1, dual-pressure method, dual-temperature method is based on the thermodynamic P, V, T equilibrium principle, the equilibrium time is longer, the shunt method is based on the precise mixing of absolute moisture 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 of the equipment, expensive, time-consuming and labor-intensive operation, mainly as a standard measurement of the use of the measurement accuracy of up to plus or minus two percent of the RH or more.
2, static method of saturated salt method, is the most common method of humidity measurement, simple and easy to implement, but saturated salt method of liquid, gas and two-phase equilibrium requirements are very strict, the stability of the ambient temperature requirements are high, with the requirement to wait for a very long time to equilibrate the low-humidity point requires a longer period of time, especially in the indoor humidity and humidity inside the bottle a larger difference in the balance of every time you turn on the balance is required 6 to 8 hours.
3, dew point method is to measure the temperature of the wet air when saturated, is a direct result of thermodynamics, high accuracy, wide measurement range, measurement of precision dew point instrument accuracy up to plus or minus 0.2 degrees Celsius or even higher, but with modern optoelectronic principle of cold mirror dew point instrument is expensive, often used in conjunction with the standard humidity generator.
4, wet and dry bulb method, which was invented in the 18th century on the method of measurement of humidity, long history, the most common use, wet and dry bulb method is an indirect method, it uses wet and dry bulb equations converted to humidity values, and this equation is conditional: that is, in the vicinity of the wet bulb of the wind speed must be reached more than 2.5 meters per second. Commonly used dry and wet bulb thermometer to simplify this condition, so its accuracy is only five to seven percent RH dry and wet bulb is not a static method, do not simply think that as long as you improve the accuracy of the measurement of the two thermometers is the same as improving the accuracy of the measurement of the hygrometer.
5, electronic humidity sensor method