From: awright@gssec.bt.co.uk (Alan Wright)
Subject: Nitrox FAQ
Date: Mon, 17 Jan 94 16:50:55 GMT
Nitrox: Questions and Answers
-----------------------------------
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. If you have any corrections or suggestions please
send them to awright@gssec.bt.co.uk
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.
Search for "Subject: [#]" to get to question number # quickly.
Questions:
[1] What is nitrox?
[1a] How do you name a nitrox mix?
[2] Is air nitrox?
[3] What are nitrox I and nitrox II?
[4] What is the equivalent air depth?
[5] Why would you want to use nitrox?
[6] What are the limitations or problems of using nitrox?
[7] How do you know which nitrox mix to use?
[8] Why is there so much talk about oxygen when talking about nitrox?
[9] What is oxygen toxicity?
[10] What is CNS oxygen toxicity?
[11] What is pulmonary oxygen toxicity?
[12] What is the oxygen clock?
[13] What are the working limits of oxygen?
[14] Will the oxygen and nitrogen stratify in the cylinder?
[15] What equipment considerations are there?
[16] What does oxygen clean mean?
[17] What does oxygen compatible mean?
[18] What does oxygen service mean?
[19] Where can I get further information?
[20] How are nitrox cylinders identified?
[21] What's in the cylinder?
[22] Should I use nitrox for decompression?
[23] Is it possible to get an oxygen bend?
[24] What effect does the CO2 content have?
Possible questions TBD:
[20] How is nitrox made?
[21] What more is there to know?
--------------------------------------------------------------------
Subject: [1] 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 (see question [2]). 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%. It was supposedly coined to
distance nitrox from other mixed gases because of the bad press that
recreational use of mixed gases was receiving at the time.
--------------------------------------------------------------------
Subject: [1a] How do you name a nitrox mix?
Nitrox mixes should always be named using the nitrogen percentage to
the left of the oxygen percentage; ie NOAA Nitrox I contains 32% oxygen
and 68% nitrogen so it should be named: nitrox68/32 or nitrox68 for
short. However, be careful because a lot of people get it wrong.
If you wish to quote the oxygen percentage use 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 | EAN | ONM |
| | | Title | Title | Title |
|_____|_____|___________|________|_______|
| | | | | |
| 32 | 68 | nitrox I | EAN/32 | ONM32 |
| 36 | 64 | nitrox II | EAN/36 | ONM32 |
|________________________________________|
--------------------------------------------------------------------
Subject: [2] 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 question
[1].
--------------------------------------------------------------------
Subject: [3] 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.
--------------------------------------------------------------------
Subject: [4] What is equivalent air depth?
The nitrogen partial pressure experienced at an actual depth on nitrox
will be equal to the nitrogen partial pressure which would have been
experienced at a, possibly, different depth had the dive been on air -
the equivalent air depth (EAD). Using an EAD enables dives on nitrox
to be planned using standard air tables. The EAD is calculated using
the formula:
fN2
EAD = ---- (d + x) - x
0.79
Where:
fN2 is the fraction of nitrogen in the nitrox mix
0.79 is the fraction of nitrogen in air (including the trace gases,
see question [2])
d is the actual depth in the appropriate units (fsw or msw)
x is the depth of water equivalent to 1 ATA in the appropriate
units (33 fsw or 10 msw)
When diving on air the EAD is the actual depth. On a hypoxic mix (<21%)
the EAD would be deeper than the actual depth. On a hyperoxic mix
(>21%) the EAD will be shallower than the actual 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 under-
water. 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.
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.
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 | ATA | msw | fsw | ATA |
|-----+-----+-----+-----+-----+-----+-----+-----|
| 10 | 33 | 13.2| 43.5| 0.64| 14.7| 48.5| 0.72|
| 20 | 66 | 24.9| 82 | 0.96| 27 | 89 | 1.08|
| 30 | 99 | 36.5|120.5| 1.28| 39.4|130 | 1.44|
| 40 | 130 | 48.1|159 | 1.6 | 51.7|170.5| 1.8 |
| 50 | 165 | 59.7|197 | 1.92| 64 |211 | 2.16|
|_______________________________________________|
>From the table it can be seen why the maximum recommended depth for
nitrox I is 40 msw (130 fsw). To appreciate this you must be aware of
the limitations imposed due to oxygen toxicity risks (see question [8
- 10]). At this depth the PO2 is 1.6 ATA (see question [7]).
