Nitrox
Frequently Asked Questions


Nitrox has become a common term in recreational diving. For those who have never come across this term before, it is only natural to be a little apprehensive, especially with some people confusing the issue by associating it in the wrong context with other diving mixtures such as trimix (a mixture of helium, nitrogen and oxygen) and terms like technical diving.

This document is a set of Questions and Answers on Nitrox. It should allow anyone who is either unfamiliar with nitrox or confused by the jargon to reach a point where they can follow a discussion on the subject. It may also serve as a starting point for those who wish to learn about nitrox.


DISCLAIMER

This document is not a substitute for nitrox training and does not provide enough information to enable someone without specific nitrox training to dive using nitrox. It is neither intended to encourage nor discourage the use of nitrox in recreational diving. Anyone who wishes to dive using nitrox should seek out proper training.


If you have any corrections or suggestions please send them to me at akerslake@gasdiving.co.uk

This FAQ is loosely based on a original document from and was copyrighted to Alan Wright  in 1994. The current editor (ie me) has unsuccesfully attempted to contact Alan to gain his agreement in taking on this update - If you know Alan's whereabouts please ask him to contact me.


History :



What is nitrox?

In diving terminology; any mixture of nitrogen and oxygen, where these two gases represent the major constituents of the gas mix, is termed nitrox. Note that mixes which contain more than trace levels of other gases in addition to nitrogen and oxygen are not nitrox. Air is considered a nitrox mix. Nitrox mixes which are hyperoxic (contain more than 21% oxygen) are variously known as; enriched air, enriched air nitrox (EAN or EANx) or SafeAir *. For the most part, sport divers will only be interested in hyperoxic nitrox mixes.

* SafeAir is copyright of ANDI and refers to any nitrox mix with an oxygen percentage between 22% and 50%.

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How long has nitrox been around?

Quite a while. The early history of nitrox is really the history of research into oxygen toxicity.

The toxic effect of enriched oxygen mixtures were first demonstrated by Paul Bert in 1878. He discovered that high partial pressures of oxygen were directly responsible for causing convulsions.

In 1899, Lorrain Smith demonstrated that animals breathing moderately increased partial pressures of oxygen over a long period develop pulmonary problems. For example, a partial pressure of about 0.8 bar breathed for more than 4 days produced severe lung problems and could be fBarl. In 1903, Hill and Macleod noted that resistance to pulmonary damage in individuals varied enormously.

Between 1910 and 1912 various experiments were carried out using nitrox including riding a bike while breathing nitrox10 and a dive to 100 fsw (30 msw) using a surface supplied 50:50 nitrogen-oxygen mixture. These may have been the first experiments in which the effects or nitrox were observed in man.

During the 1930's a great number of experiments were carried with individuals breathing PO2's in the range of 2 to 4 Bar - even one of breathing up to 7 Bar (Haldane (the younger) in 1941). When taken overall these experiments demonstrated the enormous variation in susceptibility. Some people were okay after an hour while others convulsed within minutes. One even convulsed after switching back to air (which he did because his lips were twitching). In 1933, Shilling and Adams noted the extreme variation in CNS O2 toxicity tolerance although they erroneously concluded that man should have ample warning of the onset of symptoms. It was discovered that prior to the onset of CNS O2 toxicity there is a loss of respiratory control where breathing may become jerky and irregular and then change to become prolonged and laboured. They also noted that the effect varied enormously between individuals. In 1939, Lambertson developed the first nitrox rebreather.

In Haldane's experiments some subjects said they could taste the oxygen at 5 to 7 Bar. Dr Kenneth Donald, author of Oxygen and the Diver, breathed O2 at 10 Bar for 25 to 30 seconds along with Haldane and another man. Only Haldane thought he might have tasted something.

Experiments with cats in 1944 showed that repeated CNS O2 hits produced symptoms similar to that of neurological damage but the effects apparently disappeared a few weeks after the exposures were stopped. The cats also appeared to develop a tolerance to the high partial pressures during the experiments but this also returned to normal after a few weeks break.

