Known constant humidities can be created by materials whose affinity for water regulates the water vapor pressure in the atmosphere surrounding the material.
Among the more readily controllable materials are salt solutions and acid solutions. Numerous lists of the humidities created by various salts and acid concentrations appear in literature, inferring a certain ease of producing the specified humidity conditions. An ease of maintenance of these humidity conditions with high accuracy is also inferred since the temperature dependency is very small. The ease of handling and low low cost of salts has promoted the widespread use of salt solutions as humidity generators.