An intelligent molecular flow sensor [see Note (1) on next page] enables direct gas flow measurements in the transitional and molecular flow regimes, as well as in the slip and continuum flow regimes, thereby enabling flow measurements at pressures ranging from vacuum through atmospheric. The properties of this sensor are investigated, along with the sensor's transitional operating ranges. The IMFS operates as a mass flow sensor (directly measures the mass flow) in the transitional and molecular flow regimes, and it operates as a volumetric flow sensor in the continuum and slip flow regimes. Understanding of these properties and ranges of operation are critical for demanding leak testing, sealed closure integrity testing and vacuum test applications. (C) 2004 American Vacuum Society.