Water potential is an equilibrium measure. This often is not the case with foods. Sometimes however, the water is moving through the system so slowly that equilibrium cannot be achieved in the foods normal lifttime. In this case it can for practical purposes be said that the water is bound. For example, it is hypothesised that when foods enter the glassy state the movement of water is so slow that it is effectively bound.
Water potential is a quantitative measure of the binding energy of water in food.
There are esssentially two types of sensors used for measuring water activity. This is the primary method approved by the AOAC International. It's major advantage is speed. Some can take readings in as little as 5 minutes. This compared to 30-90 minutes for the capacitive sensors.