Nitrox is often sold as a way to stay underwater longer. Sometimes it does exactly that. But if you normally end dives because your cylinder is approaching its planned reserve, Nitrox won't give you more gas. I use it on most recreational dives because reducing nitrogen exposure is worthwhile to me, provided the mixture suits the planned depth.
What Nitrox Actually Changes
Ordinary air contains about 21 percent oxygen and 79 percent other gases, mostly nitrogen. Enriched air Nitrox replaces some of that nitrogen with oxygen. The common blends EAN32 and EAN36 contain 32 and 36 percent oxygen, respectively.
Breathing less nitrogen at a given depth reduces the pressure driving nitrogen into your tissues. That can extend your no-stop time, especially on repetitive dives. It does not increase the amount of breathing gas in your cylinder. If gas supply, rather than your no-stop limit, determines when you ascend, Nitrox may not extend the dive at all.
The difference between nitrogen exposure and no-stop time matters. You can follow the same profile you would have on air and take advantage of the lower nitrogen exposure, or use the extra no-stop allowance to remain underwater longer. Both are reasonable choices, but they're not the same benefit. Equivalent Air Depth explains the nitrogen-pressure relationship if you're interested in the mathematics.
Why I Choose It
My first Nitrox dives were in Cozumel. I didn't feel more energetic on the boat afterward, and the dives themselves felt no different. What I got was less nitrogen exposure at a modest additional cost, which seemed a reasonable trade.
EAN32 is usually my choice for recreational diving. It gives me less nitrogen than air while accommodating much of the diving I do. I wouldn't select it automatically for a dive that might exceed its maximum operating depth, though. More oxygen brings an important limitation.
The Depth Limit Matters
The oxygen percentage in your cylinder stays the same throughout the dive, but the pressure of the gas you breathe increases as you descend. So does oxygen's partial pressure, or PO₂. Too much oxygen pressure can cause central nervous system oxygen toxicity, including an underwater seizure without reliable warning.
A mixture's maximum operating depth (MOD) is the depth where its oxygen partial pressure reaches your selected limit. A commonly used recreational planning limit is 1.4 ATA. At that limit, EAN32 reaches approximately 111 feet of seawater; EAN36 reaches approximately 95 feet. Those are oxygen limits, not invitations to dive that deep or promises of safety at the boundary.
I plan to stay shallower than my MOD, accounting for depth-control difficulty and conditions. For the calculation, see Calculating Maximum Operating Depth or use the Nitrox MOD Calculator. You need to understand why a richer blend has a shallower MOD even if your computer does the arithmetic.
Know What's in Your Cylinder
Nitrox requires appropriate training and one habit I don't compromise on: analyze the cylinder you're actually going to breathe. A label reading EAN32 tells you what somebody thinks is inside, not what your analyzer will measure.
Use a properly calibrated oxygen analyzer, confirm the measured percentage, and label or record the mixture and MOD according to your training and the operator's procedure. Before entering the water, make sure your dive computer has the analyzed oxygen percentage and the planned PO₂ limit. Yesterday's EAN36 setting is wrong for today's EAN32.
An incorrect computer setting can mislead you about no-stop time and oxygen exposure. I want the computer to monitor a depth limit I've already established, not introduce that limit through an underwater alarm.
For my diving, Nitrox is usually worth the extra cost when it's available and appropriate. On a trip I've already spent thousands of dollars to take, saving a few dollars per cylinder is not much of an argument against reducing nitrogen exposure. It doesn't replace gas planning, and it doesn't make an unsuitable mixture safe.