Aqua-Lung was the first open-circuit, self-contained underwater breathing apparatus (or "scuba") to achieve worldwide popularity and commercial success. This class of equipment is now commonly referred to as a twin-hose diving regulator, or demand valve. The Aqua-Lung was invented in France during the winter of 1942–1943 by two Frenchmen: engineer Émile Gagnan and Jacques Cousteau, who was a Naval Lieutenant (). It allowed Cousteau and Gagnan to film and explore underwater more easily.
The invention revolutionised autonomous underwater diving by providing a compact, reliable system capable of a greater depth range and endurance than its precursors, and was a major factor influencing the development of recreational scuba diving after WWII.
The twin-hose Aqua-Lung demand regulator is the foundation of all modern scuba regulators. A diaphragm is used to control a valve to deliver the breathing gas to the diver on demand, at ambient water pressure. However, the layout has changed to a single hose system, where the second stage is split from the first stage along with the exhaust valve, as they must be kept at the same depth, and repositioned at the diver's mouth, eliminating the need for the exhaust hose, and allowing the use of a more rugged, smaller bore hose for the intermediate pressure gas supply to the second stage valve at the mouthpiece, but increasing the load on the diver's jaw and releasing bubbles nearer the eyes and ears.
Mechanism
The Aqua-Lung is a self-contained open-circuit demand system, which means that breathing gas is provided from high-pressure storage carried by to the diver on demand, when the diver inhales and reduces the pressure in the supply hose, and the flow is shut off when not required, and that once breathed, the exhaled gas is vented to the surroundings. Scuba systems invented before the Aqua-Lung were mostly closed circuit rebreather equipment, in which breathing gas flows through an ambient pressure hose to the diver, and exhaled gas is returned through a scrubber which removes carbon dioxide, to a counter-lung reservoir. Some fresh gas is added to maintain the oxygen content and is then circulated back to the diver again in a closed loop. In open circuit free-flow systems, the air is supplied at an approximately constant rate, and the diver uses only a relatively small part of the passing gas.
The original Aqua-Lung regulator was a single stage unit, packaged in a circular brass housing mounted on the cylinder valve behind the diver's neck. High pressure gas flows into the regulator from the cylinder valve outlet, and is blocked by the demand valve. When the diver consumes ambient pressure gas, the pressure falls in the low pressure chamber and the diaphragm deforms inwards pushing against the valve lifter. This opens the high pressure valve permitting gas to flow past the valve seat into the low pressure chamber. When the diver stops inhaling, pressure in the low pressure chambers quickly rises until the diaphragm returns to its neutral position and no longer presses on the valve lifter, shutting off the flow until the next breath is taken. On a single stage regulator, the flow rate through the demand valve orifice will vary depending on cylinder pressure for the same opening size, and the opening force required will vary depending on the inlet pressure and orifice area, together making the delivery rate vary as the pressure in the cylinder changes. Flow rate is also affected by downstream pressure, which varies with depth, so the effort of breathing could vary considerably during a dive, even without taking diver attitude into account.
Later models included a first stage regulator to provide air to the demand valve at a lower pressure, compensated for depth, which allowed finer control and greater sensitivity to small pressure differences over the second stage diaphragm, but less sensitivity to cylinder pressure variation during the dive, both desirable features. Both the first and second stage valve mechanisms of the regulator are packaged in the circular brass housing mounted on the cylinder valve behind the diver's neck. High pressure gas flows into the regulator from the cylinder valve outlet, and is blocked by the first stage valve. The first stage reduces cylinder pressure to an interstage pressure, a fairly constant few bars higher than the ambient pressure, set by an adjustable spring preloading the diaphragm, The interstage breathing gas is then reduced to ambient pressure by the second stage.