Is Nitrox Worth It? The Real Bottom Time Gain at 18-30 m

How much longer can you stay down on EAN32 and EAN36?

NOAA's tables give you about 20-50% more no-stop time on EAN32 between 18 and 30 m, and about 50-100% more on EAN36 between 18 and 27 m. The biggest jump is at 18 m (60 ft), where the air limit of 60 minutes becomes 92 minutes on EAN32 and 125 on EAN36.

The reason is nitrogen. Tables and dive computers limit your time by how much nitrogen your tissues take up, and a 32% mix carries 68% nitrogen instead of the 79% in air. DAN puts it this way: 32 percent nitrox at 110 feet is equivalent to air at 90 feet. That shallower equivalent air depth is what the EAD tab of our nitrox calculator works out for any depth and mix.

Tables disagree on the exact minutes. PADI's Recreational Dive Planner gives 56 minutes at 18 m on air and 95 on EAN32, where NOAA gives 60 and 92. Your dive computer runs its own algorithm and conservatism setting, so its numbers will differ again. All of them move the same way: less nitrogen in the tank, later limit.

Does the extra time matter if your gas runs out first?

On a single dive shallower than about 20 m, often not, because the tank runs out before the air limit. Take one example diver: a surface air consumption (SAC) of 15 L/min, a 12 L tank filled to 200 bar and a plan to surface with 50 bar. That leaves 1,800 L of usable gas. Ignoring descent and ascent, the time at depth works out like this:

• 12 m (2.2 bar absolute): 33 L/min, so about 54 minutes of gas against a 163-minute air limit.
• 18 m (2.8 bar): 42 L/min, about 42 minutes against 60 on air.
• 24 m (3.4 bar): 51 L/min, about 35 minutes against 39 on air and 48 on EAN32.
• 30 m (4.0 bar): 60 L/min, about 30 minutes against 25 on air and 30 on EAN32.

Where nitrox pays: 24-30 m and the second dive of the day

At 18 m the example diver never touches the air limit, so EAN32's 92 minutes change nothing. At 24 m the air limit sits only 4 minutes past the gas, which leaves no slack for a slow ascent or a longer look at something. At 30 m, air ends the dive at 25 minutes with gas still in the tank. On EAN32, the tank and the no-stop time run out together.

Repetitive diving is the other place it counts. Nitrogen left over from the morning dive shortens the limit on the next one, and a nitrox diver starts the second dive carrying less of it. PADI's own summary of the course is that breathing less nitrogen means longer dives and shorter surface intervals. On a boat day with three or four dives at 18-25 m, this is where a nitrox fill earns its price.

Your numbers depend on how you breathe. Run a logged dive through the SAC calculator and your tank into the gas planner before deciding: a diver using 25 L/min gets a very different answer from one using 12.

Does nitrox let you dive deeper?

No. It makes your depth limit shallower, because the extra oxygen becomes toxic sooner. At the usual 1.4 bar ppO₂ limit, the maximum operating depth (MOD) is 56.6 m for air, 33.7 m for EAN32 and 28.8 m for EAN36 with our calculator's conservative rounding. DAN's article gives about the same limits in feet: 112 ft for EAN32 and 95 ft for EAN36.

Below the MOD the risk is central nervous system oxygen toxicity. DAN lists warning signs such as tunnel vision, ringing in the ears, twitching and nausea, and the worst outcome is a convulsion, which underwater can end in drowning. It is rare: in a DAN review of 399 US recreational diving deaths over 10 years, 55 divers were on nitrox and one death was judged likely due to an oxygen toxicity seizure. That diver had filled doubles with EAN32 for a dive that was cancelled, used them weeks later on a dive to about 160 ft (49 m), and had a seizure after ascending to about 130 ft.

NOAA's own EAN36 table marks 30.6 m (100 ft) and deeper as over its 1.40 limit. For dives planned below about 33 m, EAN32 is the wrong gas, not a better one.

Does nitrox reduce fatigue after diving?

