Nitrox

Nitrox refers to any gas mixture composed (excluding trace gases) of nitrogen and oxygen; this includes normal air which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon.^[1][2][3] However, in scuba diving, nitrox is normally differentiated and handled differently from air.^[3] The most common use of nitrox mixtures containing higher than normal levels of oxygen is in scuba, where the reduced percentage of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues and so extending the possible dive time, and/or reducing the risk of decompression sickness (also known as the bends).

Use

Enriched Air Nitrox, nitrox 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. One of the more significant aspects of this application is the extended no-stop time when using nitrox mixtures. The exact values of the extended no-stop times vary depending on the decompression model used to derive the tables, but as an approximation, it is based on the partial pressure of nitrogen at the dive depth. This principle can be used to calculate an equivalent air depth with the same partial pressure of nitrogen, and this depth is less than the actual dive depth for oxygen enriched mixtures. The equivalent air depth is used with air decompression tabled to calculate decompression obligation and no-stop times. 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.^{[4][5][note 1]} Nonetheless, there are people in the diving community who insist that they feel reduced narcotic effects at depths breathing nitrox.^{[note 2]} This may be due to a dissociation of the subjective and behavioural effects of narcosis.^[6] 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 divers.

There is anecdotal evidence that the use of nitrox reduces post-dive fatigue,^[7] particularly in older and or obese divers; however a double-blind study to test this found no statistically significant reduction in reported fatigue.^[1][8] 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.^[9]

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 increases exercise tolerance, during aerobic exertion.^[10] 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.

Terminology

Nitrox is known by many names: Enriched Air Nitrox, Oxygen Enriched Air, Nitrox, EANx or Safe Air.^[3][11] Since the word is a compound contraction or coined word and not an acronym, it should not be written in all upper case characters as "NITROX",^[3] but may be initially capitalized when referring to specific mixtures such as Nitrox32, which contains 68% nitrogen and 32% oxygen. When one figure is stated, it refers to the oxygen percentage, not the nitrogen percentage. The original convention, Nitrox68/32 became shortened as the first figure is redundant.^{[citation needed]}

The term "nitrox" was originally used to refer to the breathing gas in a seafloor habitat where the oxygen has to be kept to a lower fraction than in air to avoid long term oxygen toxicity problems. It was later used by Dr Morgan Wells of NOAA for mixtures with an oxygen fraction higher than air, and has become a generic term for binary mixtures of nitrogen and oxygen with any oxygen fraction,^[3] and in the context of recreational and technical diving, now usually refers to a mixture of nitrogen and oxygen with more than 21% oxygen.^[3] "Enriched Air Nitrox" or "EAN", and "Oxygen Enriched Air" are used to emphasize richer than air mixtures.^[3] In "EANx", the "x" was original the x of nitrox, but has come to indicate the percentage of oxygen in the mix and is replaced by a number when the percentage is known; for example a 40% oxygen mix is called EAN40. The two most popular blends are EAN32 and EAN36, developed by NOAA for scientific diving, and also named Nitrox I and Nitrox II, respectively, or Nitrox68/32 and Nitrox64/36.^[2][3] These two mixtures were first utilized to the depth and oxygen limits for scientific diving designated by NOAA at the time.^[12]

The term Oxygen Enriched Air (OEN) was accepted by the (American) scientific diving community, but although it is probably the most unambiguous and simply descriptive term yet proposed, it was resisted by the recreational diving community, sometimes in favour of less appropriate terminology.^[3]

In its early days of introduction to non-technical divers, nitrox has occasionally also been known by detractors by less complimentary terms, such as "devil gas" or "voodoo gas" (a term now sometimes used with pride).^[13]

American Nitrox Divers International (ANDI) uses the term "SafeAir", but considering the complexities and hazards of mixing, handling, analyzing, and using oxygen-enriched air, this name is considered inappropriate by those who consider that it is not inherently "safe", but merely has decompression advantages.^[3]

