The water activity meter method is one of the official methods for determining the moisture content of a material. This method is based on the principle that the water activity of a material is a measure of its free water content. Water activity is defined as the ratio of the vapor pressure of the water in the material to the vapor pressure of pure water at the same temperature. The water activity meter measures the vapor pressure of the water in the material and uses this measurement to calculate the water activity.
One benefit of the water activity meter method is that it can be used to measure the moisture content of a wide variety of materials. This method can also be used to measure the moisture content of materials that are difficult to dry, such as hygroscopic materials.