There is no reliable proof that it does. DAN's answer to exactly this question was that objective studies have not found a reliable difference in fatigue or cognitive performance after dives on air versus nitrox, and that the placebo effect should not be underestimated. The individual studies point in different directions:

• Harris et al. (2003): 11 divers, double-blind and randomized, 40 minutes at 18 m in a dry chamber on air or EAN36, with two exercise periods. No measurable difference in fatigue, attention or concentration.
• Lafère et al. (2010): 219 divers on open-water dives to about 21 m for about 43 minutes, 121 on air and 98 on EAN32. The nitrox group reported a smaller rise in fatigue, and a flicker-fusion test of alertness fell 6% after air dives but rose 4% after nitrox. The abstract does not say how divers were assigned to each gas.
• Brebeck et al. (2017): 108 divers randomized to air or EAN28 for a 60-minute dive to 24 m. The nitrox group made fewer mistakes on the long-term memory test only; the authors called the effect moderate.
• Germonpré et al. (2017): 8 volunteers, 20-minute dry chamber dives at 0.4 MPa (about 30 m). Test scores were better on nitrox, yet the divers could not tell which gas they had breathed.

Is nitrox safer than air?

Only when you dive it on air limits. Breathing nitrox on a profile planned for air leaves you with less nitrogen than the plan assumed. In a double-blind crossover chamber study, 12 divers picked because they produce many bubbles did a 55-minute dive to 28 m on air and on EAN36. The median cumulative bubble score was 1 on nitrox against 8 on air, and three divers had skin itching after the air dive against none after nitrox.

If you spend the gain on longer dives instead, you are back at the edge of the no-stop model with an oxygen limit added on top. NOAA's scientific diving manual allows nitrox as a margin on air tables but says the mix's MOD, oxygen exposure and time limits still apply. DAN makes the same point from the other side: less nitrogen lowers the decompression sickness risk of a given dive, but does not remove it.

What does nitrox cost?

In the US, expect around $230 for the course and $2-9 more per fill than air, going by the prices listed by the shops cited here. Blue Planet Scuba in Washington, DC lists its Enriched Air Diver course at $232. According to PADI, the course takes a few hours and can be completed without any dives, although many dive centres add two open-water dives. You need to be at least 12 and hold an Open Water certification.

Over a week of 12 boat dives, the fill premium at these three shops adds up to $24-108. Resorts and liveaboards set their own nitrox prices, so ask before you book.

EAN32 or EAN36?

Take EAN32 for dives that may reach 30 m, and EAN36 when the whole dive stays shallower than about 27 m. EAN36 gives the bigger gain (125 minutes at 18 m on NOAA's table against 92 for EAN32), but its MOD at 1.4 bar is 28.8 m. On a wall that drops past 30 m, that leaves you very little room.

Availability decides a lot of this. Aquarius Dive Shop in Monterey fills pre-blended 32% into ordinary tanks, while custom blends above 32% need an oxygen-cleaned tank and at least 24 hours' notice. Where a shop blends by adding pure oxygen, the tank has to be ready for it: East Coast Divers requires cylinders certified and stamped 'Over 40%'. To find the richest mix for a planned depth, use the Best Mix tab in the nitrox calculator.

When is nitrox not worth it?

When the dive is shallow, deeper than the mix allows, or you have no way to check the gas yourself.

• Shallow dives. At 12 m NOAA's air limit is 163 minutes and at 15 m it is 92. A 12 L tank is empty long before either.
• Dives below the MOD. Past 33.7 m, EAN32 is over 1.4 bar; past 28.8 m, EAN36 is. Air, or a leaner mix, is the right gas there.
• Sites where depth is hard to hold. A drop-off with no clear bottom makes an accidental MOD overrun easy, and DAN lists unplanned depth excursions and hard exertion among the things that raise oxygen toxicity risk.
• No analyser at the fill station. NOAA's scientific diving rules make each diver analyse their own cylinder before the dive and sign for the oxygen percentage and MOD, and DAN says every diver should analyse every cylinder. A fill you cannot verify is a guess.
• Fatigue as the only reason. The studies above do not support paying for the course on that alone.