A method of volumetrically measuring and controlling an average molecular rate of gas flow from an upstream source of gas having a pressure higher than a pressure of a destination for the gas, comprising: flowing the gas through an upstream valve into a cavity of known volume, then through a restrictor element before proceeding to the destination, wherein the upstream valve is the only valve controlled via the control electronics between the upstream source of gas and the destination for the gas while volumetrically measuring and controlling the average molecular rate of gas flow to the destination which permits the downstream gas flow rate to the destination to vary within process-compatible limits around the average molecular rate of gas flow in a repeated oscillatory manner; controlling the upstream valve such that an average pressure in the cavity correlates with an average level of the gas flow rate; characterised in that the method additionally includes the steps of, controlling the upstream valve via control electronics to repeatedly vary the flow rate of gas from the upstream source through the upstream valve in a determined oscillatory manner between a state of higher gas flow rates and a state of lower gas flow rates such that the gas flow rate through the upstream valve in the state of lower gas flow rates is no more than five percent of the gas flow rate in the state of higher gas flow rates; controlling the upstream valve 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 the cavity; monitoring a temperature in the cavity using a temperature sensing element to enable correction for gas density changes; and calculating the molecular rate of gas flow to the destination in real time when the upstream valve is in the state of lower gas flow rates using a rate of fall of pressure in the cavity to control the molecular rate of gas flow to the destination. flowing the gas through an upstream valve into a cavity of known volume, then through a restrictor element before proceeding to the destination, wherein the upstream valve is the only valve controlled via the control electronics between the upstream source of gas and the destination for the gas while volumetrically measuring and controlling the average molecular rate of gas flow to the destination which permits the downstream gas flow rate to the destination to vary within process-compatible limits around the average molecular rate of gas flow in a repeated oscillatory manner; controlling the upstream valve such that an average pressure in the cavity correlates with an average level of the gas flow rate; characterised in that the method additionally includes the steps of, controlling the upstream valve via control electronics to repeatedly vary the flow rate of gas from the upstream source through the upstream valve in a determined oscillatory manner between a state of higher gas flow rates and a state of lower gas flow rates such that the gas flow rate through the upstream valve in the state of lower gas flow rates is no more than five percent of the gas flow rate in the state of higher gas flow rates; controlling the upstream valve 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 the cavity; monitoring a temperature in the cavity using a temperature sensing element to enable correction for gas density changes; and calculating the molecular rate of gas flow to the destination in real time when the upstream valve is in the state of lower gas flow rates using a rate of fall of pressure in the cavity to control the molecular rate of gas flow to the destination.