Sunday, 23 December 2012

Enriched Air NITROX

Enriched Air Nitrox, Mixture of gases with an oxygen content above 21%, 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 for the same dive profile, or allows extended dive times without increasing the need for decompression stops for the same risk. Nitrox is not a safer gas than compressed air in all respects; although its use can reduce the risk of decompression sickness, it increases the risk of oxygen toxicity and fire, which are further discussed below.

Breathing Nitrox is not thought to reduce the effects of narcosis, as oxygen seems to have equally narcotic properties under pressure as nitrogen; thus one should not expect a reduction in narcotic effects due only to the use of Nitrox.Nonetheless, there are people in the diving community who insist that they feel reduced narcotic effects at depths breathing Nitrox. This may be due to a dissociation of the subjective and behavioral effects of narcosis. However, it should be noted that because of risks associated with oxygen toxicity, divers tend not to utilize Nitrox at greater depths where more pronounced narcosis symptoms are more likely to occur. For a reduction in narcotic effects Trimix or Heliox, gases which also contain helium, are generally used by technical divers.

In 1995 PADI (Professional Association of Diving Instructors) took nitrox training as a part of it's training program, and the course certifies diver to dive nitrox dives within the recreational limits and with blends of 22%-40% of oxygen. Copyright ©rivemar.com
In about the year 1890 scientists discovered to benefits of enriched air in diving in theory. The first real mass diving tests were conducted during the World War I and a little before. During the WW II it was used much more, and the first open circuit EANx equipment were used.
In the year 1979 USA's National Underwater and Athmospheric Administration (NOAA) published EANx diving tables.

International Association of Nitrox and Technical Divers (IANTD) bas established in the year 1985 (USA) and IANTD UK in 1992 (Europe). ANDI and TDI were also quickly in markets.
In 1995 PADI (Professional Association of Diving Instructors) took nitrox training as a part of it's training program, and the course certifies diver to dive nitrox dives within the recreational limits and with blends of 22%-40% of oxygen.

Nitrox with more than 40% oxygen is uncommon within recreational diving. There are two main reasons for this: the first is that all pieces of diving equipment that come into contact with mixes containing higher proportions of oxygen, particularly at high pressure, need special cleaning and servicing to reduce the risk of fire. Copyright © Nsyll's photostream
The two most common recreational diving nitrox mixes contain 32% and 36% oxygen, which have maximum operating depths (MODs) of 34 meters  and 29 meters  respectively when limited to a maximum partial pressure of oxygen of 1.4 bar.

Divers may calculate an equivalent air depth to determine their decompression requirements or may use Nitrox tables or a Nitrox-capable dive computer.

Nitrox with more than 40% oxygen is uncommon within recreational diving. There are two main reasons for this: the first is that all pieces of diving equipment that come into contact with mixes containing higher proportions of oxygen, particularly at high pressure, need special cleaning and servicing to reduce the risk of fire. The second reason is that richer mixes extend the time the diver can stay underwater without needing decompression stops far further than the duration of typical diving cylinders. For example, based on the Professional Association of Diving Instructors PADI Nitrox recommendations, the maximum operating depth for EAN45 would be 21 meters  and the maximum dive time available at this depth even with EAN36 is nearly 1 hour 15 minutes: a diver with a breathing rate of 20 liters per minute using twin 10 liters, 230 bar cylinders would have completely emptied the cylinders after 1 hour 14 minutes at this depth.



Divers may calculate an equivalent air depth to determine their decompression requirements or may use Nitrox tables or a Nitrox-capable dive computer.   Copyright ©ecodivecenter.com
The two most common recreational diving nitrox mixes contain 32% and 36% oxygen, which have maximum operating depths (MODs) of 34 meters  and 29 meters  respectively when limited to a maximum partial pressure of oxygen of 1.4 bar. Copyright ©aquaventurers.biz
Usage of Nitrox mixtures containing 50% to 80% oxygen is common in technical diving as a decompression gas, which by virtue of its lower partial pressure of inert gases such as nitrogen and helium, allows for more efficient (faster) elimination of these gases from the tissues than leaner oxygen mixtures.

In deep open circuit technical diving, where Hypoxic gases are breathed during the bottom portion of the dive, a Nitrox mix with 50% or less oxygen called a "travel mix" is sometimes breathed during the beginning of the descent in order to avoid hypoxia. Normally, however, the most oxygen-lean of the diver's decompression gases would be used for this purpose, since descent time spent reaching a depth where bottom mix is no longer Hypoxic is normally small, and the distance between this depth and the MOD of any Nitrox decompression gas is likely to be very short, if it occurs at all.


Diving and handling Nitrox raises a number of potentially fatal dangers due to the high partial pressure of oxygen (ppO2).Copyright ©Ahmed F.Gad 2011
There is anecdotal evidence that the use of Nitrox reduces post-dive fatigue, particularly in older and or obese divers; however a double-blind study to test this found no statistically significant reduction in reported fatigue. There was, however, 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 fatigue in both Nitrox and air divers. In 2008, a study was published using wet divers at the same depth and confirmed that no statistically significant reduction in reported fatigue is seen.

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 occurrence 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.


I have to say that nitrox diving is not dangerous when done right, but these points have to be counted when people talk about safety. So Nitrox diving is safe, but not safer than air, generally speaking. Copyright ©underwater-adventures.com
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 owing to the increased proportion of oxygen, which becomes toxic when breathed at high pressure. For example, the maximum operating depth of Nitrox with 36% oxygen, a popular recreational diving mix, is 29 meters to ensure a maximum ppO2 of no more than 1.4 bar . 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 breathe higher ppO2 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. Most dive shops, dive operators, and gas blenders require the diver to have a Nitrox certification card before selling/providing Nitrox tank to divers.

Some training agencies, such as Technical Diving International, 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 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 a greater 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 may be spat out during convulsions, which occur in conjunction with sudden unconsciousness (general seizure induced by oxygen toxicity).

Divers trained to use Nitrox may memories the acronym VENTID-C or sometimes ConVENTID, (which stands for Vision (blurriness), Ears (ringing sound), Nausea, Twitching, Irritability, Dizziness, and Convulsions). However, evidence from non-fatal oxygen convulsions indicates that most convulsions are not preceded by any warning symptoms at all. Further, many of the suggested warning signs are also symptoms of nitrogen narcosis, and so may lead to misdiagnosis by a diver. A solution to either is to ascend to a shallower 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. Most dive shops, dive operators, and gas blenders require the diver to have a Nitrox certification card before selling/providing Nitrox tank to divers. 
Copyright ©Ahmed F.Gad 2012
 Let's take a look at the diving accident statistics! DAN (Divers Alert Network) has statistics of DCS cases in the USA, which is based on DEMA/NUADC and PADI New Diver Benchmark reports. According to them, the probability to get DCS by air diving is about 0.004% (1 to 25000). Based on this number, and on the PADI Undersea Journal 3/97 magazine, the probability to get DCS by air diving even once during 500 dives is 1.98% (1 to 50). The risk is 1.49% in nitrox diving in same amount of dives. So... so far the difference is marginal, even though nitrox seems to be a bit safer.

But an accident doesn't have to be DCS only, and when diving with nitrox there are several other concerns too that air diving doesn't have. Gas blending, analyzing errors, oxygen clean equipments, CNS clock etc. may also be a source of accident. In this point I have to say that nitrox diving is not dangerous when done right, but these points have to be counted when people talk about safety. So nitrox diving is safe, but not safer than air, generally speaking.





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