Between 1942 and 1945 the Royal Navy carried out extensive work on oxygen poisoning in divers. The experiments are documented in Kenneth Donald's book. His conclusions were that diving on pure O2 deeper than 25 fsw (7.6 msw) is a pure gamble. He found that tolerance is reduced underwater (compared to dry experiments) and that the variation in symptoms, even for the same person, makes the onset impossible to predict.

In the 1950's Rev Lanphier worked on producing O2 exposure tables for the US Navy. He reached many of the same conclusions as Dr Donald but there were some discrepancies. Despite experimental evidence from Dr Donald's experiments to the contrary, Rev Lanphier concluded that oxygen was more toxic when breathed as part of a nitrox mixture. Experiments by other researchers have supported the conclusion that nitrogen has no effect on oxygen toxicity. However, Lanphier's conclusions were the basis of the US Navy's exposure tables in 1959, and for many years after that.

Rev Lanphier also tried to find a way of predicting CO2 retainers but could not find one. He realised that breathing resistance at depth, due to higher gas density, may increase CO2 levels. A higher CO2 level in the body would increase the risk of O2 toxicity, however, the fact that divers are breathing out an increased level of CO2 cannot be used to infer the arterial and body CO2 levels. Thus it does not immediately follow that nitrox divers are at an increased O2 toxicity risk due to CO2 retention.

The work to find out if divers adapt and become CO2 retainers continued through the 1960's and is still continuing today. Dr Donald's opinion is that there is no conclusive evidence that divers adapt and become less sensitive to the carbon dioxide breathing stimulus.

A major step was taken in 1970 when Dr Morgan Wells of the National Oceanic and Atmospheric Administration (NOAA) began experimenting with nitrox. He recognised the advantages of nitrox for the type of diving that NOAA divers were carrying out. Dr Wells was responsible for the (now) standard nitrox I (published in 1978) and nitrox II (published in 1990) mixes. It is largely as a consequence of NOAA's decision that we have nitrox in the recreational diving scene today.

Dick Rutkowski formed the International Association of Nitrox Divers (IAND) in 1985 to teach nitrox to sport divers. In 1992 the name was changed to the International Association of Nitrox and Technical Divers (IANTD). The T was added when the European Association of Technical Divers merged with IAND. Prior to founding IAND, Dick Rutkowski worked for Dr Wells and was director of the diver training at NOAA. This was the first organisation to offer international training to recreational scuba divers.

In 1987 or 1988 (depending on which page you read in the manual) Ed Betts, who had previously been with Dick Rutkowski at IAND, formed the second organisation for recreational nitrox training: American Nitrox Divers Inc. (ANDI).

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How do you name a nitrox mix?

Originally all Nitrox mixes where named using the nitrogen percentage to the left of the oxygen percentage; i.e. NOAA Nitrox I contained 32% oxygen and 68% nitrogen and was referred to as : nitrox 68/32 or nitrox68 for short.

Today most people quote the oxygen percentage using a term EANx (where x refers to the percentage of oxygen) or ONM (oxygen-nitrogen mixture). For example;

EAN/32 = ONM32 = nitrox68 = NOAA Nitrox I

EAN/36 = ONM36 = nitrox64 = NOAA Nitrox II

Or for those who prefer a table:

%O2 %N2 NOAA title EAN title ONM title
32 68 Nitrox I EAN32 ONM32
36 64 Nitrox II EAN36 ONM36

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Is air nitrox?

Yes. Air is, roughly, a mixture containing: 78.05% nitrogen + 20.95% oxygen + 1% trace gases including; carbon dioxide, carbon monoxide and various inert gases - mainly argon.

It thus meets the requirements of the definition given in above

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What are nitrox I and nitrox II?

Nitrox I and nitrox II are standard nitrox mixes defined by the National Oceanic and Atmospheric Administration (NOAA) in the US. NOAA have been using nitrox since the 1970's. Nitrox I is defined to be a mix containing 32% oxygen and 68% nitrogen. Nitrox II is defined to be a mix containing 36% oxygen and 64% nitrogen. The tolerance in the oxygen percentage is +/-1%.

When the nitrox is made by enriching air with oxygen, the trace gases are included in the percentage nitrogen figure.

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What is equivalent air depth?

