A system to supply breathing gas to patient airways, comprising: a gas distribution manifold comprising at least one patient interface port; at least one controllable source of a breathing gas comprising a pressurized tank of the breathing gas; a gas flow controller in fluid connection between the pressurized tank and the gas distribution manifold, the gas flow controller volumetrically measuring and controlling in real time flow of the breathing gas, the gas flow controller comprising a calibration volume, a temperature sensor in operative connection with the calibration volume, a pressure sensor in fluid connection with the calibration volume, a controllable valve in fluid connection between the source of the breathing gas and the calibration volume, and a restrictor downstream from the calibration volume and in fluid connection between the calibration volume and the gas distribution manifold; and a control system in operative connection with the gas flow controller and the gas distribution manifold, characterized in that the control system is configured to control the controllable valve to vary the gas flow rate of breathing gas from the source of breathing gas in an oscillatory manner therethrough such that an average gas pressure in the calibration volume correlates with an average level of gas flow, and wherein variations in calibration volume pressure induced by varying the gas flow rate of the breathing gas through the controllable valve in the determined manner are used by the control system to determine and control an average molecular rate of gas flow to the at least one patient interface port in real time. a gas distribution manifold comprising at least one patient interface port; at least one controllable source of a breathing gas comprising a pressurized tank of the breathing gas; a gas flow controller in fluid connection between the pressurized tank and the gas distribution manifold, the gas flow controller volumetrically measuring and controlling in real time flow of the breathing gas, the gas flow controller comprising a calibration volume, a temperature sensor in operative connection with the calibration volume, a pressure sensor in fluid connection with the calibration volume, a controllable valve in fluid connection between the source of the breathing gas and the calibration volume, and a restrictor downstream from the calibration volume and in fluid connection between the calibration volume and the gas distribution manifold; and a control system in operative connection with the gas flow controller and the gas distribution manifold, characterized in that the control system is configured to control the controllable valve to vary the gas flow rate of breathing gas from the source of breathing gas in an oscillatory manner therethrough such that an average gas pressure in the calibration volume correlates with an average level of gas flow, and wherein variations in calibration volume pressure induced by varying the gas flow rate of the breathing gas through the controllable valve in the determined manner are used by the control system to determine and control an average molecular rate of gas flow to the at least one patient interface port in real time.