A gas delivery system accurately measures and optionally regulates mass flow rate in real time. A fluid conduit connects an inlet valve, calibration volume, flow restrictor, and outlet valve in series. Pressure and temperature sensors are coupled to the calibration volume. One or more pressure sensors may be attached across the flow restrictor. Alternatively, an absolute pressure sensor may be attached upstream of the flow restrictor. One embodiment of differential pressure sensors comprises a floating reference differential pressure sensor, including a first transducer attached to the fluid conduit upstream of the flow restrictor and a second transducer attached to the conduit downstream of the flow restrictor. In this embodiment, each transducer receives a reference pressure from a reference source, and optionally, after the calibration volume is charged, the floating reference differential pressure transducers are calibrated. When gas flow is initiated, differential and/or absolute pressure measurements are repeatedly taken, and a measured mass flow rate calculated thereon. Gas flow is adjusted until the measured mass flow rate reaches a target mass flow. Using the temperature/pressure sensors at the calibration volume, repeated calculations of actual flow rate are made to uncover any discrepancy between actual and measured mass flow rates. Whenever a discrepancy is found, the manner of calculating measured mass flow is conditioned to account for the discrepancy; thus, the measured mass flow rate more accurately represents the actual mass flow rate thereby providing an actual mass flow rate more accurately achieving the target mass flow rate.