Methods and system for concentrating oxygen include providing a portable apparatus having a plurality of sieve beds, each sieve bed in the plurality of sieve beds including a first end and a second end, a reservoir storing oxygen-enriched gas exiting from the second ends of the plurality of sieve beds, a compressor, an oxygen delivery valve communicating with the reservoir via a delivery line, a flow sensor associated with the delivery line, and control electronics adapted to control operation of the oxygen delivery valve; measuring, via the flow sensor, a flow of the oxygen-enriched gas through the sensor and outputting a signal indicative thereof; and controlling, using the control electronics, the opening and closing the oxygen delivery valve to deliver the oxygen-enriched gas from the reservoir to a user based on the signal.