Nitrox

Nitrox

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Typical Nitrox cylinder marking 

Nitrox refers to any gas mixture composed (excepting trace gases) of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarilyargon.[1][2][3] In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently.[3] The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness (also known as the bends).[4]

Nitrox is used to a lesser extent in surface-supplied diving, as these advantages are reduced by the more complex logistical requirements for nitrox compared to the use of simple low-pressure compressors for breathing gas supply. Nitrox can also be used in hyperbaric treatment of decompression illness, usually at pressures where pure oxygen would be hazardous. 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.

Though not generally referred to as nitrox, an oxygen-enriched air mixture is routinely provided at normal surface ambient pressure as oxygen therapy to patients with compromised respiration and circulation.

Contents

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Use[edit]

Underwater diving[edit]


Enriched Air Nitrox diving tables, showing adjusted no-decompression times.

See also: Decompression (diving)Decompression theory and Decompression practice

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. The significant aspect of extended no-stop time when using nitrox mixtures is reduced risk in a situation where breathing gas supply is compromised, as the diver can make a direct ascent to the surface with an acceptably low risk of decompression sickness. 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 (EAD) with the same partial pressure of nitrogen as the mix to be used, and this depth is less than the actual dive depth for oxygen enriched mixtures. The equivalent air depth is used with air decompression tables to calculate decompression obligation and no-stop times. The Goldman decompression model predicts a significant risk reduction by using nitrox (more so than the PADI tables suggest).[5]

Controlled tests have not shown breathing nitrox to reduce the effects of narcosis, as oxygen seems to have similarly narcotic properties under pressure to nitrogen; thus one should not expect a reduction in narcotic effects due only to the use of nitrox.[6][7][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.[8] However, because of risks associated with oxygen toxicity, divers do not usually use 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,[9] particularly in older and or obese divers; however a double-blind study to test this found no statistically significant reduction in reported fatigue.[1][10] 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.[11]

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.[12] 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.

Therapeutic recompression[edit]

Nitrox50 is used as one of the options in the first stages of therapeutic recompression using the Comex CX 30 table for treatment of vestibular or general decompression sickness. Nitrox is breathed at 30 msw and 24 msw and the ascents from these depths to the next stop. At 18m the gas is switched to oxygen for the rest of the treatment.[13]

Terminology[edit]

Nitrox is known by many names: Enriched Air Nitrox, Oxygen Enriched Air, Nitrox, EANx or Safe Air.[3][14] 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 originally the x of nitrox,[2] 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.[15]

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).[16]

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.[17]

MOD[edit]

Main article: Maximum operating depth

Maximum Operating Depth (MOD) is the maximum safe depth at which a given nitrox mixture can be used. MOD depends on the allowed partial pressure of oxygen, which is related to exposure time and the acceptable risk assumed for central nervous system oxygen toxicity. Acceptable maximum ppO2 varies depending on the application:[3]

  • 1.2 is often used in closed circuit rebreathers.
  • 1.4 is recommended by several recreational training agencies for ordinary scuba diving.
  • 1.5 is allowed for commercial diving in some jurisdictions.
  • 1.6 is allowed for technical diving decompression stops, and is the recommended maximum according to NOAA[2]

Higher values are used by commercial and military divers in special circumstances, often when the diver uses surface supplied breathing apparatus, or for treatment in a chamber, where the airway is relatively secure.

Choice of mixture[edit]

See also: Scuba gas planning

Technical divers preparing for a mixed-asdecompression dive in BoholPhilippines. Note thebackplate and wing setup with side mounted stage tanks containing EAN50 (left side) and pure oxygen(right side).

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][18][19]

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 permitted by the capacity 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 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.

Best mix[edit]

The composition of a nitrox mix can be optimized for a given planned dive profile. This is termed "Best mix", for the dive, and provides the maximum no-decompression time compatible with acceptable oxygen exposure. An acceptable maximum partial pressure of oxygen is selected based on depth and planned bottom time, and this value is used to calculate the oxygen content of the best mix for the dive:[20]

f_{\text{O}_2,\text{max}} = \frac{p_{\text{O}_2,\text{max}}}{p} = \frac{\text{Maximum acceptable partial pressure of oxygen}}{\text{Maximum ambient pressure of the dive}}

Production[edit]

See also: Gas blending

There are several methods of production:[3][21][16]

  • 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 cylinder that is accurately weighed until the required mix is achieved. This method requires fairly large and highly accurate scales, otherwise it is similar to partial pressure blending, but insensitive to temperature variations.
  • 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 are possible, but the equipment is 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[edit]

Any diving cylinder containing a blend of gasses other than standard air is required by most diver training organizations, and some national governments,[22] to be clearly marked to indicate the current gas mixure. In practice it is common to use a printed adhesive label to indicate the type of gas (in this case nitrox), and to add a temporary label to specify the analysis of the current mix.

Training standards for nitrox certification suggest the composition must be verified by the diver by using an oxygen analyzer before use.

Regional standards and conventions[edit]

European Union[edit]

Within the EU, valves with M26x2 outlet thread are recommended for cylinders with increased oxygen content.[23] Regulators for use with these cylinders require compatible connectors, and are not directly connectable with cylinders for compressed air.

Germany[edit]

A German standard specifies that any mixture with an oxygen content greater than atmospheric air must be treated as pure oxygen.[citation needed] A nitrox cylinder is specially cleaned and identified.[20] The cylinder colour is overall white with the letter N on opposite sides of the cylinder.[citation needed] The fraction of oxygen in the bottle is checked after filling and marked on the cylinder.

South Africa[edit]

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.[22] 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.

USA[edit]

Cylinder showing Nitrox band and sticker marked with MOD and O2%

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.[24] 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 an oxygen fraction not exceeding 40% by volume.

Dangers[edit]

Oxygen toxicity[edit]

Main article: 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, withprofessional divers sometimes being allowed to breathe higher ppO