I have noticed that when I am holding a stop at a fixed depth, the loop volume steadily slowly increases until I have to vent some gas out. I cannot understand why this is the case - the electronic system is adding in enough oxygen such that it replaces the oxygen that i have metabolised. So why is the volume increasing? When at depth, this becomes a non issue, the shallower I get the more noticeable it becomes. I have tried to close the flow stop just to make sure the ADV was not putting in any gas without me noticing - no difference.