The constituent gas percentages are what the gas blender aims for, but the final actual mix may vary from the specification, and so a small flow of gas from the cylinder must be measured with an oxygen analyzer, before the cylinder is used underwater.^[14]

Choice of mixture

The two most common recreational diving nitrox mixes contain 32% and 36% oxygen, which have maximum operating depths (MODs) of 34 metres (112 ft) and 29 metres (95 ft) respectively when limited to a maximum partial pressure of oxygen of 1.4 bar (140 kPa). Divers may calculate an equivalent air depth to determine their decompression requirements or may use nitrox tables or a nitrox-capable dive computer.^{[2][3][15][16]}

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.^[2][3] 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 PADI nitrox recommendations, the maximum operating depth for EAN45 would be 21 metres (69 ft) 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 litres per minute using twin 10-litre, 230-bar (about double 85 cu. ft.) cylinders would have completely emptied the cylinders after 1 hour 14 minutes at this depth.

Use 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.

Production of nitrox

There are several methods of production:^[3][17][18]

• Mixing by partial pressure: a measured pressure of oxygen is decanted into the cylinder and cylinder is "topped up" with air from the diving air compressor. This method is very versatile and requires relatively little additional equipment if a suitable compressor is available, but it is labour intensive, and high partial pressures of oxygen are relatively hazardous.
• Pre-mix decanting: the gas supplier provides large cylinders with popular mixes such as 32% and 36%. These may be further diluted with air to provide a larger range of mixtures.
• Mixing by continuous blending: measured quantities of oxygen are introduced to air and mixed with it before it reaches the compressor inlet. The compressor and particularly the compressor oil, must be suitable for this service. If the resulting oxygen fraction is less than 40%, the cylinder and valve may not be required to be cleaned for oxygen service. Relatively efficient and quick compared to partial pressure blending, but requires a suitable compressor, and the range of mixes may be limited by the compressor specification.
• Mixing by mass fraction: oxygen and air or nitrogen are added to a partially that is accurately weighed until the required mix is achieved. Requires fairly large and highly accurate scales, otherwise similar to partial pressure blending.
• Mixing by gas separation: a nitrogen permeable membrane is used to remove some of the smaller nitrogen molecules from low pressure air until the required mix is achieved. The resulting low pressure nitrox is then pumped into cylinders by a compressor. Limited range of mixes possible, but quick and easy to operate and relatively safe, as there is never high partial pressure oxygen involved.
• Pressure swing adsorption requires relatively complex equipment, otherwise the advantages are similar to membrane separation.

Cylinder markings to identify contents

Any cylinder containing any blend of gasses other than the standard air content is required by most diving training organizations, and some national governments,^[19] to be clearly marked. Some organizations, e.g. GUE, argue that it does not make sense to have a permanent marking on a gas tank that can be filled with any gas.

Regional standards and conventions

Unspecified region, possibly USA

The standard^{[citation needed]} 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 a six-inch band around the shoulder, with a one-inch yellow band on the top and bottom, with four inches of green in the middle. This green band will also have the designation of "Nitrox" or something similar inside, in yellow or white letters.

South Africa

South African National Standard 10019:2008 specifies the colour of all scuba cylinders as Golden yellow with French gray shoulder. This applies to all underwater breathing gases except medical oxygen, which must be carried in cylinders that are Black with a White shoulder. Nitrox cylinders must be identified by a transparent, self-adhesive label with green lettering, fitted below the shoulder.^[19] In effect this is green lettering on a yellow cylinder, with a gray shoulder. The composition of the gas must also be specified on the label. In practice this is done by a small additional self-adhesive label with the oxygen fraction, which is changed when a new mix is filled.

Other markings

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 that does not supply gas to oxygen-clean standards it is then considered contaminated and must be re-cleaned before a gas containing more than 40% oxygen may again be added.^[20] Cylinders marked as 'not oxygen clean' may only be filled with oxygen-enriched air mixtures from membrane or stick blending systems where the gas is mixed before being added to the cylinder, and to oxygen fraction not exceeding 40%.

