It is also useful to be able to generate and maintain a constant humidity atmosphere within a contained volume. In addition to being able to use all of the constant flow methods to fill up a space with humid air, there are a couple more methods used for small spaces.
The two-pressure process utilises one chamber (the saturator) at high pressure, and another (the test chamber) at the desired pressure, both at the same temperature. Saturated air flows from the saturator into the test chamber. As it reduces in pressure, the desired humidity is achieved, and if the systems are at a constant temperature then the humidity in the test chamber is the ratio between the chamber pressures. This is given by:
As pressure, temperature and saturation partial pressure are related by the perfect gas relations and Antoine equation, it is unnecessary to directly measure the humidity. This can lead to faster and more accurate readings of the humidity in the test chamber.
The two-temperature process also uses a saturator and test chamber. While both of these are at the same pressure, the saturator is at a lower temperature. The air in the saturator is saturated, with the temperature of the chamber defining the wet air's dew-point. This air is then passed into the (warmer) test chamber. The mixing ratio will not change, and thus the relative humidity will decrease. By knowing the saturation vapour pressure at various temperatures, and the temperatures of the two chambers, any desired relative humidity can be achieved.
When placed in an enclosed volume, salt solutions will (given enough time) generate a certain, predictable humidity. The undissolved salt will absorb water, while water will evaporate from the solution. At a constant temperature, these rates will be constant, and thus a constant humidity can be generated. There must be both saturated solution and undissolved salt present for this to work. For given chemicals and temperatures, the humidity generated can be looked up; the humidity is given by the water activity or equilibrium relative humidity of the substance.