The diver exhales through Helmet exhaust/recovery Valve then passed through the Bell Onboard unit as well as removing water in the return hose also provides another level of safety with the Pilot Operated Safety (POS) valve in line.
After passing through the Bell Onboard Unit (BOU), the recovered/reclaimed gas is now passed through the Umbilical into the Topside gas Reprocessing Unit (TRU). Then Oxygen is injected to meet the desired level of mix for the given depth mixture.
After which the gas is compressed using either electrical or air driven pumps, delivered into the medium pressure receivers and then to the Bell control panel. Upon arrival at the control panel it is analysed again before being sent back down to the divers.