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 along with the partial pressure this depth was calculated with.^{[citation needed]} 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,^{[citation needed]} and any cylinders lacking these markings may be considered possibly unsafe. Training for nitrox certification suggests this tag must be verified by the diver by using an oxygen analyzer before use.

Dangers

Oxygen toxicity

Diving with and handling nitrox raise a number of potentially fatal dangers due to the high partial pressure of oxygen (ppO₂).^[2][3] 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 metres (95 ft) to ensure a maximum ppO₂ of no more than 1.4 bar (140 kPa). 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. Many dive shops, dive operators, and gas blenders require the diver to present a nitrox certification card before selling nitrox to divers.^{[citation needed]}

Some training agencies, such as PADI and 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 (140 kPa). The deeper depth, called the "contingency depth", is reached when the partial pressure reaches 1.6 bar (160 kPa).^{[citation needed]} 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 memorise 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.^[21] 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.

Precautionary procedures at the fill station

Many training agencies such as PADI,^[22] CMAS, SSI and NAUI train their divers to personally check the oxygen percentage content of each nitrox cylinder before every dive. If the oxygen percentage deviates by more than 1% from the value written on the cylinder by the gas blender, the scuba diver must either recalculate his or her bottom times with the actual mix, or else abort the dive to avoid increased risk of oxygen toxicity or decompression sickness. Under IANTD and ANDI rules for use of nitrox,^[23] which are followed by most dive resorts around the world,^{[citation needed]} 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 signature of the receiving diver (who should have personally measured the oxygen partial pressure before taking delivery), 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). In South Africa, the national standard for handling and filling portable cylinders with pressurised gases (SANS 10019)^[19] requires that the cylinder be labelled with a sticker identifying the contents as nitrox, and specifying the oxygen fraction. Similar requirements may apply in other countries.

Fire 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 by a pre-mixing process, so that it is pressurized as nitrox for the first time in the diving cylinder. The pre-mixing is accomplished either by a membrane system which removes nitrogen from the air during compression or by a 'stick' blending technique where pure oxygen is mixed with air in a baffled chamber attached to the compressor intake.

With the use of pure oxygen during "partial pressure blending" (where pure oxygen is added from a large oxygen cylinder to the nearly empty dive cylinder until it reaches 300–500 psi (20–30 bar) before air is added by compressor) there is an especially increased risk of fire. Partial pressure 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 dive shops and clubs.

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 some discussion over whether or not mixtures of gas which contain less than 40% oxygen may sometimes be exempt from oxygen-clean standards.^[24] Some of the controversy comes from a single U.S. regulation intended for commercial divers (not recreational divers) years ago.^[3] However, the U.S. Compressed Gas Association (CGA) and two international nitrox teaching agencies (IANTD and ANDI)^{[citation needed]} now support the standard that any gas containing more than 23.5% oxygen should be treated as nitrox (which is to say, no differently from pure oxygen) for purposes of oxygen cleanliness and oxygen compatibility (i.e., oxygen "servicability").

Among training agencies, only ANDI subscribes to the 23% guideline. This policy goes against the policies of the USCG, NOAA, U.S. Navy, OSHA, and the other nitrox training agencies. There is adequate historical evidence to support the responsible application of the "over 40% rule" that has been observed for several decades, as no accident or incident has been known to occur when this guideline has been properly applied. Tens of thousands of enriched air divers are trained each year and the overwhelming majority of these divers are taught the "over 40% rule". ^[2][3][22] Most nitrox fill stations which supply pre-mixed nitrox will fill non-oxygen clean cylinders with mixtures below 40%. For a history of this controversy^[3] see Luxfer cylinders.

