During the year the Editor spent moonlighting as a part-time dive instructor, regularly co-teaching alongside fellow instructor friends, the classes were admittedly hit-or-miss — but one thing that came up consistently was students and customers holding some truly baffling misconceptions about enriched air nitrox (EANx). Below, we've rounded up a few of the most common nitrox urban legends we kept hearing.
The Differences Between SDI Computer Nitrox / TDI Nitrox / TDI Advanced Nitrox Courses:
-
"I already have my dive certification — why do I need a separate nitrox cert? This has to be instructors milking students for cash."
-
"I'm a recreational diver, not a technical diver. I don't need nitrox."
-
"Breathing nitrox lets me dive deeper."
-
"Breathing nitrox means I use less gas, so I can stay underwater longer."
The first and second points above come from divers who haven't yet obtained their nitrox certification. As veteran divers often say: if you're heading overseas for deeper dives (under the right conditions), without nitrox your dive computer will be beeping at you in no time — warning you that you're approaching your no-decompression limit (NDL). If everyone else on your dive trip is on nitrox while you're on air, they'll still be down watching marine life while you're already slowly making your way back to the surface, waiting for them to surface and share their photos. Nitrox also shortens your surface interval, which makes scheduling multiple dives a day on an overseas dive trip far more convenient. That's why recreational divers are strongly encouraged to get their Advanced Open Water Diver (PADI cert) plus nitrox certification — with those two, you'll be set to dive just about anywhere in the world.
The third and fourth points are a little amusing. Anyone confidently repeating those claims has almost certainly already earned their nitrox cert — but apparently hasn't quite grasped the physics of nitrox or how it affects the body physiologically. The reason nitrox extends your no-decompression limit underwater is precisely because its oxygen concentration is higher than that of regular air — hence the name "enriched air nitrox (EANx)." Put simply, in terms of physics and physiology: because nitrox contains less nitrogen, as you descend, less nitrogen is absorbed into your body compared to breathing regular air — which in turn reduces the effects of nitrogen accumulation. The most common nitrox mixes for recreational diving are 32% and 36%. Technically, anything above the standard 21% oxygen in air qualifies as nitrox — and for recreational diving, the maximum allowable oxygen concentration is 40%.
Each oxygen concentration has a corresponding Maximum Operating Depth (MOD), and dive planning must incorporate the concept of oxygen exposure (OTU/CNS tracking). While nitrox offers many benefits, it also comes with real risks and requires a solid understanding of how to use it safely. If you're interested in nitrox, we recommend consulting an instructor friend for professional guidance.
A Quick Summary of the Benefits of Using Nitrox in Recreational Diving:
-
Breathing nitrox reduces the amount of nitrogen absorbed by your body, extending your no-decompression limit
-
Reduced nitrogen accumulation lowers the risk of decompression sickness (DCS)
-
Shorter surface intervals and fewer computer lock-outs between dives
But... everything discussed above is limited to the recreational diving realm. The SDI (certification agency)/TDI (certification agency) system offers progressively advanced gas-use courses for different skill levels. In the following section, we've compiled a post shared by instructor 丁楓峻 in the Facebook group 九潛一深 · Diving 揪潛水, hoping to give everyone a more comprehensive understanding of nitrox.
Within the SDI (certification agency)/TDI (certification agency) system, nitrox education is divided into three courses:
- SDI Computer Nitrox
- TDI Nitrox
- TDI Advanced Nitrox
So what exactly are the differences? Comparing the syllabi directly, TDI Nitrox goes noticeably deeper into the following topics than SDI Computer Nitrox:
- Physics: pressure and partial pressure
- Dive planning tables: Equivalent Air Depth (EAD) conversion / EAN tables / gas switching between different nitrox mixes on repetitive dives
- Gas blending procedures: partial pressure method / continuous flow method
TDI Advanced Nitrox, on the other hand, tackles the use of nitrox mixes above 40%, and many instructors design it as a combined course alongside TDI Decompression Procedures.
It's worth understanding the fundamental difference between decompression sickness (DCS) and oxygen toxicity. DCS typically manifests at the surface and can cause long-term injury — though with proper treatment, full recovery is possible. Oxygen toxicity, by contrast, tends to strike underwater. While it is less likely to cause lasting damage, the real danger is that a seizure underwater can lead to drowning. Managing an emergency underwater is far more difficult than at the surface.
In recreational diving, depths don't exceed 50m — with a hard limit of 40m — so dive computers are configured to keep you within your MOD, and the primary concerns are DCS prevention and nitrogen narcosis. TDI Nitrox takes things further by reinforcing the calculation and table-lookup skills: beyond knowing how to set a dive computer, divers must also become proficient with the Circle T formula conversions. This satisfies curious divers' desire to truly understand what's happening — and prepares them for the next step: technical diving's Advanced Nitrox and Decompression Procedures courses.
Going deeper introduces the issue of gas switching to accelerate decompression — because it's entirely possible to switch to the wrong gas at the wrong depth, meaning you could end up breathing a decompression gas at a depth beyond that cylinder's MOD. This is why decompression cylinders must be clearly labelled with their MOD, and divers must always verify depth before switching gases.
Another key topic is the upper PPO₂ limit. In recreational diving, we typically set this at 1.4 ata, but in technical diving, to accelerate decompression stops, a PPO₂ ceiling of 1.6 ata is often permitted — for example, breathing pure oxygen at a 6m stop. This demands much better neutral buoyancy control from the diver, because moving to shallower depths to breathe a higher-concentration nitrox mix means the MOD decreases — and you absolutely cannot descend below that MOD.
This also helps explain why technical divers prefer the long-hose / short-hose configuration. Beyond the obvious need to share gas in a single-file formation through tight spaces, it also prevents a critical mistake in an emergency: accidentally breathing from the wrong decompression cylinder. With a conventional octopus / alternate second stage stowed in the chest triangle, a panicked dive buddy could grab the second stage regulator from your decompression cylinder and breathe it at a depth exceeding its MOD. A long-hose setup ensures that whatever you hand to your buddy is the same gas source you're currently breathing — and one whose MOD you haven't exceeded.
Once you've completed Decompression Procedures and move on to Extended Range or rebreather (SCR/CCR) courses, you're planning to go deeper, stay longer, or breathe nitrox at sustained higher partial pressures. This means you may be using multiple different gas mixes and spending significantly more time exposed to elevated oxygen levels — think of it like taking a higher dosage of medication over a longer period. At that point, pulmonary oxygen toxicity must be taken far more seriously, and Air Break procedures become an essential consideration.
In summary:
- SDI Computer Nitrox ⇒ Set your mix, don't exceed your MOD
- TDI Nitrox ⇒ Circle T formula / EAD conversion and table lookup
- TDI Advanced Nitrox ⇒ Equipment considerations for mixes above 40%, avoiding MOD exceedance during gas switches, flexible PPO₂ limits, neutral buoyancy control skills, pros and cons of long/short hose configurations, pulmonary oxygen toxicity, Air Break procedures
Beyond that, things get even more complex — trimix and Extended Range diving are a whole other rabbit hole to explore...
Header image credit: Image by Monica Volpin from Pixabay.
Related links:
