Abstract: A pair of beds (10, 12) are connected by a cross-over valve (20) cyclically with a source of air under pressure (22) and a source of vacuum (24). The beds are further interconnected by a pressure equalization flow path (40) and a pressure equalization valve (42) for selectively opening and closing the flow path. The beds are filled with a physical separation material which allows oxygen to pass more readily than other components of air. The separated oxygen is collected in a surge tank (30). A timing and control circuit (50) controls the cross-over valve such that it reverses positions twice per cycle. The timing and control circuit further controls the pressure equalization valve to open 1 percent of the cycle duration before the cross-over valve reverses positions and to close 2 percent of the cycle duration after the cross-over valve changes positions.