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Nitrox Diving. Nitrox is a breathing gas consisting of oxygen and nitrogen (similar to air), but with a higher proportion of oxygen than the normal 20.95%. Nitrox is mainly used in scuba diving to reduce the proportion of nitrogen in the breathing gas mixture. Reducing the proportion of nitrogen by increasing the proportion of oxygen reduces the risk of decompression sickness, allowing extended dive times without increasing the need for decompression stops. Nitrox is not a safer gas than compressed air in all respects: although its use reduces the risk of decompression sickness,it increases the risk of oxygen toxicity and fire, which are further discussed below. It is commonly believed that breathing nitrox can reduce the effects of nitrogen narcosis, but this has not been fully studied. In fact, there is some suggestion that oxygen may also have some narcotic properties under pressure; thus one should not expect a reduction in narcotic effects due only to the use of nitrox. In any case, the issue rarely arises, since depths which begin to induce narcosis in sensitive divers (> 35-40 m) are beyond the maximal operating depth of standard nitrox mixes anyway. There is anecdotal evidence that the use of nitrox reduces post-dive fatigue, particularly in older divers; however the only known double-blind study to test this found no statistically significant reduction in reported fatigue. There has, however, been some suggestion that post dive fatigue is due to sub-clinical decompression sickness (DCS); the fact that the study mentioned was conducted in a dry chamber with an ideal decompression profile may have been sufficient to reduce sub-clinical DCS and prevent fatigue in both nitrox and air divers. Further studies with a number of different dive profiles, and also different levels of exertion, would be necessary to fully investigate this issue. For example, there is much better scientific evidence that breathing high-oxygen gases increase exercise tolerance, during aerobic exertion. Though even moderate exertion while breathing from the regulator is a relatively uncommon occurance in scuba, as divers usually try to minimize it in order to conserve gas, episodes of exertion while regulator-breathing do occasionally occur in sport diving. Examples are surface-swimming a distance to a boat or beach after surfacing, where residual "safety" cylinder gas is often used freely, since the remainder will be wasted anyway when the dive is completed. It is possible that these so-far un-studied situations have contributed to some of the positive reputation of nitrox. Nitrox is known by many names: Enhanced Air Nitrox, Oxygen Enriched Air, Nitrox, EANx or Safe Air. The name nitrox has been written in upper and lower case in English; in this article the lower case convention will be used except when specific mixtures are referred to (such as Nitrox32). In its early days of introduction to non-technical divers, nitrox it was occasionally also been known by detractors by less complementary terms, such as "devil gas" or "voodoo gas" (a term now sometimes used with pride). In EANx, the "x" indicates the percentage of oxygen in the mix and is dropped when the percentage is known; for example a 32% EANx mix is called EAN32. The two most popular blends are EAN32 and EAN36 (also named Nitrox I and Nitrox II, respectively, or Nitrox32 and Nitrox36). These percentages are what the cylinder-filler (gas blendor) aims for in partial-pressure blending, but the final actual mix in such cases will be unique, and so a small flow of gas from the cylinder must be measured with a hand held oxygen analyzer meter, before use of the cylinder. Nitrox with more than 40% oxygen is uncommon within entry-level recreational diving. The reason is that the oxygen is expensive, and higher oxygen mixtures severely limit the maximum operating depth at which a mix can be used. Nitrox with 50% to 80% oxygen is common in technical diving as a decompression gas, which eliminates inert gases, such as nitrogen and helium, from the tissues more quickly than leaner oxygen mixtures eliminate them. Nitrox with around 30% oxygen, with a maximum operating depth of around 35 metres/115 feet, is sometimes used as "travel mix". The main reasons for using "travel mix" are to conserve the "bottom mix" for the deep part of the dive, to reduce nitrogen tissue loading and avoid breathing a hypoxic "bottom" gas in shallow water. Any cylinder containing any blend of gas other than the standard air content is required by the dive community to be clearly marked. The standard nitrox cylinder is yellow in color and marked with a green band around the shoulder of the tank, with "Nitrox" or "Enriched air" marked in white or yellow letters inside. Tanks of any other color are generally marked with six inch band around the shoulder, with a one inch green band on the top and bottom, with four inches of green in the middle. This green band will also have the desigation of "NITROX" or something similar inside, in yellow or green letters. Every nitrox cylinder should also have a sticker stating whether or not the cylinder is oxygen clean and suitable for partial pressure blending. Any oxygen clean cylinder may have any mix up to 100% oxygen inside. If by some accident an oxygen clean cylinder is filled at a station which does not supply gas to oxygen-clean standards, it is then considered contaminated and must be recleaned before a gas containing more than 40% oxygen may again be added. Cylinders marked as not-oxygen clean may only be filled with enriched oxygen mixtures from membrane or stick blending systems, where the gas is mixed before being added to the cylinder. Finally, all nitrox cylinders should have a tag that, at minimum, states the oxygen content of the cylinder, the date it was blended, the gas blender's name, and the maximum operating depth. Other requirements may be made as to what is marked on the cylinder, but these markings are considered standard and safe by the diving community, and any cylinders lacking these markings should be considered possibly unsafe. Diving and handling nitrox raises a number of potentially fatal dangers due to the high partial pressure of oxygen (ppO2). Nitrox is not a deep-diving gas mixture due the increased proportion of oxygen in Nitrox: oxygen becomes toxic when breathed at high pressure. For example, the maximum operating depth of nitrox with 36% oxygen, a popular recreational diving mix, is generally around 30 metres/100 feet. The exact value of the maximum allowed ppO2 and maximum operating depth varies depending on factors such as the training agency, the type of dive, the breathing equipment and the level of surface support, with professional divers sometimes being allowed to breath higher ppO2s than those recommended to recreational