Humidity measurement technology has a long history. With the development of electronic technology, modern measurement technology has also made rapid progress. Humidity measurement can be divided into as many as 20-30 types based on principles. The methods for representing humidity include humidity, relative humidity, dew point, the ratio of moisture to dry gas (weight or volume), and so on. However, humidity measurement has always been one of the challenges in the field of metrology worldwide. A seemingly simple quantity, when delved into, involves quite complex physical chemical theoretical analysis and calculations. Novices may overlook many factors that must be taken into account in humidity measurement, thus affecting its rational use.
Common humidity measurement methods include: dynamic method (dual pressure method, dual temperature method, split current method), static method (saturated salt method, sulfuric acid method), dew point method, dry wet bulb method, and electronic sensor method for shape and color.
The double pressure method and double temperature method here are based on the thermodynamic principles of P, V, and T equilibrium, with longer equilibrium times, while the split flow method is based on the mixing of moisture and dry air. Due to the use of modern measurement and control methods, these devices can be made quite precise. However, due to their complexity, high cost, and time-consuming operation, they are mainly used as standard measuring devices, with measurement accuracy ranging from ± 2% RH to ± 1.5% RH.
The saturated salt method in static methods is a common and simple method for humidity measurement. However, the saturated salt method requires strict equilibrium between liquid and gas phases, and has high requirements for stable environmental temperature. It takes a long time to balance when used, and even longer for low humidity points. Especially when there is a large difference between indoor humidity and the humidity inside the bottle, it takes 6-8 hours to balance each time it is opened.
The dew point method is a direct result of thermodynamics that measures the temperature at which humid air reaches saturation, with high accuracy and a wide measurement range. The precision dew point meter used for measurement can achieve an accuracy of ± 0.2 ℃ or even higher. However, cold mirror dew point meters based on modern photoelectric principles are expensive and often used in conjunction with standard humidity generators.
The dry wet bulb method is a moisture measurement method invented in the 18th century. Long history and widespread use. The dry wet bulb method is an indirect method that converts humidity values using the dry wet bulb equation, which is conditional: the wind speed near the wet bulb must be above 2.5m/s. The commonly used dry wet bulb thermometer simplifies this condition, so its accuracy is only 5-7% RH, significantly lower than electronic humidity sensors. Obviously, the dry wet bulb does not belong to the static method. Do not simply think that improving the measurement accuracy of two thermometers is equivalent to improving the measurement accuracy of the hygrometer.
This article wants to emphasize two points: because humidity is a function of temperature, changes in temperature decisively affect the measurement results of humidity. Regardless of the method, temperature measurement and control are crucial. It should be noted that even a well insulated constant temperature and humidity chamber has a certain temperature gradient in its working chamber. So the humidity in this space is also difficult to be uniformly consistent.
Secondly, due to significant differences in principles and methods, it is difficult to directly calibrate and identify various measurement methods, and most can only be compared indirectly. So it is very difficult to cross check the measurement results of the full humidity range (relative humidity 0~RH) between the two humidity measurement methods, or to calibrate the measurement results of each point in all temperature ranges. For example, a ventilated dry wet bulb hygrometer requires flowing air with a specified wind speed, while the saturated salt method requires strict sealing, and the two cannot be compared. A good approach is to follow the transmission method and verification regulations specified by the national humidity measuring instrument verification system (standard) for hierarchical recognition.