Similarly the maximum depth for Nitrox II is 30 msw (100 fsw). If you
look only at the EAD you may be misled by the fact that these are
reasonable depths to dive to on air. The oxygen partial pressure is
very important (see question [8]).
--------------------------------------------------------------------
Subject: [5] Why would you want to use nitrox?
In short, the correct nitrox mix is 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 (see question
[6]). 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 (see question [6]).
1. Longer NDL. Because we work to an EAD (see question [4]) the NDL for
our actual depth on nitrox is the one which applies to our EAD. This
will be shallower than our actual depth, thus the NDL will be longer
than if we were using air.
2. Reduced nitrogen narcosis due to the lower percentage of nitrogen in
your breathing mix. The nitrogen partial pressure is governed by your
EAD which will be shallower than your actual depth. It has been
suggested recently that there may be a narcotic contribution from high
PO2, but this has yet to be verified. It may turn out to be a trade-
off.
3. Reduced decompression penalty due to the lower level of nitrogen
absorbed during the dive. This may be realised by surfacing according
to the nitrox tables or as an additional safety factor by following the
standard air tables. For older or overweight divers (or those with poor
circulation) this additional safety feature may be very desirable.
4. Shorter surface intervals and longer subsequent dives due to the
lower residual nitrogen level following a dive. The surface interval is
followed for the EAD not the actual depth. Alternatively it may be used
as safety padding by following the standard air surface interval
recommendation. Again this may be advantageous to older or overweight
divers.
5. Accelerated decompression and alternative to pure O2 on surface
TBD.
The following claims are also made of nitrox but have been disputed.
6. The reduced level of nitrogen in your system has also been claimed
to reduce the feeling of lethargy or tiredness following a dive.
Personally, I haven't noticed any difference, however, on a recent
dive trip a friend insisted that he felt much more alert after dives
on nitrox - just before he dropped off to sleep on the way home :-)
7. A lower gas consumption due to the higher percentage of oxygen in
the mix. Again, I haven't noticed any difference. It may be just
coinicidence but this friend also insists he gets better consumption.
8. The effects of a barotrauma may be reduced. This is supposition
based on improved circulation due to high blood oxygenation and lower
nitrogen level implying fewer nitrogen bubbles. This sounds plausible
but I don't know of any research evidence to support this claim.
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 a
situation where the mix in your cylinder is not what you hoped for then
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 will probably die.
--------------------------------------------------------------------
Subject: [6] What are the limitations or problems of using nitrox?
There are a few limitations when using nitrox, however, these are not,
IMHO, justifications for banning the use of nitrox.
1. Concrete maximum operating depth (MOD) and risk of acute/CNS oxygen
toxicity (see questions [8 - 10]). However, 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. There are two advantages to
having nitrogen narcosis as your governing factor; the first is that it
gradually increases with depth and up to a point you can take action to
reduce it (ie ascend), the second is that if deep water blackout occurs
the victim's regulator tends to stay in place. However, if you continue
down to depths in excess of 200 fsw (66 msw) on air you carry a
significant risk of an acute O2 attack.
On nitrox, the risk of an acute O2 attack may be the same or higher
than the risk from nitrogen narcosis at certain depths. Remember that
reduced nitrogen narcosis is one of the benefits. 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 ATA - 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 ATA 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 (see question [13]).
2. For those doing advanced (read "deep") diving who wish to use nitrox
as a travel and/or decompression gas there is the added complexity of
carrying multiple cylinders and doing gas switches. However, this is
nothing new amongst that fraternity. Pure O2 decompression has been
used for quite some time and the risk of switching to pure O2 at the
wrong depth is probably higher than switching to a nitrox mix. If it is
new then extensive training, mental conditioning and preparation is in
order (see references 7 and 8). The comment above about not convulsing
instantly does give those who make mistakes a chance, so once you've
switched it may be worth a second check :-)
3. Those who do not go beyond NDL limits should be aware of the
predisposing factors to DCS (see any of the references). Every diver
should be aware of those factors but with extended bottom times it is
important to keep in mind that cold and dehydration are predisposing
factors. The following example, using the Buhlmann tables, illustrates
what is possible.
