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Chapter 1- Diving Equipment Diving Equipment 7 larger the cylinder, the more buoyancy change you will experience as the air is breathed out. You may have noticed in your diving that some cylinders will actually float at the end of a dive. The less the change in cylinder buoyancy, the easier your buoyancy adjustments will be. Care of Scuba Cylinders Beyond preventing physical damage to your cylinder from dropping, banging, or overheating it; the most important consideration is keeping moisture from entering the cylinder. Water will cause corrosion by combining with the high pressure oxygen inside the cylinder. In a steel cylinder, this corrosion is called ferrous oxide, or rust. In an aluminum cylinder the corrosion is called aluminum oxide. Keeping the Water Out Water may enter a scuba cylinder from: • An improperly operated or maintained air compressor. • Droplets in the cylinder valve or filler attachment during filling. • Back flow through the regulator when the cylinder is empty. • Changes in temperature when the cylinder is empty and the valve is left open. • Bleeding cylinder air rapidly, causing internal condensation. A serious problem, in addition to corrosion, can result from the prolonged storage of air in a steel cylinder containing water. The oxidation process will, over time, consume some of the oxygen in the air, leaving a higher nitrogen concentration. This is one of the reasons cylinders should be stored with only a small amount of air in them and filled prior to use. To prevent moisture accumulation, always maintain a small amount (e.g. 20 bar or 300 psi) of pressurized air in your cylinder. If whitish mist is detected when the valve is opened, if sloshing can be heard when the cylinder is tipped back and forth, or if the air has an odor; do not use the cylinder, and have it inspected as soon as possible. All scuba cylinders should be inspected externally FIGURE 1-2. VISUAL CYLINDER INSPECTION REQUIRES SPECIAL TOOLS AND TRAINING and internally by a qualified professional at least once a year (figure 1-2). This testing requires special training, procedures, and equipment. NAUI sanctions a special cylinder inspection program (PSI; Professional Scuba Inspectors, Inc.). If internal inspection reveals corrosion in a scuba cylinder, it may have to be cleaned by tumbling. The tumbling process involves filling the cylinder approximately half full of an abrasive material such as carbide chips, or aluminum oxide chips, and rotating it for a number of hours. The abrasive materials remove rust and polish the inside surface of the cylinder, which is then rinsed to remove loose material and dehydrated internally to remove all traces of moisture. At least once every five years in the USA, and even more frequently in some countries, scuba cylinders must be pressure tested. There are several methods of hydrostatic testing, including direct expansion, pressure recession, and the water jacket method. The latter of which involves filling the cylinder with water, placing it in a water-filled pressure chamber, raising the pressure inside the cylinder with a hydraulic pump, and measuring


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