The following references for oxygen cleaning specifically cite the "over 40%" guideline that has been in widespread use since the 1960s, and consensus of attendees and professional panelists at the 1992 Enriched Air Workshop was to accept that guideline and continue the status quo.^[3]

• Code of Federal Regulations, Part 1910.430 (i) - Commercial Diving Operations
• OSHA Oxygen Specifications 1910.420 (1)
• NOAA Oxygen Specifications (appendix D)
• U.S. Navy Oxygen Specifications U.S. MIL-STD-777E (SH) Note K-6-4, Cat. K.6
• U.S. Coast Guard Oxygen Specifications Title 46: Shipping, revisions through 10-1-92. 197.452 Oxygen Cleaning 46 CFR 197.451

Much of confusion appears to be a result of misapplying PVHO (pressure vessel for human occupancy) guidelines which prescribe a maximum ambient oxygen content of 25% when a human is sealed into a pressure vessel (chamber). The concern here is for a fire hazard to a living person who could be trapped in an oxygen-rich burning environment.^[3]

Of the three commonly applied methods of producing enriched air mixes (continuous blending, partial pressure blending, and membrane separation systems, only partial pressure blending would require the valve and cylinder components to be oxygen cleaned for mixtures with less than 40% oxygen. The other two methods ensure that the equipment is never subjected to greater than 40% oxygen content.

History

In the 1920s or 1930s Draeger of Germany made a nitrox backpack independent air supply for a standard diving suit.

In World War II or soon after, British commando frogmen and work divers started sometimes diving with oxygen rebreathers adapted for semi-closed-circuit nitrox (which they called "mixture") diving by fitting larger cylinders and carefully setting the gas flow rate using a flow meter. These developments were kept secret until independently duplicated by civilians in the 1960s.

In the 1950s the United States Navy (USN) documented enriched oxygen gas procedures for military use of what we today call nitrox, in the USN Diving Manual.^[25]

In 1970, Morgan Wells, who was the first director of the National Oceanographic and Atmospheric Administration (NOAA) Diving Center, 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 Wells' invention was the only practical alternative to partial pressure blending. In 1979 NOAA published Wells' procedures for the scientific use of nitrox in the NOAA Diving Manual.^[2][3]

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 dangerous by some, and met with heavy skepticism by the diving community.

In 1991, in a watershed moment, the annual DEMA show (held in Houston, Texas that year) banned nitrox training providers from the show. This created a backlash, and when DEMA relented, a number of organizations took the opportunity to present nitrox workshops outside the show. In 1992 BSAC banned its members from using nitrox.

In 1992 IAND's 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 1990s, these agencies were teaching nitrox, but the main scuba agencies were not. Additional new organizations, including the American Nitrox Divers International (ANDI) - which invented the term "Safe Air" for marketing purposes - and Technical Diving International (TDI) were begun.

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 that nitrox gave, the incentive for the sport diver to use the gas increased. An intersection of economics and scientific validity had occurred.

In 1993 Skin Diver magazine, the leading recreational diving publication at the time, published a three-part series arguing that nitrox was unsafe for sport divers.^{[note 3]} Against this trend, in 1992 NAUI became the first existing major sport diver training agency to sanction nitrox.

In 1993 Dive Rite manufactured the first nitrox-compatible dive computer, called the Bridge.^[26]

In 1994 BSAC reversed its policy on Nitrox and announced BSAC nitrox training to start in 1995^[27]

In 1996, the Professional Association of Diving Instructors (PADI) announced full educational support for nitrox.^[22] While other mainline scuba organizations had announced their support of nitrox earlier,^[27] it was PADI's endorsement that put nitrox over the top as a standard sport diving option.^[28]

Nitrox in nature

Sometime in the geologic past the Earth's atmosphere contained much more than 20% oxygen: e.g. up to 35% in the Upper Carboniferous era. This let animals absorb oxygen more easily and influenced evolution.^[29][30]

See also

• Other breathing gases:
  • Argox
  • Heliox
  • Hydreliox
  • Hydrox
  • Trimix
• Methods of Gas blending:
  • Membrane method
• Diving cylinders
• Equivalent air depth
• Partial pressure
• Maximum operating depth
• Nitrogen narcosis
• Oxygen toxicity

References

Footnotes

External links

• Nitrox - frequently asked questions
• Useful Luxfer FAQ on the CGA and "40% rule" controversy
• Online Nitrox calculator for EAD, MOD and PPO2; warns if a critical limit is reached
• Diving Nitrox

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