Gas flow calibration is typically accomplished by measuring the rate at which volume is displaced by gas flowing at constant pressure. An alternative methodology is described below which uses the measurement of pressure drop at constant volume to define volumetric flow rates. The pressure of a gas leaking from a fixed volume through a small conductance decays exponentially. The time constant for the decay is directly proportional to the size of the fixed volume and inversely proportional to the conductance. Thus, for measurement intervals short compared to the exponential time constant, the pressure and consequentially the rate of gas flow from the volume falls linearly to first order. This greatly simplifies the calculation of flow rate. The calibrator design provides for various configurations of volume, conductance and gas pressure to generate a wide range of linear pressure drops in time intervals sufficient for collecting meaningful data. Measuring the pressure drop differentially, as well as absolutely, affords a high level of resolution and precision. A flow meter based on this concept has already been proven effective for an orifice flow vacuum calibration system.@footnote 1@ This paper describes a scaling up of that flow meter design to provide known gas flow rates approaching those generated by piston provers, with the possibility for even further extension. Details of the design and the overall calibration methodology are described. Sources of uncertainty are identified and the inherent advantages and practical limitations are discussed. @FootnoteText@ @footnote 1@P.D. Levine, J.R. Sweda "A Precision Gas Flowmeter For Vacuum Calibration", J. Vac. Sci. and Technol. A 15(3) May/June 1997; pp 747-752.