Mailing List Archive Mailing List Archive
As many of you will remember I first published this on techdiver about
a year ago. Unfortunately I've been really busy and I've only been
able to support it on an adhoc basis. Many people have said it was
useful and I've had a good bit of feedback. Some are even using it as
a teaching aid.
I'd like to thank all those who took the time to comment on the
document. It was really useful. I'm sorry I can't remember
everyone's name but in particluar I'd like to thank Tracy Baker,
Ralph Landry and Michael Lynch. I'd also thank the person from
Italy who took the time to check the figures - I'm sorry I'm
hopeless with names and I lost all my email records when I moved
from BT.
As I'm going to be away for a few months I thought I'd put it out
again. It has been updated and corrected in a number of places. I
also added a copyleft notice for those who asked about publishing
bits or all of it.
As always comments and corrections are welcome. If you have any
comments from now until April please CC them to:
IRVINE!ENG1!Alan@be*.at*.co*
I may not be able to read my usual address while I'm away.
Hope this is useful.
Alan
----------- Cut here ----------
Nitrox: Questions and Answers
-----------------------------
Nitrox is rapidly becoming a common buzzword 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).
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.
*************************** DISCLAIMER ***************************
If you have any corrections or suggestions please send them to me at
alan@mi*.de*.co*.uk* or IRVINE!ENG1!Alan@be*.at*.co*
Copyright 1994, Alan Wright
Commercial redistribution is allowed and encouraged, provided that the
author is notified of any such distributions and given the opportunity
to provide a more up-to-date version.
Noncommercial redistributions of a verbatim copy in any medium physical
or electronic are permitted without express permission of the author.
Translations are similarly permitted without express permission if they
includes a notice on who translated it.
Short quotes may be used without prior consent by the author.
Derivative work and partial distributions of the document must include
either a verbatim copy of this file or make a verbatim copy of this
file available. If the latter is the case, a pointer to the verbatim
copy must be stated at a clearly visible place.
The intention is to promote dissemination of this information through as
many channels as possible, however, I retain copyright on this document.
If you have questions, please contact me at alan@mi*.de*.co*.uk* or
IRVINE!ENG1!Alan@be*.at*.co*
Last modified: Sun Jan 15 04:09:52 GMT 1995
Search for "Subject [#]" to get to question number # quickly.
Contents:
[1] What is nitrox?
[2] How long has nitrox been around?
[3] How do you name a nitrox mix?
[4] Is air nitrox?
[5] What are nitrox I and nitrox II?
[6] What is equivalent air depth?
[7] Why would you want to use nitrox?
[8] What are the limitations or problems of using nitrox?
[9] How do you know which nitrox mix to use?
[10] Why is there so much talk about oxygen when talking about
[11] What is oxygen toxicity?
[12] What is CNS oxygen toxicity?
[13] What is pulmonary oxygen toxicity?
[14] What is the oxygen clock?
[15] What are the working limits of oxygen?
[16] Is it possible to get an oxygen bend?
[17] What effect does the CO2 content have?
[18] What has nitrox got to do with trimix?
[19] What's in the cylinder?
[20] Will the oxygen and nitrogen stratify in the cylinder?
[21] What are the equipment considerations for oxygen/nitrox?
[22] How are nitrox cylinders identified?
[23] What does oxygen clean mean?
[24] What does oxygen compatible mean?
[25] What does oxygen service mean?
[26] What more is there to know?
[27] Where can I get further information?
Appendix A - Acronyms
--------------------------------------------------------------------
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. 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: [2] How long has nitrox been around?
Quite a while. The early history of nitrox is really the history of
research into oxygen toxicity. The following information is taken
largely from the references given at the end of this document.
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
fatal. 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 (see
references), 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 realized 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 recognized 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 organization 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 organization for recreational nitrox training: American Nitrox
Divers Inc. (ANDI).
