At equilibrium, the relationship between water content and equilibrium humidity of a material can be displayed graphically by a curve, the so-called
moisture sorption isotherm. For each
humidity value, a
sorption isotherm indicates the corresponding
water content value at a given, constant temperature. If the composition or quality of the material changes, then its sorption behaviour also changes. Because of the complexity of sorption processes, the isotherms cannot be determined by calculation, but must be recorded experimentally for each product.