What does the water activity (aw) output from the moisture in oil instruments indicate?

Water activity (aw) is a measurement indicating moisture content in a fluid on a scale from 0…1 (0 being completely dry, 1 being completely saturated). It provides a relative measurement describing how saturated the oil is. For example, a reading of 0.90 aw would mean the oil is 90% saturated with water.

Every fluid has the ability to hold water in the dissolved state below the saturation point. Once the saturation point of that fluid has been reached, any additional water that enters the fluid will separate out into “free water” forming a distinct layer usually below the oil.

For more information on this subject, please see the article entitled Understanding water activity.