['< sod > * closed - circuit or semi - closed circuit scuba systems allow recycling of exhaled gases. this reduces the volume of gas used, so that a smaller cylinder, or cylinders, than under open circuit scuba may be used for the equivalent dive duration, and giving the ability to spend far more time underwater compared to open circuit for the same gas consumption. professional rebreathers also produce far less bubble volume and less diver noise than scuba, which makes them attractive to military, scientific and media divers. < ref name = " usn " / > < eod > < sod > * closed - circuit or semi - professional closed circuit [ [ rebreather diving | rebreather ] ] scuba systems allow recycling of exhaled gases. this reduces the volume of gas used underwater compared together with open circuit, so thus that a smaller cylinder, or cylinders, may be used for the equivalent dive duration. they provide the capacity to spend far more time underwater compared to open circuit for the same gas consumption. rebreathers also produce generally far less bubble volume and less noise than scuba. this makes them attractive to specialist covert military divers, who wish to avoid detection, scientific divers who do not also want to disturb the marine animals, and media divers who do not want their work obscured by bubbles. < ref name = " usn " / > < eod >']