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Okay, here it is. I think I would have preferred a private review
rather than a public flogging, but what the hell :-)
Going on the flimsy supposition that the technical content is okay
I know there are a few simplifications, for example, suggesting that
UPTD and CPTD are exactly the same when in reality the CPTD is all the
daily UPTD's added together - is this sort of detail really relevant?
To answer to wrolf:
> As a FAQ, it might be more useful to people on rec.scuba - by definition,
> this list is for people who are already at least interested in technical
> diving.
I agree that we don't need this as an FAQ here. I wrote it to use locally
as an introduction to the subject - the TV at Christmas was pish :-( I
always find it easier to digest information when it is presented in the
small chunks of an FAQ than trawl through an article sprawling over many
pages.
As far making an FAQ on rec.scuba - I'd be happy to submit it there if
it gets a good reception here. I guess it would be up to Nick whether
it gets absorbed into the FAQ or whether it should remain independent.
At the moment there isn't really a lot of nitrox traffic on rec.scuba
so it may be a little premature. I can certainly post it there.
So here you are ...
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@gs*.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?
Possible questions TBD:
[20] Should I use nitrox for decompression?
[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.
--------------------------------------------------------------------
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|
|_______________________________________________|
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