On air the NDL for 21msw (69 fsw) is 35 mins.
On nitrox60 (40% O2) the EAD = 13.5 msw (45 fsw), so we use the
15 msw (50 fsw) table. The NDL is 75 mins.
4. Equipment cleaning is a consideration but should not worry the
nitrox diver in the water. It may be a consideration during dive
planning. This is covered in questions [15 - 18].
5. Handling pure oxygen, during the making of nitrox, is obviously an
increased risk over standard compressed air. But if properly trained
people do the mixing using the appropriate equipment, then the risks
are minimised. People have been making nitrox for years, the use of
nitrox dates back to the beginning of the century (Draeger-Werk circa
1912 using 40% N2/60% O2). If you are not properly trained, don't do
it.
--------------------------------------------------------------------
Subject: [7] 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 * AP
where:
PO2 is the partial pressure
fO2 is the fraction of oxygen in the gas
AP 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 1.45 ATA,
then the fO2 is:
PO2 1.45
fO2 = --- = ---- = 0.39
AP 3.7
Thus, to obtain the maximum benefit from the use of nitrox, you'd
choose nitrox61, a mix with 39% O2 and 61% N2.
--------------------------------------------------------------------
Subject: [8] 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.
See questions [9 - 11].
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 70 msw (230 fsw) and whatever decompression
that you will be compelled to do? Standard sport diving tables don't
even go that far. My Buhlmann air tables only go to 21 minutes at 60msw
(200 fsw). The DCIEM tables go to 40 min. at 72msw (236 fsw) and the
decompression penalty for that is 286 min. on air (almost 5 hours) or
129 min. (over 2 hours) if the last stop is done on pure O2. By my
reckoning you'd be carrying at least 4 cylinders and I'd put this dive
well beyond even your adventurous sport diver. It is probably more
likely that you'll convulse in a chamber after getting bent.
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):
At a depth = 30 msw (100 fsw); the PO2 = 1.6 ATA and the EAD = 20 msw
(66 fsw). The NDL is around 35 min. 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 min. 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. See question [11].
At a depth = 40 msw (130 fsw); the PO2 = 2.0 ATA. 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. See question [10].
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.
--------------------------------------------------------------------
Subject: [9] 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 (see question [10]). Pulmonary toxicity is caused by longer
term exposures to moderate oxygen partial pressures and leads to
pulmonary problems (see question [11]).
--------------------------------------------------------------------
Subject: [10] What is CNS oxygen toxicity?
Central nervous system (CNS) toxicity, aka acute oxygen toxicity or the
Paul Bert Effect (who researched this problem in the 1870s), 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.
1. Pre-tonic build-up or pre-tonic premonition. This is the lead up to
convulsions and may or may not be present or noticed. The symptoms are
very similar to DCS and may include muscle twitching, nausea, hearing
problems, tunnel vision, light headedness and breathing problems. If
the symptoms do appear they may be followed so quickly by the next
phase that the victim has no time to deal with the problem. It is also
unlikely that the buddy will notice them either with all that diving
gear (could you spot if your buddy's pupils were dilated?), especially
if the victim is wearing a hood.
2. Tonic or rigid phase. The body goes completely rigid and the victim
will stop breathing and lose consciousness. This phase may last from 30
seconds to 2 minutes. If the victim is taken to a shallower depth
during this phase he may suffer lung barotrauma.
3. Convulsions. The victim is still unconscious but breathing
restarts. It is at this point that the risk of drowning becomes a
serious problem. As the body relaxes the victim's regulator could fall
out of his mouth.
4. Post convulsive depression. The victim is still unconscious and
breathing may be rapid and heavy. This can last anywhere from 5 to 30
minutes.
5. Conscious recovery. During this phase the victim may suffer amnesia,
exhaustion, confusion and lethargy.
CNS oxygen toxicity should be avoided at all costs. Your chance of
survival is minimal at best.
--------------------------------------------------------------------
Subject: [11] What is pulmonary oxygen toxicity?
Pulmonary oxygen toxicity goes by a variety of names; chronic toxicity,
whole body toxicity or the Lorrain Smith Effect (see question [12]).
