NAUI Master Scuba Diver 12 Diving Equipment Open Upstream Diaphragm Regulator Balanced Diaphragm Regulator Downstream Piston Regulator Balanced Piston Regulator piston first stage or in a balanced diaphragm first stage, a failure of the seal tends to cause the valve to fail in the open or free flow position. In all scuba regulators, external pressure is admitted into the spring chamber and contributes to the spring pressure. Thus, the intermediate pressure remains at a constant above the ambient pressure as the diver descends or ascends. Formerly, diaphragm regulators were often preferred in dirty water conditions because the water could not get beyond the diaphragm into the workings of the first stage. Currently, several manufacturers of piston regulators have developed methods to admit ambient pressure to the spring chamber while excluding the water. With any regulator, a program of scheduled preventive maintenance is needed to preserve peak performance. Second-Stage Valves The downstream second-stage valve is connected to the first stage by a low pressure hose (figure 1-8). A reduction in pressure in the second-stage chamber causes the second-stage diaphragm to bulge inward and depress a lever, which opens the valve and admits air into the mouthpiece until it reaches ambient pressure. As long as you continue to inhale, air will continue to flow. When inhalation ceases, the diaphragm returns to a flat position releasing pressure on the lever and closing the valve due to spring pressure. Upon exhalation, pressure in the second stage chamber unseats the non-return exhaust valve, allowing used air to be exhausted into the water. With a downstream second-stage valve, any build up of pressure in the first stage of the regulator will simply push air past the second-stage valve. An upstream second-stage valve, although rarely encountered, should be understood due to its potential hazards and inefficiency (figure 1-8). A dangerous feature of this regulator is that if there is a build-up of pressure in the first stage, the valve will be held tightly against its seat and shut off the flow of air. The low pressure hose will rupture if its burst pressure is exceeded. Regulators with upstream second-stage valves must be equipped with a pressure relief valve in the intermediate pressure chamber of the first stage to relieve any excess pressure, or an extra second stage with a downstream valve must be attached to the first stage. An upstream second-stage valve operates in a manner similar to a downstream second-stage valve, but the diaphragm pushes against a stem instead of a lever. The stem is attached to the valve seat, and the valve is tilted or partially unseated when the stem is Closed O-ring Spring High Pressure Intermediate Pressure Water Pressure FIGURE 1-7. REGULATOR FIRST STAGES
20047TC
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