--------------------------------------------------------------------
Subject: [3] 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: [4] 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: [5] 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: [6] 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
Where:
fN2 is the fraction of nitrogen in the nitrox mix
0.79 is the fraction of nitrogen in air (including the trace gases)
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)
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 | ATA | msw | fsw | ATA |
|-----+-----+-----+-----+-----+-----+-----+-----|
| 10 | 33 | 13 | 44 | 0.42| 15 | 49 | 0.89|
| 20 | 66 | 25 | 82 | 1.12| 27 | 89 | 1.33|
| 30 | 99 | 37 |121 | 1.49| 40 | 130 | 1.78|
| 40 | 130 | 48 |159 | 1.86| 52 | 171 | 2.22|
| 50 | 165 | 60 |197 | 2.23| 64 | 211 | 2.67|
|_______________________________________________|
This table also shows an important point about CNS oxygen toxicity (CNS
toxicity is discussed later). It is recommended that maximum PO2 is
kept below 1.6 ATA. At 121 fsw on Nitrox I the PO2 is 1.49, this is
getting near the recommended limit and, in fact, you'll find that the
recommended maximum operating depth (MOD) for Nitrox I is 130fsw
(40msw). Similarly the recommended maximum operating depth for Nitrox
II is 100 fsw (30msw).
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.
--------------------------------------------------------------------
Subject: [7] 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.
1. Longer NDL. Because we work to an EAD 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 realized by surfacing according
to the nitrox tables or as an additional safety factor by following the
standard air tables. For anyone in a higher predisposition to DCS
category (older, overweight, poor circulation etc) 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. Again it may be used as
safety padding by following the standard air surface interval.
5. Nitrox provides faster off-gassing than air during decompression. 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 factor by following an air decompression
schedule. This is similar to the use of pure oxygen during
decompression.
On the surface it also provides an alternative to pure O2 in situations
where it is desirable to breath a mix with a higher O2 percentage than
air.
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 may not come back from the dive.
--------------------------------------------------------------------
Subject: [8] 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. 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.
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 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.
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 subsequent questions.
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 minimized. 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: [9] 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:
PO2 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 1.45 ATA,
then the fO2 is:
PO2 1.45
fO2 = --- = ---- = 0.39
P 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: [10] 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. My Buhlmann air tables only go to 21 minutes at 200
fsw (60 msw). 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. 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):
[Note: Oxygen toxicity is explained below.]
At a depth = 100 fsw (30 msw); the PO2 = 1.6 ATA and the EAD = 66 fsw
(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 = 130 fsw (40 msw); 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.
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: [11] 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 | 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 | 2.5 | 180 | 3 |
| 1.5 | 120 | 2 | 180 | 3 |
| 1.6 | 45 | 0.75| 150 | 2.5 |
|_______________________________|
--------------------------------------------------------------------
Subject: [12] 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 characterized
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: [13] 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 ATA 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.
--------------------------------------------------------------------
Subject: [14] 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.
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)/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 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.
--------------------------------------------------------------------
Subject: [15] 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 (Scottish Sub-Aqua Club) 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) |
+---------------------------------------------------------------+
(*) technical Diver, 3.2, October 1992, pp16-19. aquaCorps, PO Box
4243, Key West, Florida 33041.
--------------------------------------------------------------------
Subject: [16] 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 metabolized 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 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.
I've also had a recommendation for the following article:
Lillo, R.S., and MacCallum, M.E., "Decompression Comparison of
N2 and O2 in Rats," Undersea Biomedical Research, [18:4] July
1991, page 317.
--------------------------------------------------------------------
Subject: [17] 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.
--------------------------------------------------------------------
Subject: [18] 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 200 fsw (60 msw) down to around 800 fsw (245 msw). Although
it is perfectly reasonable from a physiological point of view to use
helium on dives shallower than 150 fsw (45 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 800 fsw 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 minimize 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.
--------------------------------------------------------------------
Subject: [19] What's in the cylinder?
