prototype had two large filter stones mounted on the back of the cylinder with a hose connected to the exhaust ports of the second-stage regulator. The filter stones were mounted on a hinged arm to float above the diver, to set up a depth-pressure-differential suction effect to counteract the extra exhalation pressure needed to breathe out through the diffuser. The scuba muffler was claimed to cut the exhalation noise by 90%. Closed circuit rebreathers proved more useful in letting divers get near sharks. Gas endurance of a scuba set Gas endurance of a scuba set is the time that the gas supply will last during a dive. This is influenced by the type of scuba set and the circumstances in which it is used. Open circuit The gas endurance of open-circuit-demand scuba depends on factors such as the capacity (volume of gas) in the diving cylinder, the depth of the dive and the breathing rate of the diver, which is dependent on exertion, fitness, physical size of the diver, state of mind, and experience, among other factors. New divers frequently consume all the air in a standard "aluminum 80" cylinder in 30 minutes or less on a typical dive, while experienced divers frequently dive for 60 to 70 minutes at the same average depth, using the same capacity cylinder, as they have learned more efficient diving techniques. An open-circuit diver whose breathing rate at the surface (atmospheric pressure) is 15 litres per minute will consume 3 x 15 = 45 litres of gas per minute at 20 metres. [(20 m/10 m per bar) + 1 bar atmospheric pressure] × 15 L/min = 45 L/min). If an 11-litre cylinder filled to 200 bar is to be used until there is a reserve of 17% there is (83% × 200 × 11) = 1826 litres available. At 45 L/min the dive at depth will be a maximum of 40.5 minutes (1826/45). These depths and times are typical of experienced recreational divers leisurely exploring a coral reef using standard 200 bar "aluminum 80" cylinders as may be rented from a commercial recreational diving operation in most tropical island or coastal resorts. Semi-closed rebreather A semi-closed circuit rebreather may have an endurance of about 3 to 10 times that of the equivalent open-circuit dive, and is less affected by depth; gas is recycled but fresh gas must be constantly injected to replace at least