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Chapter 1- Diving Equipment Diving Equipment 17 regulator first stage. Cylinder pressure gauge types include spiral Bourdon movements or digital instrumentation. A digital gauge uses a pressure-sensitive transducer, a battery, electronic circuitry, and a liquid crystal display to provide information. Should a leak develop in the Bourdon movement, high pressure air will flow into the gauge housing which could cause a rupture in the housing. This is why you are instructed not to look at your gauge when opening your cylinder valve. To prevent injury from a rupture or failure of the housing in this instance, a rubber plug is incorporated into the housing of most cylinder pressure gauges. Avoid obstructing the area over the plug so the plug will be free to blow out and release pressure if necessary. If water is visible inside a cylinder pressure gauge, the gauge should not be used until repaired and tested. Pressure gauges require professional service and repair. Cylinder pressure gauges should not be subjected to shock and abuse and should be inspected and verified for accuracy annually, or more often if damage or inaccuracy are suspected. Depth Gauges There are several types of depth gauges, including capillary, Bourdon tube, diaphragm, and electronic (figure 1-14). The simplest and least expensive of all gauges is the capillary gauge. During descent air is compressed in a length of clear tubing that is sealed at one end. Water enters the open end when the tube is submerged and compresses the column of air in proportion to the depth. The scale is non-linear, so the graduations on the scale become more closely spaced and are difficult to read in deeper water. A well-constructed capillary gauge can be extremely accurate in shallow water, and is also of value for altitude diving because it reads deeper than the actual depth, very nearly compensating for the reduced pressure at altitude. Problems associated with capillary gauges include clogging of the tube, air bubbles in the tube, and difficulty in reading at low light levels as well as deeper depths. For these reasons, a capillary gauge should generally not FIGURE 1-13. TWO-GAUGE CONSOLE be used as your primary depth gauge, except perhaps in shallow water. Open Bourdon tube depth gauges are the least expensive form of Bourdon movement depth gauge. The tube is mounted inside a case with the open end exposed to the water, which enters the tube. This type of gauge is subject to corrosion, silting, and blockage by salt crystals. They are no longer readily available and are unpopular due to maintenance problems. The sealed Bourdon tube depth gauge was developed to overcome the problems associated with the open tube design. This gauge is fundamentally the same as the open Bourdon tube, except the tube is fluid-filled and the end is sealed with a rubber diaphragm. Water pressure is transmitted through the diaphragm to the oil, which exerts pressure on the tube. The oil-filled Bourdon tube gauge consists of a Bourdon movement sealed at both ends and enclosed in an oil-filled case (figure 1-15). Either a pliable case or a diaphragm transmit pressure through the case and the oil to the Bourdon movement. The diaphragm depth gauge is the only mechanical gauge that does not use a Bourdon movement (figure 1-16). It is an expensive but accurate gauge. A thin, metal diaphragm is mounted in a case which is rigid and hermetically sealed. Most of the air is removed from the case to form a partial vacuum. Water pressure on the diaphragm moves mechanical linkage attached to the


20047TC
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