The ability to equate the actual depth to an equivalent air depth is one of the fundamental principles underlying nitrox diving. One of the limitations in scuba diving is the inert gas we absorb while underwater. It governs our decompression obligation. By reducing the fraction of inert gas in our breathing mix we reduce the partial pressure we experience of that gas at any depth when compared to air at that same depth. Since the absorption of the inert gas is controlled by the difference between the partial pressure in our tissues and the ambient partial pressure it follows that we will absorb less inert gas than we would on air at the same depth over the same period of time. Thus the equivalent air depth is the depth on air at which we would experience the same nitrogen partial pressure, absorb the same amount of nitrogen and incur the same decompression penalty for our actual depth on nitrox.

The equivalent air depth (EAD) is calculated using the formula:

  FN2
EAD = (
* (d + x)
)
- x
  0.79

Another Form of the equation can be shown as

FN2 * (d + x)
EAD =
- x
0.79

Where:

Using an EAD enables dives on nitrox to be planned using standard air tables. When diving on air the EAD is the actual depth. On a hypoxic mix (<21% O2) the EAD would be deeper than the actual depth. On a hyperoxic mix (>21% O2) the EAD will be shallower than the actual depth.

This is how nitrox dive profiles are calculated. For a given nitrox mix and a planned maximum actual depth (or partial pressure) the dive is planned using the EAD to get a bottom time and decompression obligation. The EAD is also used to calculate surface intervals and repetitive dive penalties.

Tables are available for nitrox I and nitrox II which have already taken the EAD information into account. It is not difficult to generate a nitrox table for a different mix with either custom or commercially available software.

The following table equates some actual depths with their EAD and also shows the importance of considering the PO2 when selecting a nitrox mix.

EAD Actual D PO2 Actual D PO2
Nitrox I Nitrox II
msw fsw msw fsw Bar msw fsw Bar
10 33 13 44 0.74 15 49 0.89
20 66 25 82 1.12 27 89 1.33
30 99 37 121 1.49      

This table also shows an important point about CNS oxygen toxicity. It is recommended that maximum PO2 is kept below 1.4 Bar. At 37m/121 fsw on Nitrox I the PO2 is 1.49, this is passed the recommended limit

When planning a dive on nitrox it is vitally important to consider the PO2. If you look only at the EAD you may be misled by the fact that these are reasonable depths to dive to on air.

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Why would you want to use nitrox?

In short, the correct nitrox mix can be safer than air for the diver. However, we need to qualify that; by correct I mean the most appropriate mix for your dive and it's safer provided you follow the guidelines for its use. There are some additional guidelines to follow (when compared to air) and some priorities have changed. Some of the benefits are listed below, for hyperoxic mixes (i.e. EANx), but it should be noted that some of these are a double edged sword and could also be disadvantages if the guidelines are not followed.

The following claims are also made of nitrox.

I have also heard the shallower maximum depth proposed as a benefit.  The basis being that having a shallower maximum depth means you are nearer the surface in case of emergency (for no stop dives). However, I would assume that you dive to the depth that you planned and that the breathing mixture is appropriate for that depth. If you follow the guidelines, the maximum depth is largely irrelevant because you have factored that into your dive plan. If you do happen to be in situation where the mix in your cylinder is not what you hoped for the you should either change your dive plan or not dive at all. Pushing the oxygen toxicity limits of nitrox is as risky as pushing the oxygen toxicity limits of air - you may not come back from the dive.

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What are the limitations or problems of using nitrox?

There are a few limitations when using nitrox.

Bear in mind the risk of acute oxygen toxicity with nitrox is no greater than that with air. The difference is the changed priority between nitrogen and oxygen. On air, nitrogen narcosis is generally the governing factor in choosing a maximum depth for most sport divers.

On nitrox, the risk of an acute O2 attack may be the same or higher than the risk from nitrogen narcosis at certain depths. The major problem with oxygen is that you may get little or no warning of an attack and your chances of surviving one are remote. However, all of the information to avoid this problem is available before the dive so why should you risk it? Why would you be diving to, for example, 150 fsw (45 msw) on nitrox II? At this depth the PO2 is 2 Bar - equivalent to 280 fsw (85 msw) on air.

