Abstract: A pressure swing adsorber (PSA) air prepurifier includes at least a first adsorber and a second adsorber and a controller for controlling air feed to the first adsorber and the second adsorber. The invention is a "realtime" method for controlling cycle times for each adsorber and includes the following steps. During a determined time period, flow is measured to an adsorber that is coupled to an air inlet. An actual totalized flow to the on-line adsorber is accumulated, based upon the measured flow values. During this time, air feed conditions (e.g., temperature, pressure, relative humidity) are monitored and periodically, a maximum totalized flow to the adsorber is calculated, determined, at least in part, based upon the monitored air feed conditions.