A gas fractionalization apparatus and methods for providing oxygen rich air to patients are disclosed. The apparatus comprises a variable output compressor, a PSA unit, a control system, and one or more pressure sensing devices. Pressure of the oxygen rich gas is maintained within a selected window by adjusting the output of the compressor in response to changes in the pressure of the oxygen rich gas. Methods of feedback control are further provided for calibration of the compressor under factory and in-service conditions, as well as under high-altitude conditions. Methods for providing diagnostic estimates and alarms of time to service are further provided.