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Before diving with
Enriched
Air get the
appropriate training, the probability of killing yourself WILL increase
dramatically without it. The SAA run a Nitrox course throughout the
various Regions. Please check the downloads page for more information
on the course.
Nitrox,
so what's the big deal about it?
Strange
how that tends to be the first question and not, "What is it?"
Nitrox
is just a name that has been applied to a gas mix of oxygen and
nitrogen... you're breathing it as you read this article, Enriched Air
Nitrox (EANx) is what
you get when you use a gas blending technique to fill your cylinder.
There's more to it than that (not a lot though) but I'll try and keep
it relatively simple for now.
When divers refer to Nitrox however they are normally talking about an
oxygen enriched mix so EAN32 would contain 32% oxygen and 68% nitrogen.
More
oxygen = less nitrogen = thats got to be better for you?
Sort
of, as we already know (through school or from when we first
learnt to dive) the very element that keeps us alive, oxygen, can also
kill us at depth; this isn't something that concerns
normal recreational divers on air, because the depth at
which oxygen toxicity affects us starts at 56m, and of course nobody
reading this would ever consider diving beyond their certified level.
Now
we're going to add more oxygen to our cylinder which means our
bodies will be exposed to more and therefore we need to calculate what
our maximum depth is before we face oxygen toxicity e.g. with EAN40%
the risks start at 25m so you wouldn't be planning a dive in the
deepest part of Stoney Cove with that would you?
There
is also a limit to how much oxygen exposure our bodies can
tolerate in a 24 hour period and when you do the course through the SAA
you'll get the corresponding tables for this as well.
Now,
that's the scary stuff out of the way, what are the benefits?
Longer
bottom time (NB: this does NOT mean that a cylinder will
last longer, you will still consume the same quantity of gas over the
same period of time) and an added safety factor because our bodies are
absorbing less nitrogen (although as the books say, this has not been
statistically proven).
When
we plan dives on air we're calculating our exposure to nitrogen
because that is the primary limiting factor to our dive time; it is the
same with EAN but because we have less nitrogen to worry about we can
stay down longer.
What
do we need to calculate?
First
we need to know the depths at which oxygen toxicity will affect
us and to bring in some new terminology.
Air
breathed at 1 atmosphere
on the surface consists of approximately 0.21 oxygen and 0.79 nitrogen,
the component parts of the total gas mix are said to have a Partial
Pressure (PP), i.e. 0.21ata and 0.79ata respectively.
At
10m we breathe at 2 atmospheres so the PP's of oxygen and nitrogen
are 0.42ata and 1.58ata, at 20m they are 0.63ata and 2.37ata and so on.
The
diving agencies will teach you that a Maximum Operating Depth is
reached when the PP of oxygen reaches 1.4ata or 1.6ata, why the
difference? To be honest I can't give an exact answer to this one, it
is generally considered that for a working dive you calculate your MOD
at 1.4ata for resting or deco it is calculated at 1.6ata.
SAA
consider the maximum depth is when the PP reaches 1.4ata and that
you have a contingency depth up to the point where the PP reaches
1.6ata.
**Hint**
- When talking to somebody else
about a Nitrox
mix, planning a dive or getting a fill you'll often be talking about
the MOD, don't confuse the issue with maximum and contingency depths
stick to the figures.
Secondly
we need to know how great our total oxygen exposure is going
to be during a 24 hour period so even if we aren't planning to go to
our MOD we need to know what the PP of oxygen is going to be during
each dive.
Finally,
to plan our dive effectively we need to know our Equivalent
Air Depth, our bodies absorb less nitrogen with enriched air but we
still have to plan our dives according to our exposure and without
having a table for every mix we can plan our dives using our 'normal'
table.
For
example: If you planned a dive to 20m on EAN36, your nitrogen
absorption would be equivalent to you diving at 14.3m if you were on
air,
so you could use your air dive planner based on a 16m dive (this
obviously
increases the safety factor as well).
***This
calculator is for information only***
and must not be used as a substitute for training in using Enriched Air.
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