned on.

The Dew point method measures the temperature when the humid air reaches saturation, which is a direct result of thermodynamics, with high accuracy and a wide measurement range. The accuracy of the precision dew point meter used for measurement can reach ±0.2℃ or even higher. However, the cold mirror dew point meter using the modern photo-electric principle is expensive, and it is often used in conjunction with a standard humidity generator.

Wet and dry bulb method: It 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 indirect. It uses the wet and dry bulb equation to convert the humidity value, and this equation is conditional: the wind speed near the wet-bulb must reach 2.5m/s or more. The standard dry and wet bulb thermometer simplifies this condition, so its accuracy is the only 5~7%RH, which is significantly lower than that of the electronic humidity sensor. The wet and dry bulb does not belong to the static method. Don't simply think that as long as the measurement accuracy of the two thermometers is improved, the measurement accuracy of the hygrometer is improved.

Regarding the Measurement Method, Two Points are Emphasized:

First, because humidity is a function of temperature, temperature changes decisively affect the measurement results of humidity. Regardless of the method, accurately measuring and controlling the temperature is the priority. Note that even if it is a well-insulated constant temperature and humidity box, the temperature in the working room also has a specific gradient. Therefore, the humidity in this space is also brutal to be completely uniform. 

Second, due to the significant differences in principles and methods, it is difficult to calibrate and confirm various measurement methods directly. Most of them can only be compared by indirect methods. Therefore, it is tough to calibrate the measurement results of the whole humidity range (relative humidity 0~99.9%RH) between the two humidity measurement methods or to calibrate the measurement results of each point in all temperature ranges. For example, the ventilated wet and dry bulb hygrometer requires flowing air with a specified wind speed, while the saturated salt law requires strict sealing, and the two cannot be compared. The best way is to confirm it step by step according to the transmission method and verification procedures stipulated by the country for the verification system (standard) of humidity measurement instruments.