A method of measuring a molecular rate of gas flow from an upstream source of gas at a pressure higher than a pressure of a destination includes flowing the gas through an upstream valve into a cavity of known volume, then through a restrictor element before proceeding to the destination, controlling the upstream valve to vary a flow rate of the gas through the upstream valve in a determined manner such that its average pressure in the cavity correlates with an average level of gas flow and such that variations in pressure in the cavity induced by varying the flow rate of gas through the upstream valve in the determined manner induce instantaneous pressure variations smaller than 30% of the average pressure in that cavity; and using the variations in the pressure in the cavity associated with varying the flow rate of gas through the upstream valve in the determined manner to determine the molecular rate of gas flow to the destination in real time.