An oxygen concentrator having one or more molecular sieve beds for selectively adsorbing nitrogen from atmospheric air for increasing the oxygen concentration of a product gas, a reservoir for receiving the product gas, a surge tank, a compressor for charging atmospheric air to the sieve beds and surge tank and a valve and valve switching means for directing atmospheric air from the compressor to the sieve beds and surge tank includes means for sensing the withdrawal rate of product gas from the reservoir and a microprocessor having means for switching the valve for charging the surge tank for a miniumum time required to achieve a product gas having a selected oxygen concentration at the sensed rate of withdrawal.

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