The influence of the salt/water mixing ratio on the climate control of salt solutions

Hygric property measurement has always been a heavily researched topic within Building Physics since accurate material properties are required for reliable hygrothermal simulations. Some widely accepted experimental techniques for measuring these properties require conditioning a volume of air to a specific relative humidity, for example, to determine a material’s sorption isotherm or when performing cup tests. This is typically done using salt/water mixtures, where the chosen salt dictates the obtained humidity. However, the mixing process is poorly documented. Several guidelines exist but often contradict each other, particularly regarding the water-to-salt ratio. Literature on salt solubilities varies, rarely specifying whether the solution should be undersaturated, saturated, or supersaturated. In the case of supersaturation, available information on excess salt content differs considerably. This paper therefore investigates the maintained relative humidity in function of the water to salt mixing ratio. This is done by mixing different salt solutions in a cup test setup and measuring the relative humidity inside this cup over time. The experimental part of this paper shows that there are two different types of salts. Some salts require a mixing ratio between certain limits to provide a constant relative humidity over time. This stable relative humidity could still diverge slightly from the intended value and hence measuring the relative humidity inside the cup during or after the experiment is advised. Other salts condition the air to a relative humidity which is heavily dependent on the solution’s mixing ratio. These salts can result in a changing relative humidity over time when applied in cup tests, especially for vapour open materials where lots of vapour is added or removed to the salt solution during the experiment. This varying relative humidity renders the assumption of a stationary cup test inapplicable and these salts should therefore be avoided at all cost.