Nitrox

[6] 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.

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.

[8][9][note 1] Nonetheless, there are people in the diving community who insist that they feel reduced narcotic effects at depths breathing nitrox.

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.

Diving with and handling nitrox raise a number of potentially fatal dangers due to the high partial pressure of oxygen (ppO2).

[11][3] Nitrox is not a deep-diving gas mixture owing to the increased proportion of oxygen, which becomes toxic when breathed at high pressure.

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.

[14] Some training agencies, such as PADI and Technical Diving International, teach the use of two depth limits to protect against oxygen toxicity.

[12] 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).

Use of nitrox may cause a reduced ventilatory response, and when breathing dense gas at the deeper limits of the usable range, this may result in carbon dioxide retention when exercise levels are high, with an increased risk of loss of consciousness.

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, and unplanned contingencies due to currents or buoyancy problems.

[25] American Nitrox Divers International (ANDI) uses the term "SafeAir", which they define as any oxygen-enriched air mixture with O2 concentrations between 22% and 50% that meet their gas quality and handling specifications, and specifically claim that these mixtures are safer than normally produced breathing air for the end user not envolved to the mix production which.

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

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.

[11][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.

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:[29] There are several methods of production:[3][25][30] Any diving cylinder containing a blend of gasses other than standard air is required by most diver training organizations, and some national governments,[32] to be clearly marked to indicate the current gas mixture.

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.

In practice this is done by a small additional self-adhesive label marked with the measured oxygen fraction, which is changed when a new mix is filled.

The USCG, NOAA, U.S. Navy, OSHA, and the other recreational training agencies accept the limit as 40% as no accident or incident has been known to occur when this guideline has been properly applied.

[41] Christian J. Lambertsen proposed calculations for nitrogen addition to prevent oxygen toxicity in divers utilizing nitrogen–oxygen rebreather diving.

[42] In World War II or soon after, British commando frogmen and clearance divers started occasionally 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.

[6] In 1989, the Harbor Branch Oceanographic institution workshop addressed blending, oxygen limits and decompression issues.

[citation needed] NAUI became the first existing major recreational diver training agency to sanction nitrox.

[46] 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.

[citation needed] 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.

[note 2][citation needed] DiveRite manufactured the first nitrox-compatible dive computer, called the Bridge,[47] the aquaCorps TEK93 conference was held in San Francisco, and a practicable oil limit of 0.1 mg/m3 for oxygen compatible air was set.

[6] In 1994, John Lamb and Vandagraph launched the first oxygen analyzer built specifically for Nitrox and mixed-gas divers, at the Birmingham Dive Show.