Fluid flow controlling

This invention relates to fluid flow controlling and, more particularly, to systems and methods for controlling gas flow. A method and system for controlling the rate of fluid flow. A flow restrictor having known pressure drop and flow rate characteristics provided in a passage through which the fluid, preferably a gas, flows. An upstream pressure sensor determines the pressure of fluid in the flow passage upstream of the flow restrictor. A downstream pressure sensor determines the pressure of fluid in the flow passage downstream of said flow restrictor. A pressure regulator adjusts the pressure of fluid upstream or downstream of the flow restrictor based on the pressure drop across the flow restrictor so that the actual pressure drop across the flow restrictor closely corresponds to the pressure drop associated with a desired rate of fluid flow.