Water activity is nearly always measured by an instrument in which a sample is loaded into a sealed chamber. A sensor inside the chamber then measures the relative humidity of the air above the sample. A few things to keep in mind:
- Samples must reach vapor equilibrium (meaning their humidity must stop changing) before a water activity reading can be taken. Typically, this take a few minutes.
- Temperature affects water activity, but it is possible to get accurate readings without having temperature control. Standardizing all readings on a single temperature eliminates that as a variable. Water activity meters with internal temperature control tend to cost more than those that don’t.
- Most “fresh” products will have a water activity of close to 1.0 aw. Anything with greater than 50% moisture content probably falls into this category.