divers. To dive safely with nitrox, the diver must learn good buoyancy control, a vital part of scuba diving in its own right, and a disciplined approach to preparing, planning and executing a dive to ensure that the ppO2 is known, and the maximum operating depth is not exceeded. Reputable dive operators and gas blenders insist on the diver having recognised nitrox training (which appears as an extra notation on a certification card) before selling nitrox to divers. Some training agencies teach the use of two depth limits to protect against oxygen toxicity. The shallower depth is called the "maximum operating depth" and is reached when the partial pressure of oxygen in the breathing gas reaches 1.4 bar. The second deeper depth, called the "contingency depth", is reached when the partial pressure reaches 1.6 bar. Diving at or beyond this level exposes the diver to the risk of central nervous system (CNS) oxygen toxicity. This can be extremely dangerous since its onset is often without warning and can lead to drowning, as the regulator is spat out during convulsions which occur in conjunction with sudden unconsciousness (general seizure induced by oxygen toxicity). Precautionary procedures at the fill station: Many training agencies such as PADI and NAUI train their divers to personally check the oxygen percentage content of each nitrox cylinder before every dive. If the oxygen percentage is 1% or over the value written on the cylinder by the gas blender, the scuba diver must either recalculate his or her bottom times with the new mix, or else abort the dive to remain safe and avoid oxygen toxicity or decompression sickness. Under IANTD and ANDI [2] rules for use of nitrox, which are followed by most dive resorts around the world, filled nitrox cylinders are signed out personally in a gas blender log book, which contains, for each cylinder and fill, the cylinder number, the measured oxygen percent composition, the signiture of the recieving diver (who should have personally measured the oxygen percent with an instrument at the fill-shop), and finally a calculation of the maximum operating depth for that fill/cylinder. All of these steps minimize danger but increase complexity of operations (for example, personalized cylinders for each diver must generally be kept track of on dive boats with nitrox, which is not the case with generic compressed air cylinders). Fire or and toxic cylinder contamination from oxygen reactions: Diving cylinders are usually filled with nitrox by a gas blending technique such as partial pressure blending or premix decanting (in which a nitrox mix is supplied to the filler in pressurized larger cylinders). A few facilities have begun to fill cylinders with air which has been enriched with oxygen to nitrox by a diffusion process, so that it is pressurized as nitrox for the first time in the diving cylinder. With the use of pure oxygen during "partial pressure blending" (where pure oxygen is added to the nearly empty dive cylinder to 300-500 p.s.i., from a large pure oxygen cylinder before air is added, by compressor) there is an especially increased risk of fire. Partial blending using pure oxygen is often used to provide nitrox for multiple dives on live-aboard dive boats, but it is also used in some smaller diver shops. However, any gas which contains a significantly larger percentage of oxygen than air is a fire hazard. Furthermore, such gases can also react with hydrocarbons or incorrect lubricants inside a dive cylinder to produce carbon monoxide, even if a recognized fire does not happen. At present, there is controvery over whether or not mixtures of gas which contain less than 40% oxygen may sometimes be exempt from oxygen clean standards, especially if the gas supplied to them has been made by a diffusion process, and not previously compressed. Some of the controversy comes from a single U.S. regulation intended for commercial divers (not recreational divers) years ago. However, the U.S. Compressed Gas Association (CGA) and both major international nitrox teaching agencies (IANTDANDI) now support the standard that any gas containing more than 23.5% oxygen should be treated as nitox (which is to say, no differently from pure oxygen) for purposes of oxygen cleanliness and oxygen compatability (i.e., oxygen "servicability"). In 1959 the United States Navy (USN) documented enriched oxygen gas procedures for the military use of what we today call nitrox, in the USN Diving Manual. In 1970, Dr. Morgan Wells, the former Director of the National Oceanographic and Atmospheric Administration (NOAA)began instituting diving procedures for oxygen-enriched air. He also developed a process for mixing oxygen and air which he called a continuous blending system. For many years Dr. Wells' invention was the only practical alternative to partial pressure blending. In 1979 the National Oceanic and Atmospheric Administration (NOAA) published Wells' procedures for the scientific use of Nitrox in the NOAA Diving Manual. In 1985 Dick Rutkowski, a former NOAA diving safety officer, formed IAND (International Association of Nitrox Divers) and began teaching nitrox use for recreational diving. This was considered heresy by some, and met with heavy skepticism by the diving community. In 1992 the name was changed to the International Association of Nitrox and Technical Divers (IANTD), the T being added when the European Association of Technical Divers (EATD) merged with IAND. In the early 1990's, the agencies teaching nitrox were not the main scuba agencies. New organizations, including Ed Betts' ANDI (American Nitrox Divers International, which invented the term "Safe Air" for marketing purposes), and Bret Gilliam's TDI (Technical Divers International) gave scientific credence to nitrox. Meanwhile, diving stores were finding a purely economic reason to offer nitrox: not only was an entire new course and certification needed to use it, but instead of cheap or free tank fills with compressed air, dive shops found they could charge premium amounts of money for custom-gas blending of nitrox to their ordinary moderately experienced divers. With the new dive computers which could be programmed to allow for the longer bottom-times and shorter residual nitrogen times which nitrox gave, the incentive for the sport diver to use the gas increased. An intersection of economics and scientific validity had occurred. However, in the meantime during the early 90's, a number of feelings were hurt. While other main line scuba organizations had announced their support of nitrox earlier, it was PADI's endorsement that put nitrox over the top as a standard sport diving "option." PADI 5 Star National Geographic Instructor Development Center. 198/12 Rat-U-Thit Road, Patong, Phuket, Thailand. Phone: (+66) 076292052 Fax: (+66) 076293034
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