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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 now, 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 = good for you, right?
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 (and I use that word cautiously!) 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 on the Rosalie Moller 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 a recognised agency 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 (Qualifier: 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.
PADI 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 I planned a dive to 20m on EAN36, my nitrogen absorption would be equivalent to me diving at 14.3m if I was on air, so I could use my air dive planner based on a 16m dive (this obviously increases the safety factor as well).
and must not be used as a substitute for training in using Enriched Air.