The energy level of water in liquid media is a powerful parameter which can be measured through the water activity of the solution. A sensor for on-line measurement of this parameter has been developed recently, but measurement of this parameter remains difficult for high water activity values. In this paper a sensor based on the measurement of the capillary ascension induced by a difference in osmotic pressure between two systems is developed theoretically, and the results of the application of a sensor of this type to osmotic flux measurement are presented.