Over the past year of moonlighting as a part-time dive instructor, the Editor has assisted fellow instructor friends with various courses. Though the teaching gigs have been sporadic, one thing that keeps coming up is the astonishing number of misconceptions students and guests have about nitrox. Let's run through some of the most common nitrox urban legends we keep hearing.
The Differences Between SDI Computer Nitrox / TDI Nitrox / TDI Advanced Nitrox Courses:
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I already have a dive cert — why do I need to add a nitrox cert? Surely that's just instructors bleeding me dry.
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I'm a recreational diver with no interest in technical diving, so I don't need nitrox.
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Using a nitrox tank lets me dive deeper.
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Using a nitrox tank means I use less air, so I can stay underwater longer.
The first and second points above come from divers who haven't yet earned their nitrox certification. Seasoned divers will tell you: if you're planning to go abroad and push greater depths (under the right conditions), without nitrox your dive computer may start beeping at you far too soon (approaching the no-decompression limit). If everyone else in your group is on nitrox while you're breathing air, you'll find yourself slowly heading back to the surface while your buddies are still down there watching marine life — left waiting to see their photos. Using nitrox also shortens your surface interval, which is a huge bonus on multi-dive-per-day international dive trips, making your schedule far more flexible. That's why we strongly recommend every recreational diver get their AOW plus nitrox — with those two certs, you're set to dive virtually anywhere in the world.
Points three and four are good for a chuckle. Anyone repeating these lines has clearly already earned their nitrox cert, yet doesn't seem to fully grasp the physics behind nitrox or its physiological effects on the body. The reason nitrox extends your no-decompression limits underwater is precisely because its oxygen concentration is higher than regular air — hence the name. In simple physical and physiological terms: because nitrox contains less nitrogen, the total amount of nitrogen that dissolves into your body as depth increases is lower than with regular air, which in turn reduces the effects caused by nitrogen accumulation in the body. The most common nitrox mixes in recreational diving are 32% and 36%. Anything above the 21% found in regular air qualifies as enriched air nitrox — and, for the record, the maximum oxygen concentration permitted in recreational diving is 40%.
Each oxygen concentration has a corresponding Maximum Operating Depth (MOD), and dive planning must incorporate the concept of oxygen exposure (OTU/CNS). While nitrox offers many advantages, it also carries real risks and requires a proper understanding. If you're curious about nitrox, reach out to an instructor friend for professional guidance.
A Quick Summary of the Benefits of Using Nitrox in Recreational Diving:
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Breathing nitrox reduces the amount of nitrogen absorbed by the body, extending your no-decompression limits.
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Less nitrogen accumulation means a reduced risk of decompression sickness (DCS).
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Shorter surface intervals and fewer "locked-out" dive computers.
But... everything discussed above is limited to the recreational diving realm. The SDI/TDI system offers progressively advanced courses for different levels of gas use. The following section features a contribution from instructor 丁楓峻 shared in the Facebook group 九潛一深 · Diving 揪潛水, with the hope of giving everyone a more comprehensive understanding of nitrox.
Within the SDI/TDI system, nitrox training 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 deeper than SDI Computer Nitrox on the following topics:
- Physics: pressure and partial pressure
- Dive planning tables: EAD calculations / EANx tables / switching between different gas mixes on repetitive dives
- Gas blending procedures: partial pressure method / continuous flow
TDI Advanced Nitrox, meanwhile, addresses the use of nitrox mixes above 40%, and many instructors design this course to be taken in combination with TDI Decompression Procedures.
The fundamental difference between decompression sickness (DCS) and oxygen toxicity is this: DCS typically manifests at the surface and can cause longer-term injury — though with proper treatment, full recovery is possible. Oxygen toxicity, on the other hand, tends to strike underwater. While it generally causes no lasting damage, the danger lies in the risk of drowning if a convulsion occurs beneath the surface. Managing an emergency underwater is considerably harder than managing one at the surface.
In recreational diving, because depths don't exceed 50m — with 40m as the absolute limit — you set your dive computer and make sure not to exceed MOD, with the remaining focus being on preventing DCS and nitrogen narcosis. TDI Nitrox builds on this by strengthening the calculation and table-lookup component: rather than simply following a dive computer, divers must also become fluent in the Circle T formula conversions. This satisfies the diver's curiosity while also laying the groundwork for the next step — technical diving's Advanced Nitrox and Decompression Procedures courses.
Going deeper brings with it the issue of switching gas sources to accelerate decompression, because there is a very real possibility of switching to the wrong gas at the wrong depth — meaning you could potentially be breathing a gas at a depth that exceeds that cylinder's MOD. This is why decompression cylinders must be clearly labelled with the MOD, and why divers must check their current depth before making any gas switch.
Another key issue is the PPO2 ceiling. In recreational diving, this is typically set at 1.4 ata. In technical diving, however, to accelerate decompression stops, a PPO2 limit of up to 1.6 ata is often permitted — for example, breathing pure oxygen during a stop at 6m. This demands a higher level of neutral buoyancy control from the diver, because ascending to shallower depths to breathe a higher-concentration nitrox mix means the MOD decreases, and you are no longer allowed to descend below that MOD.
This also helps explain why technical divers favour the long-hose/short-hose configuration. Beyond needing to pass single-file through tight spaces while sharing a gas source, there is the added concern of accidentally breathing from the wrong decompression cylinder in an emergency. With a standard octopus / alternate second stage tucked into the chest's triangle zone, a panicking buddy might grab the second stage from your decompression cylinder — and breathe it at a depth that exceeds its MOD. The long-hose setup ensures that what you hand off is exactly the gas you yourself are breathing, at a depth that hasn't exceeded its MOD.
After completing the Decompression Procedures course, if a diver moves on to Extended Range or Semiclosed/Closed-Circuit Rebreather (SCR/CCR) training — meaning they want to go deeper, stay longer, or sustain a higher nitrox partial pressure — multiple different gas mixes will likely come into play, and the diver's total exposure time to elevated oxygen partial pressures will increase significantly. Think of it like taking a higher cumulative dose of medication. At that point, pulmonary oxygen toxicity must be taken far more seriously, and Air Break procedures need to be factored into the dive plan.
In summary:
- SDI Computer Nitrox => Set your mix, don't exceed MOD.
- TDI Nitrox => Circle T formula / EAD calculations and table lookups.
- TDI Advanced Nitrox => Equipment considerations for mixes above 40% nitrox, avoiding MOD exceedance during gas switches, flexible PPO2 limits, neutral buoyancy control skills, pros and cons of the long-hose/short-hose setup, pulmonary oxygen toxicity, Air Break procedures.
And beyond all this lies the even more complex territory of trimix (helium-oxygen-nitrogen) and Extended Range diving...
Cover image credit: Image by Monica Volpin from Pixabay.
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