During the filling process, the dryer cartridge absorbs the flowing CO2. When there is no more flow after the compressor shuts down, the cartridge releases the CO2 again and the entire dryer cartridge housing is filled with CO2.
To ensure this CO2 does not enter the breathing apparatus, the CO2 in the dryer housing is purged for about 30 seconds when the compressor starts, and only after that is the air routed to the filling device for filling.