Know Your Scuba Equipment

Practically all of the scuba in sports use is of the open-circuit type. This type derives its name from the fact that the breathing medium is used only once; the expired gases are discharged into the water during exhalation. Normally compressed air is the breathing medium used in open-circuit scuba, but it is possible to used mixed gases for deep dives. However, oxygen alone must never be used in open-circuit scuba.

Once you are under water, your life depends on the operation of your scuba, and you should fully understand its operation before using it. There are two basic systems in open-circuit scuba, the continuous-flow system and the demand system. The demand system is the one currently favored, and is the only one which has approval of the United States Navy. The 3 types which are in Navy use are the United States Divers aqualung; the Scott Hydro-Pak, and the Northill air lung.

The continuous-flow system is somewhat simpler in construction. Basically, it is a self-contained substitute for the ordinary air hose with continuous flow. A simple example is a shallow-water face mask furnished with air from a cylinder carried by the diver rather than from a surface supply through an air hose. Since the flow has to meet the demands of inspiration, and since it continues during expiration, the cylinder must provide at least twice the volume of “breathable” air. This waste will soon exhaust a portable cylinder. In some such scuba, a reservoir bag is arranged to accumulate incoming air during the expiratory phase, to permit larger air flow and reduce waste, and it is necessary to adjust the flow to meet changes in the diver’s respiration.

The more widely used demand system provides for periodic release of the compressed air as it is needed for the inspiration of the diver, and shuts off the flow during expiration. A demand regulator controls the release automatically. This is a special low-pressure regulator which maintains the breathing system at surrounding depth pressure, opening to slight negative pressure at the start of inspiration and remaining open only until the end of inspiration.

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