NAUI Master Scuba Diver 252 Technical Diving predetermined based on the planned depth and specific environmental considerations of the dive. Adding helium to the breathing mixture can significantly counteract the narcotic effects of nitrogen. Technical divers can develop gas mixtures with specific Equivalent Narcosis Depths (END) by using a formula that considers the percentage of nitrogen in the mixture, as well as the depth of the dive. For example, a diver breathing a trimix mixture containing 14% oxygen, 50% helium, and 36% nitrogen at a depth of 92 msw (302 fsw) would have an END of 37 msw (120 fsw). A typical trimix dive to 77 msw (253 fsw) for 30 minutes might utilize a bottom mix of 17% oxygen, 50% helium, and the balance nitrogen for the deepest working portion of the dive. The oxygen concentration is below the 21% concentration found in air in order to avoid oxygen toxicity problems at depth (figure 11-3). The 50% helium replaces a large portion of the nitrogen found in air and creates an equivalent narcosis depth of 26.3 msw (86.5 fsw) on air. During the decompression phase of the dive, a nitrox mixture would typically be breathed, finishing with either pure oxygen from 6 msw (20 fsw) or EAN 80 from 9 m (30 ft) to the surface. Due to the considerable decompression obligation, a diver on a dive such as this might consume as much as 5,500 to 11,000 L (200 to 400 cubic feet) of gas (see “Physics” chapter). Obviously, specific, unique dive tables or computers must be used in accordance with the percentages of nitrogen and helium in such mixtures. Several types of custom dive tables are used by technical divers. Some even use computer programs to plan mixed gas dives. EQUIPMENT FOR TECHNICAL DIVING Most technical divers use specially configured conventional open circuit scuba equipment. A highly precise gas switching procedure requires detailed planning, multiple cylinders with special manifolds, high performance regulators with custom-length hoses, and may require specialized equipment such as full face masks with communications, programmable decompression computers, reels, and decompression lines (figure 11-4). Diver propulsion vehicles are sometimes used due to the distance traveled and the weight and bulkiness of the equipment worn (figure 11-6). FIGURE 11-3. ANALYSIS OF OXYGEN CONCENTRATION FIGURE 11-4. TECHNICAL DIVER WITH MULTIPLE CYLINDERS, MANIFOLD, AND LONG-HOSED REGULATORS
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