What nitrox is
Air is roughly 21 percent oxygen and 79 percent nitrogen. Enriched air nitrox, usually shortened to nitrox or EAN, is any breathing gas with more oxygen than air. The number after EAN is the oxygen percentage, so EAN32 is 32 percent oxygen and EAN36 is 36 percent. These two are by far the most common blends at recreational dive centers.
Because every breath of nitrox contains less nitrogen than a breath of air at the same depth, your body takes up nitrogen more slowly. That is the whole point of nitrox for recreational diving: it extends your no-decompression limits.
What it does and what it does not do
Nitrox gives you longer no-decompression time at the same depth, which is most noticeable between about 18 and 30 meters. On a typical dive at 25 meters, the difference between air and EAN32 can be a large share of your bottom time. It can also shorten the surface interval you need before a repetitive dive, which makes it popular on liveaboards with three or four dives a day.
Nitrox does not let you dive deeper. In fact it is the opposite: the extra oxygen lowers the depth you can safely reach. It is not a cure for narcosis in any practical sense, and it does not replace good ascent habits or safety stops. Many divers report feeling less tired after a day of nitrox diving, but that is anecdotal and should not change how you plan.
Oxygen limits and maximum operating depth
Oxygen becomes toxic to the central nervous system when its partial pressure gets too high. Recreational training agencies set a working limit of 1.4 bar and a contingency limit of 1.6 bar. The maximum operating depth, or MOD, is the depth at which your blend reaches that limit.
You can calculate it in meters with a simple formula: MOD = (limit ÷ oxygen fraction − 1) × 10. For EAN32 at 1.4 bar, that is (1.4 ÷ 0.32 − 1) × 10, or about 33 meters. For EAN36 it is about 29 meters. Your dive computer shows the same figure once you set the right mix, and many dive centers write it on the tank label.
Plan every nitrox dive so the deepest point stays shallower than the MOD, with some margin for a descent that goes a little deeper than expected.
Checks before every nitrox dive
Analyze your own tank. Whoever filled it should have checked it, but the diver who breathes the gas is the one responsible for knowing what is inside. Use an oxygen analyzer, read the percentage yourself, and write it on the tank label with the MOD and your initials.
Set your dive computer to the analyzed mix before you get in the water. A computer left on air will be conservative, but one set to a richer mix than you are breathing will give you more no-decompression time than you really have.
Tell your buddy and guide what you are breathing. If you are diving with people on air, you will have different limits on the same dive, so agree on a plan that suits both.
Getting certified
Every major agency offers an enriched air course, usually one of the quickest and most useful specialties you can take. It covers oxygen limits, MOD, analyzing a tank and setting your computer, and most dive centers will not fill a nitrox tank without seeing the card.
In Frey Dive, choose nitrox as the gas on a dive and enter the oxygen percentage. Your logbook keeps the mix with the dive, so you can see later how much of your diving was on enriched air, and Best Mix in the tools suggests the richest blend that suits a planned depth.