The air charge is contained within the tank, whilst the liquid from the main flows into and out of the bladder. Like the air in bladder design, the liquid does not come into contact with the tank walls nor the air charge, so again, the air charge cannot be absorbed by the pumped liquid and so air losses due to absorption are minimal.
The pumped liquid from the main flows into and out of the bladder and will not come into contact with the tank walls, again compatible materials must be selected.
As for the air in bladder design, the bladder is often made from a highly elastic rubber, such as butyl rubber, and may be sized to expand to fit the entire vessel. For this design the whole tank is pre-charged at installation with a pre-determined pressure to give the correct operating level in the tank when the unit is connected to the main and at the operating pressure.