Intermediate 9 minutes
Air and nitrox: what the extra oxygen actually buys you
More decompression margin in exchange for less depth, and none of the other advantages it's credited with
Nitrox has a reputation as "souped-up air," and that phrase sums up pretty much everything people get wrong about it. It isn't air on steroids: it's a trade-off, and like any trade-off, it's paid for with something.
In this article you'll see exactly what an enriched-air mix is, how much decompression margin it adds over air, what it costs in terms of maximum depth, which advantages are credited to it without real basis, and what the two checks are that concentrate all of its real risk.
Nitrox is the first gas mix nearly every recreational diver encounters, and also the first time gas choice becomes part of dive planning. This article is educational material and doesn't replace the dedicated nitrox course, which is where the procedures are actually practiced.
What it is and what it changes
Nitrox, or enriched air, is simply a mix of nitrogen and oxygen with more oxygen than air contains. The usual proportions are 32% and 36%, though the whole range in between exists, along with custom blends.
The change is arithmetic and direct: more oxygen means less nitrogen. And since nitrogen is the inert gas that dissolves into the tissues and governs decompression (per the mechanism covered in the article on dissolved gases and saturation) breathing less nitrogen at the same depth means accumulating a smaller load.
That's where the real benefit comes from, and it can be used two ways. The first is more no-stop time at a given depth: in the fifteen-to-thirty-meter range, the difference over air is substantial and it's especially noticeable on days with several dives. The second, and probably better, is diving the same profile you would on air but with a larger decompression margin, which is what many experienced divers do with nitrox without using a single extra minute. The concrete calculation behind that extra margin (how you enter an air table with an enriched mix) is worked through with a numerical example in the article on planning with tables.
That extra margin is also the reason many users report less fatigue at the end of a diving day. It's a widespread perception, though studies that have tried to measure it haven't produced conclusive results.
And there's a side effect that is objective: on repetitive dives, the residual nitrogen you carry over from one dive to the next is smaller, so the benefit accumulates over the course of the day, per the logic covered in the article on repetitive diving.
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