There are as many as twenty to thirty principles for humidity measurement using thermometers and hygrometers. However, humidity measurement has always been one of the famous 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 considered in humidity measurement, thus affecting the rational use of sensors.
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.
① The dual pressure method and dual temperature method are based on the thermodynamic principles of P, V, and T equilibrium, with longer equilibrium times. The split flow method is based on precise 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 of over ± 2% RH.
② The saturated salt method in static method is a simple and feasible method commonly used in 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.
③ 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.
④ The dry wet bulb method is a moisture measurement method invented in the 18th century. have a long history.. 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, and 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.