The other thing which is often missed is that you don't automatically convulse if the PO2 goes above a certain value. Some days you might others you might not. Most people can breath a PO2 of 2 Bar for several minutes without any adverse effects. Of course you won't know if you are "most" people unless you push your luck.

It is right that the above hazard is made known and the high risk associated with breaking the guidelines is pressed home, but if the guidelines are applied there is no increased risk. Keep the maximum partial pressure of oxygen at a safe level.

On air the NDL for 69 fsw (21 msw) is 35 mins.

On nitrox60 (40% O2) the EAD = 45 fsw (13.5 msw), so we use the 50 fsw (15 msw) table. The NDL is 75 mins.

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How do you know which nitrox mix to use?

Pick your depth and your O2 toxicity risk and use the standard formula for calculating the partial pressure, given below. This is becoming known as the "best mix" formula when talking about nitrox.

PO2 = FO2 * P

where: PV is the partial pressure, FO2 is the fraction of oxygen in the gas, P is the absolute pressure

So, to illustrate with an example, if we wish to limit the maximum depth to 90 fsw (27 msw) and the oxygen partial pressure to 1.4 Bar, then the FO2 is:

PO2 1.4
FO2 =
=
= 0.38
P 3.7

Thus, to obtain the maximum benefit from the use of nitrox, you'd choose EAN38, a mix with 38% O2 and 62% N2.

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Why is there so much talk about oxygen when talking about nitrox?

Use of nitrox generally involves being exposed to higher than normal partial pressures of oxygen (when compared to diving on air). On air there is virtually no danger of oxygen toxicity on normal sport dives.

On air, CNS/acute toxicity doesn't come into play until you get below 200 fsw (60 msw) and it is highly likely that nitrogen narcosis, decompression obligation and air supply will be far more limiting factors. It is also likely that air supply limitations will make pulmonary toxicity highly unlikely - do you carry enough air to remain for more than 45 minutes at 230 fsw (70 msw) and whatever decompression that you will be compelled to do? Standard sport diving tables don't even go that far. The DCIEM tables go to 40 minutes at 236 fsw (72msw) and the decompression penalty for that is 286 minutes on air (almost 5 hours) or 129 minutes (over 2 hours) if the last stop is done on pure O2.

Diving on nitrox, however, brings the depths and dive times for oxygen toxicity well into the range for sport divers. Take an example, using nitrox60 (40% O2):

[Note: See also Oxygen toxicity.]

At a depth = 30 msw; the PO2 = 1.6 Bar and the EAD = 20 msw. The NDL is around 35 minutes depending on which tables you work on and for a minor decompression penalty you could remain for around 1 hour. The maximum recommended exposure time  for this PO2 is 45 minutes. It is thus quite possible to go beyond the limits recommended for pulmonary toxicity. Although this transgression on its own probably won't be a problem, it may cause problems with intensive repetitive diving.

At a depth = 40 msw; the PO2 = 2.0 Bar. This is not an exceptional depth for many sport divers but it is in the range where there is a serious risk of an O2 induced convulsion.

Thus people who intend to use nitrox must be made aware of these "new" risks and limits which they should consider when planning and executing their dives.

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What is oxygen toxicity?

Oxygen toxicity is precisely what it suggests; oxygen poisoning the human body. There are two types of oxygen toxicity; central nervous system (CNS) toxicity and pulmonary toxicity. CNS toxicity is caused by short term exposure to high oxygen partial pressures and can result in convulsions. Pulmonary toxicity is caused by longer term exposures to moderate oxygen partial pressures and leads to pulmonary problems.These two topics are considered separately in the following questions.

The table below (published by NOAA) gives the recommended maximum oxygen exposure time limits for nitrox diving. The values in this table take both CNS and pulmonary toxicity into consideration.

PO2 Max single exposure duration Max total exposure in any 24 hours
  Min Hour Min Hour
0.6 720 12 720 12
0.7 570 9.5 570 9.5
0.8 450 7,5 450 7.5
0.9 360 6 360 6
1.0 300 5 300 5
1.1 240 4 270 4.5
1.2 210 3.5 240 4
1.3 180 3 210 3.5
1.4 150 2.5 180 3
1.5 120 2 180 3
1.6 45 0.75 150 2.5
         
         

What is CNS oxygen toxicity?

