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The water activity of a material is defined by the equilibrium water vapor pressure above the product (pw), divided by the vapor pressure of pure water (p0). The aw-value equals the relative humidity of the surrounding atmosphere in equilibrium with the material divided by 100. Thus, aw-values are dimensionless and can take values between 0 and 1.
Further helpful information about the behavior of the material can be obtained from the shape of an isotherm that is influenced by the interaction between the vapor molecules and the solid. Thus, the course of an isotherm can give information about the present sorption mechanism. In addition, some materials show hysteresis effects which can also be evaluated with the help of a sorption isotherm. Based on this information, valuable insights into moisture-related material properties and processes can be obtained.
