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