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.