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