north-east American wreck diving community. The challenges of deeper dives and longer penetrations and the large amounts of breathing gas necessary for these dive profiles and ready availability of oxygen sensing cells beginning in the late 1980s led to a resurgence of interest in rebreather diving. By accurately measuring the partial pressure of oxygen, it became possible to maintain and accurately monitor a breathable gas mixture in the loop at any depth. In the mid-1990s semi-closed circuit rebreathers became available for the recreational scuba market, followed by closed circuit rebreathers around the turn of the millennium. Rebreathers are currently manufactured for the military, technical and recreational scuba markets, but remain less popular, less reliable, and more expensive than open circuit equipment. Equipment Scuba diving equipment, also known as scuba gear, is the equipment used by a scuba diver for the purpose of diving, and includes the breathing apparatus, diving suit, buoyancy control and weighting systems, fins for mobility, mask for improving underwater vision, and a variety of safety equipment and other accessories. Breathing apparatus The defining equipment used by a scuba diver is the eponymous scuba, the self-contained underwater breathing apparatus which allows the diver to breathe while diving, and is transported by the diver. It is also commonly referred to as the scuba set. As one descends, in addition to the normal atmospheric pressure at the surface, the water exerts increasing hydrostatic pressure of approximately 1 bar (14.7 pounds per square inch) for every 10 m (33 feet) of depth. The pressure of the inhaled breath must balance the surrounding or ambient pressure to allow controlled inflation of the lungs. It becomes virtually impossible to breathe air at normal atmospheric pressure through a tube below three feet under the water. Most recreational scuba diving is done using a half mask which covers the diver's eyes and nose, and a mouthpiece to supply the breathing gas from the demand valve or rebreather. Inhaling from a regulator's mouthpiece becomes second nature very quickly. The other common arrangement is a full face mask which covers the eyes, nose and mouth, and often allows the diver to breathe through the nose. Professional scuba divers are more likely to use full face masks, which protect the diver's airway if the diver loses consciousness. Open-circuit Open circuit scuba has no provision for using the breathing gas more than once for respiration. The gas inhaled from