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Scuba Diving and breathing underwater [click here to return to previous page]

Early diving experimenters quickly discovered it is not enough to simply supply air in order to breathe comfortably underwater. As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs - approximately 1 bar or 14.7 psi for every 33 feet or 10 meters of depth - so the pressure of the inhaled breath must exactly counter the surrounding or ambient pressure in order to safely and efficiently inflate the lungs.  By always providing the breathing gas at ambient pressure, modern demand valve regulators ensure the diver can inhale and exhale naturally and virtually effortlessly, regardless of depth.  As the diver's nose and eyes are inside a diving mask, the diver cannot breathe in through his nose, except when wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature very quickly.  The most commonly used scuba set today is the open circuit 2-stage diving regulator, coupled to a single pressurized gas cylinder. This 2-stage arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 design, known as the Aqua-lung, in which the cylinder's pressure was reduced to ambient pressure in a single stage. The 2-stage system has significant advantages over the original single-stage design.  In the 2-stage design, the first stage regulator reduces the cylinder pressure of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi). The second stage demand valve regulator, connected via a low pressure hose to the first stage, delivers the breathing gas at the correct ambient pressure to the diver's mouth and lungs. The diver's exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one "high pressure" outlet, which delivers breathing gas at unreduced tank pressure. This is connected to the diver's pressure gauge or computer, since both need to "see" the actual tank pressure in order to function.  Less common (but becoming increasingly so) are the closed and/or semi-closed rebreather units. Unlike the open circuit arrangements which vent all exhaled gases to the surrounding environment, rebreathers capture each exhaled breath and recycle it for re-use by removing the carbon dioxide buildup and replenishing the oxygen used up by the diver. Rebreathers release few or no gas bubbles into the water which has advantages for research, military, photography and other applications. The body uses only a portion of the oxygen taken in with each breath, and in open-circuit systems the rest is wasted as exhaust. Rebreathers, on the other hand, re-circulate the unused gas and inject fresh oxygen as needed, allowing the diver greatly increased dive times with an identical quantity of gas.  For some diving gas mixtures other than normal atmospheric air are used, such as air with enriched oxygen content known as nitrox for less risk of decompression illness and less nitrogen narcosis or for deeper or more prolonged dives, oxygen with helium and a reduced percentage of nitrogen, known as trimix.  In cases of technical dives multiple cylinders may be carried, each containing a different gas mixture for a distinct phase of the dive, typically designated as Travel, Bottom and Decompression. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects and reduce decompression times. 

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Reference material for this scuba diving related informational article: wikipedia – the free online encyclopedia, scuba diving category

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