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 (see
question [10]). 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 ATA for several days you will probably want to speak to a
doctor. The oxygen clock (see question [12]) is used to track
pulmonary oxygen toxicity. Taking air breaks can reduce the risk to
virtually nil. The following table gives the NOAA normal exposure
limits.
_______________________________
| | | |
| PO2 | Max single | Max total |
| | exposure |exposure in |
| | duration |any 24 hours|
|_____|____________|____________|
| | | | | |
| ATA | 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 | 3.5 | 180 | 3 |
| 1.5 | 120 | 2 | 180 | 3 |
| 1.6 | 45 | 0.75| 150 | 2.5 |
|_______________________________|
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Subject: [12] 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 ATA 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. See
question [11].
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 after one of the original researchers). 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)/0.5 ]^0.83
where:
t is the exposure time in minutes
PO2 is the partial pressure of oxygen in ATA
0.5 is the threshold below which no significant pulmonary oxygen
toxicity has been observed.
0.83 is the exponent which gives the best fit to experimental
observations.
However, very roughly, 1 OTU is equivalent to 1 ATA exposure per
minute.
_____________________________
| | | |
| Period | Dose/day | Total |
| (days) | (units) | (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 for 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 ATA for long periods, especially if they are
doing repetitive diving. It would be in their interest to track the OTU
build-up.
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Subject: [13] 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 ATA,
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 ATA. My
own organisation recommends 1.45 ATA.
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 - 1.45 | 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 conditions |
| | (ie DCS or gas gangrene) |
---------------------------------------------------------------
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Subject: [14] Will the oxygen and nitrogen stratify in the cylinder?
It would appear not. Some (influential) 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 (see question [3]).
Of course you should still analyse the mix after the fill and again
before you dive you.
--------------------------------------------------------------------
Subject: [15] What are the oxygen considerations as regards equipment
and 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 is made oxygen clean (see question [16])
prior to use. The equipment should also be oxygen compatible (see
question [17]).
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 (see
question [18]).
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.
--------------------------------------------------------------------
Subject: [16] 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 (see question [17]), but that contaminates
which react violently with pure oxygen have been removed from the
equipment.
--------------------------------------------------------------------
Subject: [17] 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 (see question [16]).
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Subject: [18] What does oxygen service mean?
Oxygen service refers to equipment that is both oxygen compatible (see
question [17] and oxygen clean (see question [16]). 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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Subject: [19] Where can I get further information?
The following references should provide enough information for a
good background education on nitrox. Of course, you will still need
training before you go dive with the stuff.
1. NOAA Diving Manual: Diving for Science and Technology
US Dept of Commerce
National Oceanic and Atmospheric Administration,
Oceanic and Atmospheric Research,
Office of Undersea Research, 1991
Best Publishing Company
ISBN: 0-16-035939-2
Arguably the best general reference on diving. Much of the material
you see in other diving books and the two nitrox manuals named here
is based on the material in the NOAA manual.
2. Nitrox Manual
A Guide to the Applied use of Enriched Air Mixtures for Diving
IANTD
The International Association of Nitrox and Technical Divers (IANTD)
was formed in 1985 by Dick Rutkowski as the International Association
of Nitrox Divers (IAND). The T was added in 1992 when the European
Association of Technical Divers merged with IAND. This was the first
organisation to offer international training to recreational scuba
divers.
US Address: 490 Caribbean Drive, Key Largo, FL 33037.
UK Address: Neighbourne Cottage, Neighbourne, Somerset, BA3 5BQ
The information presented here is mostly an enhancement of the NOAA
manual. One main difference is the use of the Buhlmann tables instead
of the USN tables used in the NOAA manual. If you include the
technical nitrox section I'd say this was more geared towards the
aspiring technical diver than your average recreational diver. But
don't let that put you off if you don't think you are a techie.
3. The Application of Enriched Air Mixtures
ANDI
American Nitrox Divers Inc. (ANDI) was formed by Edward Betts in 1987
or 1988 (depending on which page you read in the manual). Very similar
to the IANTD material but possibly slightly more commercial - aimed at
the mass recreational market (using terms such as SafeAir). As with
the IANTD manual, the information presented here is an enhancement of
the NOAA manual. ANDI still use the USN tables.
