Chapter 11- Technical Diving Technical Diving 251 pressure, since nitrogen is the source of decompression problems, by replacing it with oxygen, much of which is used metabolically. No-stop times can be greatly increased and decompression times can be reduced using nitrox gas (table 11-1). Special techniques must be used in the mixing of breathing grade oxygen and oil free compressed air. One method involves injecting a specific volume of oxygen into a compressor intake, where it combines with atmospheric air prior to compression. Another more common procedure, called the partial pressure blending method, involves adding a predetermined amount of oxygen into a bank of storage cylinders or a scuba cylinder and then topping off the cylinder(s) with oilfree compressed air. All blending system components and cylinders must be properly cleaned and designated for oxygen service. The EANx is analyzed for its oxygen content by the blending technician and again by the diver who purchases the gas (figure 11-2). There are some potential risk factors associated with breathing nitrox. Central nervous system oxygen toxicity can result from relatively short exposures to increased partial pressures of oxygen. Factors associated with this include the percentage of oxygen in the mixture, the dive depth, and the duration of exposure. NOAA has established time exposure limits based on the partial pressure of oxygen in the breathing gas. The diver should not exceed partial pressure of oxygen of 1.6 ata, and this can only be breathed for a certain amount of time. NAUI recommends an 02 limit of 1.4 ata. Total diving time is limited in any 24 hour period. Moderate exertion, thermal stress, or lengthy dives will require that the maximum allowable partial pressure of oxygen be reduced. The transition from compressed air diving to EANx is quite simple for the traditional sport diver. Training in nitrox diving is becoming increasingly more popular and is widely available. NAUI offers entry level and specialty courses in EANx which provide the diver with the information and practical skills that are required to utilize EANx as a breathing medium. Divers using EANx with conventional open circuit scuba use the same diving skills, techniques, and procedures as compressed air scuba divers. They must have additional training in physics, physiology, decompression, and EANx table use, and use a dedicated EANx cylinder filled at a participating dealer. Advanced training in nitrox covers the use of custom EANx blends, diving to greater depths, and decompression diving. Training is gradually intensified to develop and reinforce a more comprehensive understanding of mixed gas diving, physics, physiology, stress prevention and management, equipment selection and use, dive planning, and gas management. Trimix Diving For diving at greater depths, generally beyond about 60 m (190 ft), where air or nitrox are neither safe nor practical other gas mixtures are used. Trimix gas consists of a mixture of two inert gases and oxygen. Helium, nitrogen, and oxygen are commonly used by technical divers. The individual gas concentrations are FIGURE 11-2. NITROX CYLINDERS COMPARISON OF NO-DECOMPRESSION LIMITS FOR AIR (U.S. NAVY) AND NOAA NITROX I (68% N2, 32% O2). METERS AIR NO-DECOMPRESSION NITROX I NO-DECOM- LIMITS USN (min) PRESSION LIMITS (min) 15 100 200 18 60 100 21 50 60 24 40 50 27 30 40 30 25 30 33 20 25 36 15 25 40 10 20 TABLE 11-1. NO-DECOMPRESSION LIMITS FOR AIR VS. EAN 32
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