A theoretical development is presented showing that the quality of saturated mixture inside a heat-insulated discharging container may be expressed as a function of pressure and a parameter defined as instant flow work per unit flow mass, leaving the container. Since both the parameter and the pressure inside the container are easily measured, the derived relation may be used to create a device for measuring the vapor quality. Possible schemes and characteristics of such device as well as some properties of discharging process and accuracy are discussed. (c) 2006 Elsevier B.V. All rights reserved.