Address: 74 Woodcleft Avenue, NY 11520
4. Oxygen and the Diver
Kenneth Donald
The SPA, 1992
ISBN: 1-85421-176-5
An excellent book on oxygen and its dangers to the diver. Based on the
work of the author over the past 50 years. It is probably one of the
few references here which does not draw heavily from NOAA. Includes a
lot of work which you don't see in the many US diving books.
5. Deeper Into Diving
John Lippmann
J L Publications, 1990
ISBN: 0-9590306-3-8
Another excellent book. Contains a plethora of information on the
physiological dangers facing divers and some useful information on
various decompression tables. Read it and weep - how come diving is
so detrimental to us? :-)
6. The Essentials of Deeper Sport Diving
John Lippmann
Aqua Quest Publications Inc., 1992
ISBN: 0-9623389-3-1
Much the same sort of information as the weightier volume of reference
3, but not as detailed. This book is aimed at the more casual reader
but it still contains a great deal of useful information.
7. Deep Diving: An Advanced Guide to Physiology Procedures and Systems
Gilliam, Von Maier, Crea, Webb
Watersport Books, 1992
ISBN: 0-922769-30-3
This is a good read. There's very little new here on the technical
front if you've seen the NOAA manual or Deeper Into Diving but it
contains much useful anecdotal comment from the vastly experienced
authors.
8. Mixed Gas Diving: The Ultimate Challenge For Technical diving
Mount, Gilliam, et al
Watersport Books, 1992
ISBN: 0-922769-41-9
The same type of book as reference 7 and the same comments apply.
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Subject: [20] 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.
The standard colour markings are: a yellow body with a 4" wide green
band near the top, the green band may include the neck portion of the
cylinder. The cylinder should also be tagged with the nitrogen and
oxygen percentages. As an added safety assurance you may also want to
mark it with the MOD, the fill pressure and the fill date. Appropriate
labels, tags and stickers are available from ANDI. Any shop which
supplies nitrox fills should also have these markers and should insist
on adequate markings on the cylinder.
_
| |___
/XXX\ /\ <- Contents tag
4" green -> |XXXXX|\/
band | |
| N |
| I <---- Nitrox label
| T |
| R | <- Yellow body
| O |
| X |
|_____|
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Subject: [21] What's in the cylinder?
Due to the danger of exceeding the maximum operating depth, nitrox
fills should always be checked after filling and again before
diving with the cylinder. You should never dive without being
absolutely sure what is in your cylinder.
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Subject: [22] Should I use nitrox for decompression?
Use of nitrox in preference to air is advantageous. It may be used to
reduce the length of the decompression penalty by following a nitrox
decompression schedule, or it may be used as padding to increase the
safety of the decompression by following an air schedule. This is
similar to the use of pure oxygen during decompression.
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Subject: [23] 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
ATA). 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 occurences 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 ATA 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 ATA there is no risk as the
decompression rate is carefully controlled according to a well defined
schedule.
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Subject: [24] What effect does the CO2 content have?
As yet there is no conclusive evidence that the CO2 level contributes
to hyperbaric acute O2 toxicity in hyperoxic nitrox mixes. Extensive
tests were carried out in the 1940's and 1950's and compared divers,
non-divers and ex-divers in the same test scenarios. The researchers
tried to find a means of identifying CO2 retainers and to find out if
divers build up a tolerance to CO2. Nothing conclusive was found and
this area requires further study, but it is highly unlikely to affect
recreational SCUBA nitrox diving. Nitrox closed circuit rebreather
designers will have to address this issue in order ensure sufficient
expired CO2 absorption.
There are some individuals whose breathing is regulated by oxygen
levels rather than CO2, although most of these individuals probably
shouldn't be diving anyway. This is known as "hypoxic drive", and often
occurs in COPD (chronic obstructive pulmonary disease, ie emphysema)
patients. The body adjusts to chronic elevated CO2 levels by ignoring
that as a breathing stimulus, at which point the body's normal "backup"
stimulus - lowered O2 levels - takes over as the primary means of
regulating respiration. This, in itself, is not really a problem,
although some of these people may stop breathing if exposed to high
concentrations of oxygen.
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Alan Wright (awright@gssec.bt.co.uk)