Due to the danger of exceeding the maximum operating depth, nitrox fills
should always be checked, using an O2 analyzer, 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 analyzers. Some are more accurate
than others. You should know the tolerance limits of the O2 analyzer
being used and whether or not you need to add a correction to the
measured O2 level when planning your dive.
--------------------------------------------------------------------
Subject: [20] 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. Of
course you should still analyze the mix after the fill and again
before you dive you.
--------------------------------------------------------------------
Subject: [21] 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.
--------------------------------------------------------------------
Subject: [22] 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 standardization:
In the US (CGA)
green for oxygen
black for nitrogen
yellow for air
brown for argon
blue for helium
brown and green for heliox
In Europe and Canada:
white for oxygen
black for nitrogen
black and white for air
brown for helium
In Australia:
black for oxygen (possibly with white on shoulder)
grey for nitrogen
grey with black and white quarters on shoulder for air
brown for helium
brown with white and brown quarters on shoulder for 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 references given below also suggest that the cylinder should 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 and many shops which provide nitrox fills.
_
| |___
/XXX\ /\ <- Contents tag
4" green -> |XXXXX|\/
band | |
| N |
| I <---- Nitrox label
| T |
| R | <- Yellow body
| O |
| X |
|_____|
Many of the nitrox cylinders I have seen in the UK are either all white
or all yellow with nitrox stickers across them. The main thing is to
make it very clear what is in the cylinder.
--------------------------------------------------------------------
Subject: [23] 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.
--------------------------------------------------------------------
Subject: [24] 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.
--------------------------------------------------------------------
Subject: [25] 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.
--------------------------------------------------------------------
Subject: [26] 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 a look at some of the
references below and think about doing one of the available courses.
--------------------------------------------------------------------
Subject: [27] 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
Available from the U.S. Government Printing Office.
Stock no. 003-017-00543-7
Price was $44 inside US, $60 overseas.
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: 9628 N.E. 2nd Ave. - Suite D, Miami Shores, FL 33138-2767
The information presented here is mostly an enhancement of the NOAA
manual. In the US it they use the USN tables, however, in the UK the
Buhlmann tables are used. 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.
9. Technical Diving International
This is a relatively new organization set up by various well known
members of the technical diving community. Although I have requested
information, as yet I haven't received any nor have I seen any of their
literature.
US Address: 9 Coastal Plaza, Suite 300, Bath, Maine 04530
--------------------------------------------
Appendix A - Acronyms
This appendix lists the acronyms used in this FAQ.
AEDU Admiralty Experimental Diving Unit
ANDI American Nitrox Divers Inc.
ATA Atmospheres Absolute
CGA Compressed Gas Association
CNS Central Nervous System
CO2 Carbon Dioxide
CPTD Cumulative Pulmonary Toxic Dose
DCIEM Defence and Civil Institute of Environmental Medicine (Canada)
DCS Decompression Sickness
EAD Equivalent Air Depth
EAN or EANx Enriched Air Nitrox (the x is the percentage of oxygen)
EATD European Association of Technical Divers
FAQ Frequently Asked Questions
FO2 Fraction of Oxygen
FSW Feet of Sea Water
HPNS High Pressure Nervous Syndrome
IAND International Association of Nitrox Divers
IANTD International Association of Nitrox and Technical Divers
IMHO In My Humble Opinion
MOD Maximum Operating Depth
MSW Metres of Sea Water
N2 Nitrogen
NDL No Decompression Limit
NOAA National Oceanic and Atmospheric Administration
O2 Oxygen
ONM Oxygen-Nitrogen Mixture
OTU Oxygen Tolerance Unit
P Pressure
PO2 Partial pressure of oxygen
SCUBA Self-Contained Underwater Breathing Apparatus
TDI Technical Diving International
UK United Kingdom
UPTD Units of Pulmonary Toxic Dose
US United States
--------------------------------------------------------------------
Alan Wright
alan@mi*.de*.co*.uk*
IRVINE!ENG1!Alan@be*.at*.co*
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