How do we measure Water Activity on cannabis product?
Water activity (Aw) is defined as the partial vapor pressure of water in a substance § divided by the saturation vapor pressure (P0), also known as the standard state partial vapor pressure of water:
Aw = P/P0
Water activity can range from 0.000, which would be the measurement of a dry sample devoid of water content, to 1.000, which would be the measurement of pure aqueous water free of any bonding forces.
Water activity is measured by obtaining the saturation vapor pressure and the vapor pressure of water in the sample at a specific temperature. Common instruments take these measurements from a small sample of the product that is inserted into a water activity meter (the instrument labs have to measure water activity).
First, the meter measures the temperature of the sample, which directly correlates to the saturation vapor pressure (P0). Once vapor equilibrium is established, an infrared beam focuses on a small mirror in the instrument’s chamber to measure the vapor pressure of the water in the sealed headspace above the sample tray. This equilibrium vapor pressure equals the vapor pressure of water in the sample §.
These two measurements are then used to calculate the water activity (Aw). Accuracy can be determined by measuring calibration standards (salt solutions of varying concentrations) with known water activity values. KB Labs tests Water Activity in Cannabis products.*