the oxygen used, and any excess gas from this must be vented. Although it uses gas more economically, the weight of the rebreather encourages the diver to carry smaller cylinders. Still, most semi-closed systems allow at least twice the duration of average sized open-circuit systems (around two hours) and are often limited by scrubber endurance. Closed circuit rebreathers An oxygen rebreather diver or a fully closed circuit rebreather diver consumes about 1 litre of oxygen corrected to atmospheric pressure per minute. Except during ascent or descent, the fully closed circuit rebreather that is operating correctly uses very little or no diluent. A diver with a 3-litre oxygen cylinder filled to 200 bar who leaves 25% in reserve will be able to do a 450-minute = 7.5 hour dive (3 litres × 200 bar × 0.75 litres per minute = 450 minutes). This endurance is independent of depth. The life of the soda lime scrubber is likely to be less than this and so will be the limiting factor of the dive. In practice, dive times for rebreathers are more often influenced by other factors, such as water temperature and the need for safe ascent (see Decompression (diving)), and this is generally also true for large-capacity open-circuit sets. Hazards and safety Scuba sets contain breathing gas at high pressure. The stored energy of the gas can do considerable damage if released in an uncontrolled manner. The highest risk is during charging of cylinders, but injuries have also occurred when cylinders have been stored in excessively hot environment, which can increase the gas pressure, by the use of incompatible cylinder valves, which can blow out under load, or by rupture of regulator hoses in contact with the user, as a pressure of more than can rupture the skin, and inject gas into the tissues, along with possible contaminants. Scuba is safety-critical equipment, as some modes of failure can put the user at immediate risk of death by drowning, and a catastrophic failure of a scuba cylinder can instantly kill or severely injure persons in the vicinity. Open circuit scuba is considered highly reliable if correctly assembled, tested, filled, maintained and used, and the risk of failure is fairly low, but high enough that it should be considered in dive planning, and where appropriate, precautions should be taken to allow appropriate response in case of a failure. Mitigation options depend on the circumstances