VPSA technology (Vacuum Pressure Swing Adsorption) is based on an energy-efficient, low pressure technique.
A VPSA can both circulate and inject. During circulation, the residual oxygen level reduces along with the oxygen in the storage cell. Circulation is the most energy efficient mode for lowering the oxygen level in a storage area. So-called pull-down time is an indication of the time necessary for circulation to bring your storage cells to the required oxygen levels. During circulation, the VPSA reduces the oxygen concentration in the cell up to 30% quicker than when injecting.
The advantage of injection is that a cell can be kept at slight over-pressure so that ULO conditions (ultra low oxygen, <3%) are even possible in a storage area with a minor leak (ask your cool cell constructor or Van Amerongen for further advice). With the VPSA, you can switch between injecting and circulating and vice versa at any given moment.