main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, increased work of breathing at depth, reliability of gas mixture control, and the requirement to be able to safely bail out at any point of the dive. Rebreathers are generally used for scuba applications, but are also occasionally used for bailout systems for surface supplied diving. The possible endurance of a rebreather dive is longer than an open-circuit dive, for similar weight and bulk of the set, if the set is bigger than the practical lower limit for rebreather size, and a rebreather can be more economical when used with expensive gas mixes such as heliox and trimix, but this may require a lot of diving before the break-even point is reached, due to the high initial and running costs of most rebreathers, and this point will be reached sooner for deep dives where the gas saving is more pronounced. Breathing gases for scuba Until Nitrox, which contains more oxygen than air, was widely accepted in the late 1990s, almost all recreational scuba used simple compressed and filtered air. Other gas mixtures, typically used for deeper dives by technical divers, may substitute helium for some or all of the nitrogen (called Trimix, or Heliox if there is no nitrogen), or use lower proportions of oxygen than air. In these situations divers often carry additional scuba sets, called stages, with gas mixtures with higher levels of oxygen that are primarily used to reduce decompression time in staged decompression diving. These gas mixes allow longer dives, better management of the risks of decompression sickness, oxygen toxicity or lack of oxygen (hypoxia), and the severity of nitrogen narcosis. Closed circuit scuba sets (rebreathers) provide a gas mix that is controlled to optimise the mix for the actual depth at the time. Diving cylinders Gas cylinders used for scuba diving come in various sizes and materials and are typically designated by material – usually aluminium or steel, and size. In the U.S. the size is designated by their nominal capacity, the volume of the gas they contain when expanded to normal atmospheric pressure. Common sizes include 80, 100, 120 cubic feet, etc., with the most common being the "Aluminum 80". In most of the rest of the world the size is given as the actual internal volume of the cylinder, sometimes referred to as