Central nervous system (CNS) toxicity, aka acute oxygen toxicity or the Paul Bert Effect (who published his research in 1878), manifests itself as convulsions, often with very little in the way of warning signs. The cause of these convulsions is attributed to oxidants and the resulting compounds produced in our body at elevated PO2. At some point our body will fail to cope and it reacts by convulsing. The phases before, during and following convulsions may be characterised by the steps below.

CNS oxygen toxicity should be avoided at all costs. Your chance of survival is minimal at best.


What is pulmonary oxygen toxicity?

Pulmonary oxygen toxicity goes by a variety of names; chronic toxicity, whole body toxicity or the Lorrain Smith Effect (who published his research in 1899). High partial pressures of oxygen damage lung tissue over a period of time. The result is similar to flu or pneumonia symptoms; coughing, breathing difficulty, lack of co-ordination, sore throat and chest. Unless the exposure is extremely long recovery is not a problem.

This is generally not considered to be a problem for sport divers. There is not the same risk of drowning as for CNS toxicity. If symptoms do become apparent they will probably do so after the dive. It is more of a problem for divers working in a saturation environment. For example, if the inspired PO2 is greater than 0.6 Bar for several days you will probably want to speak to a doctor. The oxygen clock is used to track pulmonary oxygen toxicity. Taking air breaks can reduce the risk to virtually nil.


What is the oxygen clock?

The oxygen clock is a mechanism for monitoring oxygen exposure over time. When diving at oxygen partial pressures above 0.5 Bar for long periods of time it becomes as important to monitor your oxygen exposure as it is to monitor your nitrogen exposure, although for quite different reasons. Whereas there is a saturation level for nitrogen after which you incur no additional decompression penalty and can remain underwater almost indefinitely given adequate facilities, with oxygen this is not the case. Over time, exposure to elevated partial pressures of oxygen is detrimental to the pulmonary system.

The theory behind the oxygen clock has been around for about 30 years and concerns pulmonary oxygen toxicity (aka whole body toxicity or the Lorrain Smith Effect). It is measured in units of pulmonary toxic dose (UPTD). There are various other names; the oxygen tolerance unit (OTU) and the cumulative pulmonary toxic dose (CPTD). Dr Bill Hamilton has suggested that we use the term OTU as he feels it gives more positive vibes. The OTU is based on empirical data from which the following best fit formula has been derived:

OTU = t * (PO2 - 0.5) 083

0.5

where:

However, very roughly, 1 OTU is equivalent to 1 Bar exposure per minute.

Period (days) Dose/day (units) Total (units)
1 850 850
2 700 1400
3 620 1860
4 525 2100
5 460 2300
6 420 2520
7 380 2660

The thing to remember, however, is that the values are not exact, hard limits, they are only guidelines. For most sport divers the oxygen clock is not a concern. However, for those who dive to partial pressures in excess of 0.5 Bar for long periods, especially if they are doing repetitive diving. It would be in their interest to track the OTU build up.


What are the working limits of oxygen?

There is a great deal of debate over the working partial pressure limits of oxygen in diving, however, the following table gives some generally accepted guidelines. The maximum partial pressure to which each person is willing to subject themselves should be made with an understanding of the relative dangers or advantages. There are some advantages to breathing slightly hyperoxic mixes, i.e. 0.22 - 1.45 Bar, but pushing the exceptional exposure limits can be dangerous. There is an excellent article in Technical Diver 3.2 (*) by Dr Bill Hamilton on oxygen limits in which he points out the absurdity of thinking that the oxygen exposure limits are hard boundaries to which we can dive. He recommends that we should keep the partial pressure below 1.5 Bar. My own organisation (Scottish Sub-Aqua Club) recommends 1.45 Bar. There is also the standard recommendation that 5 minute air breaks are taken every 20-25 minutes when breathing pure oxygen, for example, during decompression. Some extend this to be any mix with greater than 50% oxygen. This significantly reduces the risk of convulsions.

