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 (2018) manufactured for the military, technical and recreational scuba markets. Types Scuba sets are of two types: In open-circuit scuba the diver inhales from the equipment and all the exhaled gas is exhausted to the surrounding water. This type of equipment is relatively simple, economical and reliable. In closed-circuit or semi-closed circuit, also referred to as a rebreather, the diver inhales from the set, and exhales back into the set, where the exhaled gas is processed to make it fit to breathe again. This equipment is efficient and quiet. Both types of scuba set include a means of supplying air or other breathing gas, nearly always from a high pressure diving cylinder, and a harness to attach it to the diver. Most open-circuit scuba sets have a demand regulator to control the supply of breathing gas, and most rebreathers have a constant-flow injector, or an electronically controlled injector to supply fresh gas, but also usually have an automatic diluent valve (ADV), which functions in the same way as a demand valve, to maintain the loop volume during descent. Open-circuit Open-circuit-demand scuba exhausts exhaled air to the environment, and requires each breath to be delivered to the diver on demand by a diving regulator, which reduces the pressure from the storage cylinder and supplies it through the demand valve when the diver reduces the pressure in the demand valve slightly during inhalation. The essential subsystems of an open-circuit scuba set are; diving cylinders, with cylinder valves, which may be interconnected by a manifold, a regulator mechanism to control gas pressure, a demand valve with mouthpiece, full-face mask or helmet, with supply hose, to control flow and deliver gas to the diver. an exhaust valve system to dispose of used gas, A harness or other method to attach the set to the diver. Additional components which when present are considered part of the scuba set are; external reserve valves and their control rods or levers, (currently uncommon) submersible pressure gauges, (almost ubiquitous) and secondary (backup) demand valves (common). The buoyancy compensator is generally assembled as an integrated part