Enriched Air Nitrox
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Typical decal used on scuba cylinders containing Nitrox
Nitrox is a breathing gas consisting of oxygen and nitrogen (similar to air), but with a higher proportion of oxygen than the normal 20.9%.
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Purpose
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.
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 (ref: Harris RJ, Doolette DJ, Wilkinson DC, Williams DJ. Measurement of fatigue following 18msw dry chamber dives breathing air or enriched air nitrox. Undersea Hyperb Med 2003;30(4):285-91). There has, however, been some suggestion that post dive fatigue is due to sub-clinical decompression sickness (DCS) (i.e. micro bubbles in the blood insufficient to cause symptoms of 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 fatigure in both nitrox and air divers. Further studies with a number of different dive profiles would be necessary to better investigate this issue.
Naming
Nitrox is known by several names: Enhanced Air Nitrox, Oxygen Enriched Air, Nitrox, EANx or Safe Air. 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 or simply Nitrox32 and Nitrox36).
Richness of mix
Nitrox with more than 40% oxygen is uncommon within entry-level recreational diving.
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.
Cylinder markings
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 will 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 pure oxygen inside. If by some accident an oxygen clean cylinder was not filled at a nitrox station, it is then considered contaminated and must be recleaned before pure oxygen may be added again.
Finally, all nitrox cylinders will have a tag or decal 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.
Dangers
Oxygen toxicity
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. See the main articles: oxygen toxicity and maximum operating depth.
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 before selling Nitrox services.
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 hits 1.4 Bar. The second deeper depth, called the "contingency depth", is reached when the partial pressure hits 1.6 bar. Diving at or beyond this level exposes the diver to the risk of Central Nervous System 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.
Finally, many training agencies such as PADI train their divers to personally check the Oxygen 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 abort the dive to remain safe and avoid oxygen toxicity or decompression sickness.
Fire
Diving cylinders are filled with Nitrox by a gas blending technique such as partial pressure blending or premix decanting. With the use of pure oxygen during partial pressure blending there is a risk of fire, which means that only oxygen clean and oxygen compatible cylinders and regulators should be used. There have been reports of fires and explosions where the oxygen content was well below the usual 40% clean and compatible limit.
History
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.
See also
- BSAC
- CMAS
- CFT - Irish Underwater Council
- International Association of Nitrox and Technical Divers
- PADI
- Trimixda:Nitrox
de:Nitrox fr:Nitrox it:Nitrox nl:Nitrox no:Nitrox pl:Nitrox ru:Нитрокс (дайвинг) sv:Nitrox