0.1 Below the threshold for life support
0.12 Threshold for serious hypoxia
0.16 Threshold for minor hypoxia
0.21 Normoxic
0.35 Normal saturation exposure
0.5 Maximum saturation exposure
1.4 Maximum normal diving exposure
1.6 Exceptional exposure for work diving
1.8 USN exceptional exposure (was 2.0 until recently)
2.2 Belgian Navy limit (was 2.3 until recently)
3.0 Medical limit for life threatening condition (i.e. DCS or gas gangrene)

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Is it possible to get an oxygen bend?

Yes, but in practical terms it can be ignored. To get an oxygen bend you'd have to go well beyond all of the guidelines, omit a substantial amount of decompression obligation and be lucky enough not to have had an acute oxygen toxicity attack during the dive.

Experiments carried out on goats at the Admiralty Experimental Diving Unit (AEDU) in 1945 demonstrated that oxygen bends are possible. The tests were based on immediate decompression (at 75 feet/min) to atmospheric pressure after one hour at the maximum depths (PO2 > 2.0 Bar). Severe bends resulted including pulmonary oedema and bubble embolism - identical to those caused by nitrogen. The symptoms disappeared within 10 to 15 minutes demonstrating that these were indeed oxygen bends. The oxygen was metabolised by the body. One out of seven occurrences did not clear up naturally and required recompression for a full cure. Note that this procedure included substantial amounts of missed decompression and was at partial pressures well above the maximum recommendations for nitrox diving.

It was concluded that the maximum PO2 that can be added safely to the tolerable PN2 lies between 2.0 and 3.5 Bar for immediate decompression. Since this is well above the maximum recommended PO2, due to the risk of acute oxygen toxicity, there is effectively no risk of an O2 bend in nitrox diving. Even in therapeutic recompression where the PO2 may be as high as 3.0 Bar there is no risk as the decompression rate is carefully controlled according to a well defined schedule.

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What effect does the CO2 content have?

Needs updating

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What has nitrox got to do with trimix?

Nothing really. Trimix is used for deep diving, nitrox is used for shallow diving. So why are they often mentioned together? Trimix should be the subject of another FAQ but we will introduce it here, very briefly, for this answer. Trimix is a mixture of helium, nitrogen and oxygen. It is used for deep diving because helium has a lower narcotic effect than nitrogen. Deep diving in this case would be from about 44 msw down to around 245 msw. Although it is perfectly reasonable from a physiological point of view to use helium on dives shallower than 44 msw it is unlikely you'd want to do that because of the price of helium.

When doing a trimix dive you are essentially trading off a number of factors: narcosis, oxygen toxicity, cost and the amount of decompression you will have to do. Although if you get down to around 250 msw you also have to start considering things like HPNS, not to mention truck loads of equipment and support staff, so let's not worry about that.

There are two problems with using trimix at these depths. You are going to incur a fair old decompression penalty and in order to ensure that there is no significant risk of CNS O2 toxicity at your maximum depth there may not be enough oxygen in the mix to support your life at the surface. Remember that it is the partial pressure of oxygen that is important not the percentage. So at depth you can live on a breathing mixture with a much lower percentage than you can on the surface.

So you have a problem how do you get from the surface to a depth where you can breath your trimix. Ah, you guessed! This is where nitrox comes in. Nitrox is used as a travel mix. You could use air but an enriched O2 mix has advantages on the way back up - it reduces the decompression penalty if it is used on the stops on the way back to the surface so you may as well use it on the way down as well. It is also normal for trimix divers to use pure O2 on the final two stops to further minimise their decompression.

So, as you can see, nitrox is used on trimix dives but only because it has advantages for the diver during the shallow part of the dive.

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What's in the cylinder?

Due to the danger of exceeding the maximum operating depth, nitrox fills should always be checked, using an O2 analyser, after filling and again before diving with the cylinder. You should never dive without being absolutely sure what is in your cylinder.

There are commercially available O2 analysers. You should know the tolerance limits of the O2 analyser being used and whether or not you need to add a correction to the measured O2 level when planning your dive.

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Will the oxygen and nitrogen stratify in the cylinder?

