Yes, this answer is the same for LiveO2, LiveO2 AC and the ExtremeO2 systems. Mainly the air separator is left on to provide make up volume for the LiveO2 system. For the ExtremeO2 system the Air separator also creates the oxygen depleted air so it is required to be ON during use. Leaving the separator on just prior to use and during use also assures that the oxygen rich air concentration in the reservoir is also maintained. The internal membrane is designed to be slightly permeable and some mixing with air occurs. When the separator is not running the volume will drop and if left long enough the reservoir will slowly deflate on its own.