Abstract: A vacuum pressure swing absorption system provides portable oxygen to medical patients. A pair of sieve beds are alternately pressurized with air and evacuated by vacuum by coordinated operation of a selector valve and a vent valve. Product gas flows from both sieve beds through respective check valves to a product tank. An equalization valve is operated in coordination with the selector valve and the vent valve to open a flow path between the sieve beds and allow pressure to equalize therebetween. Operation of the vent valve is time to occur after pressure in the sieve beds has equalized. Components of the concentrator are tuned so that the equalization pressure occurs at about zero pounds per square inch (psi). This reduces energy loss due to free expansion of gasses and reduces noise created by the venting of gasses to the ambient atmosphere.