Once the grain has been placed in a bin, flat store or big-bag, an equilibrium is established over a period of time between the relative humidity of the air (hygrometry) in the grain mass and the water content of the grain as measured by a moisture meter. This correspondence between grain water activity and measured water content can be represented graphically by a "sorption isotherm" (figure 3).
Example: When the relative humidity between grains is stable at 70% saturation (= water activity in the grain of 0.70), this corresponds to a water content measured at 14.5% for wheat, 11.5% for soya and 8.2% for sunflower (at a fixed temperature of 20°C).