Some people have suggested that a possible problem with nitrox is that it could stratify, after mixing, in the cylinder. This would obviously be a serious problem if it did indeed happen. However, this has not been observed in practise and common sense suggests that it won't happen. Air is nitrox and in all the millions of years that this planet has had an atmosphere it has not stratified. Once mixed, the gases will remain mixed.

Depending on the preparation method there may be a slight problem getting the gases to mix properly during preparation, but this is easily overcome by tumbling, or otherwise causing turbulence, in the cylinders. You're only likely to see this if you are mixing directly in the cylinder with which you intend to dive, for example, when preparing a custom mix. If you receive a fill from a storage tank then it will probably already be mixed and there will be no problem. This is likely to be the case if you use the standard nitrox mixes. Of course you should still analyse the mix after the fill and again before you dive you.

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What are the equipment considerations for oxygen/nitrox?

There are a number of viewpoints. In industrial situations any hyperoxic gas mix may be treated as pure oxygen, others suggest that 23.5% oxygen is the limit. For scuba purposes it is generally recommended that any mix with greater than 50% oxygen is treated as pure oxygen and that any equipment which may be exposed to a mixture with greater than 40% oxygen be oxygen compatible and made oxygen clean prior to use.

When considering which pieces of equipment may be exposed to high percentages of oxygen remember that pure oxygen may be pumped into your cylinder during nitrox fills. Thus cylinders, pillar valves and hoses used for gas transfer should all be prepared for oxygen service.

Regulators and ancillary equipment which may come into contact with the mixture from the cylinder should be prepared to work with the percentage of oxygen in the final mix. For mixes with less than 40% oxygen it is not regarded as essential to have this equipment made oxygen clean, however, it is up to you. It won't hurt to have it done. The same is true for oxygen compatibility. If in any doubt follow the recommendations of the equipment manufacturer with regard to use with nitrox and/or oxygen.

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How are nitrox cylinders identified?

It is important to mark nitrox cylinders in a distinctive way due to the risks of diving without being sure of the contents of the cylinder. There is no international standard for marking nitrox cylinders so the best advice is probably to put the word nitrox in large friendly letters on the cylinder regardless of the colour coding used. Some examples to show the lack if standardisation:

In the US (CGA)

green oxygen
black nitrogen
yellow air
brown argon
helium brown
green heliox

In Europe and Canada:

white oxygen
black nitrogen
black and white air
brown helium

In Australia:

black (possibly with white on shoulder) oxygen
grey nitrogen
grey with black and white quarters on shoulder air
brown helium
brown with white and brown quarters on shoulder heliox

Nitrox cylinders often have a yellow body with a 4" wide green band near the top, the green band may include the neck portion of the cylinder.

The diagram belowshows that the cylinder should be tagged with the Maximum usable depth and the oxygen percentages. As an added safety assurance you may also want to mark the fill pressure, yout name and the fill date.

PICTURE OF CYLINDER NEEDED

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What does oxygen clean mean?

Oxygen clean refers to equipment which has been cleaned for use with pure oxygen. This does not mean that the material itself is suitable for use with pure oxygen, but that contaminates which react violently with pure oxygen have been removed from the equipment.

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What does oxygen compatible mean?

Oxygen compatible refers to the materials comprising the equipment which is intended for use with pure oxygen. It implies that the material is suitable for use with pure oxygen.

Note that describing a material as oxygen compatible does not mean that the material is ready for use with pure oxygen, merely that the material itself is suitable, if properly prepared, for use with pure oxygen.

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What does oxygen service mean?

Oxygen service refers to equipment that is both oxygen compatible and oxygen clean. Such equipment is ready for use with pure oxygen. Of course, all the safety precautions which should be followed when handling pure oxygen must also be followed to provide an appropriate level of safety. Note that oxygen service equipment will, most likely, have temperature and pressure limitations to its oxygen serviceability.

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What more is there to know?

A lot. What about how nitrox is made, how to calculate decompression schedules on nitrox, how to cope with a victim of an O2 convulsion and how to do gas switches between dives. If you're interested in learning more about nitrox I'd suggest having thinking about doing one of our available courses.


Appendix A - Acronyms

This appendix lists the acronyms used in this FAQ.


Copyright © 2001 [Gas - Diving]. All rights reserved.
Revised:3 April, 2005