Open-circuit scuba are generally made up of the following basic components: cylinders, air reserve mechanism, demand regulator, breathing tubes, check valves, mask or mouthpiece (or both), and exhaust valve.
Cylinders
The cylinders, sometimes called bottles or flasks, contain compressed air and are made of either galvanized steel or aluminum. They are specially constructed to withstand the high internal pressure of compressed air, usually 2,150 pounds per square inch for the galvanized cylinders, and 3,000 pounds per square inch for the aluminum cylinder. The volume of the cylinders varies with the different arrangements in use. The general type is a 2-cylinder arrangement, although some European scuba use a 3-cylinder arrangement. The cylinders are usually worn on the back and fastened to the diver by an arrangement of waist, shoulder and crotch straps. A means for quick release of these straps for emergency ditching of the cylinders is provided. The cylinders are fitted to permit easy, rapid replacement of cylinders on the surface or under water.
The Air-Reserve Mechanism
An essential part of the scuba is an air-reserve mechanism to provide a positive warning to the diver when his air supply is becoming critically low. The most commonly used mechanism is an air-reserve valve which permits free flow of air to the regulator until the cylinder pressure falls to a critical level-approximately 300 to 500 pounds per square inch-depending on the type of scuba used. At this pressure the valve restricts the air, causing increased breathing resistance. At this signal the diver opens the air-reserve mechanism, restoring a free flow of air from the reserve air supply. This reserve air supply should be sufficient to allow a safe return to the surface, provided that the dive is not prolonged and that decompression is not needed. The means of opening the air-reserve mechanism varies with different types of scuba.
The Demand Regulator
The demand regulator is a device which controls the flow of compressed air from the cylinders to the diver. An efficient scuba should have a demand mechanism allowing the release of compressed air at a pressure equal to that of the surrounding water and at the rate required by the diver.
The sketch illustrates the basic principle of the demand regulator. When inhalation reduces the pressure in the air chamber below the pressure in the surrounding water, the diaphragm deflects toward the air chamber, depressing the lever and opening the valve on the air supply. So long as inhalation continues, the valve remains open and admits air to the system. When inhalation stops, rising pressure in the air chamber returns the diaphragm to its original position, and the air-supply valve closes.
Demand regulators are of 2 general types, single stage and double stage. The single-stage regulator furnishes the air directly to the diver in a single reduction from cylinder
pressure to a pressure equal to that of the surrounding water. The double-stage regulator is merely a single-stage unit mounted upon another single-stage unit called the first-stage regulator. The first-stage regulator provides an intermediate reduction in the gas pressure through a high-pressure valve to approximately 100 pounds per square inch more than the depth pressure. The second-stage regulator then provides the final reduction through a low-pressure valve prior to the release of compressed air to the diver.
Demand regulators may be mounted either on the cylinders or in the face mask. Cylinder-mounted demand regulators have an advantage in that they minimize the need for medium-pressure tubing, but because of the difference in water pressure between the location of the demand regulator and the effortless breathing point of the diver (at the base of the throat), they make inhalation slightly difficult until the body adjusts. Mask-mounted regulators minimize this water-pressure difference, making breathing comparatively easy and eliminating the need for a mouthpiece.
Tags: water sports
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