In order to meet the test requirements, it is inevitable to humidify and dehumidify the constant temperature and humidity test box. At present, various methods are widely used in the constant temperature and humidity test box to analyze, pointing out their respective advantages and disadvantages and advocating conditions for application.

Humidity is indicated in many ways. As far as experimental equipment is concerned, the concept of relative humidity is usually used to describe humidity. Relative humidity is defined as the ratio of vapor partial pressure in air to saturated vapor pressure of water at that temperature and expressed by percentage. From the nature of water vapor saturation pressure, it can be seen that water vapor saturation pressure is only a function of temperature and has nothing to do with the air pressure where water vapor can be located. Through many experiments and sorting out, the relationship between water vapor saturation pressure and temperature has been found. Among them, the Gouglier formula has been widely used in engineering and measurement. It has been selected by the meteorological department in compiling the humidity checklist.

The process of humidification is actually to increase the water vapor pressure. The initial method of humidification is to spray water on the wall of the experimental box. The saturation pressure of the water surface is controlled by controlling the water temperature. The water on the surface of the box wall constitutes a larger surface. The method of diffusing the water vapor pressure into the box increases the relative humidity of the experimental box. This method appeared in the 1950s. Because the control of humidity at that time was mainly based on the simple switch adjustment of mercury electric contact type conductive meter, the control adaptability of water temperature of hot water tank with large lag was poor, so the transition process of control was long, and the requirement of humidification quantity for alternating humidity and heat was not satisfied. More importantly, when spraying on the wall of the box, water droplets inevitably sprayed on the test product. Products constitute different levels of pollution. There are also certain requirements for drainage in the box. This method was quickly replaced by steam humidification and shallow water dish humidification.

But there are some advantages to this approach. Despite its long process of control transition, the humidity fluctuation of the system after stabilization is relatively small, so it is more suitable for doing stable humidity and heat experiments. In addition, during the process of humidification, water vapor is not heated enough to add extra heat to the system. Moreover, when the spray water temperature is controlled to lower the critical temperature required by the experiment, the spray water has the dehumidification effect. With the development of wet-heat experiment from stable wet-heat to alternating wet-heat, it is required to have faster reaction ability. When spray humidification can not satisfy the requirement, steam humidification and shallow water dish humidification methods are selected and developed.