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February 2017    Download the Entire Issue (PDF) Available to the Public Vol. 32, No. 2   RSS Feed for Undercurrent Issues
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That Devil Gas Is Not So Dangerous!

putting the myths to rest

from the February, 2017 issue of Undercurrent   Subscribe Now

A couple of subscribers took issue with my exhortation for everyone to start using nitrox. It obviously touched a nerve, reawakening old arguments put forward in the early '90s by the editor of Skin Diver Magazine, Bill Gleason, when he vowed that nitrox was a devil gas and he would never use it.

Long-time subscriber and diver, Chet Heddon, wrote to say that the additional time gained is "not supported by any evidence. Slightly more time, perhaps, but certainly not 'far more' time."

Well, the no-stop time on my Suunto computer, set without any cautionary personal adjustment, is 37 minutes with nitrox 21 (air) at 68 feet (21m) against 58 minutes with nitrox 32. I'd say an extra 21 minutes' dive time is significant. The relationship expands as you go deeper.

Chet also wrote that my contention that nitrox is safer than air is simply not true. "It is more dangerous than air, because it introduces oxygen toxicity and depth limitations."

(This was echoed in another email, which Bret Gilliam, founder of training agencies SDI/TDI and past president of computer manufacturer, UWATEC answers next.)

One must keep in mind that air divers are breathing nitrox 21. That is what air is, and as such, is similarly subject to oxygen toxicity and depth limitations -- currently suggested by many diving computer manufacturers to be 182 feet (56m), as opposed to nitrox 32, where the suggested depth limit is around 130 feet (40m). That many of us older divers occasionally went deeper on air simply demonstrates that exposure to oxygen toxicity is a product of both time and depth (pressure).

Larry wrote:

"I just read your article Get off the Air, Will You? I am disappointed with the very biased statement, 'First, nitrox is safer, much safer, especially for an aging diver.' Clearly this is one-sided reporting."

"Yes, from a nitrogen-loading standpoint, the statement is 100 percent correct. However, from an oxygen exposure standpoint, the opposite is true. As a long-time PADI scuba instructor, I agree with the article about PADI's 'dry' Nitrox course being inadequate. But a significant number of divers I have encountered who dive Nitrox, PADI certified or otherwise, are either unaware or unknowledgeable of the dangers of oxygen. I have witnessed several potentially dangerous situations because the diver is not taking the oxygen exposure or maximum operating depth of the mix into account. Further, other certified Nitrox divers aboard boats have seemed surprised that this is something they had to be aware of."

"Undercurrent should not make statements that are based on half of the facts."

Dear Larry,

First, let's address the issue of whether nitrox is inherently 'safer' than air as a breathing mix. The short answer is no.

Decompression models produce exactly the same risk factor for DCS for both gasses, since the algorithm assumes the uptake of the inert gas nitrogen to be at the same rate because the model is derived from the existing underlying rates of uptake and release. Yes, nitrox will allow longer nodecompression times due to the increased oxygen/ reduced nitrogen content ... but the overall DCS risk factors are the same.

That only changes if you were to use 32 O2 percentage (for example) but set your computer to normoxic air at 21% O2. This is often referred to as the 'physiological advantage' in practical usage. This effectively eliminates any real statistical possibility of DCS.

Nitrox, however, has significant advantages for longer dive times with no decompression and shorter surface intervals allowing faster returns to diving. Overall, nitrox is simply more efficient, but the DCS risk models are the same.

As for the risks of oxygen toxicity, based on oxygen dosage, they are identical between air and nitrox. O2 toxicity is a phenomenon based on time of exposure and partial pressure. These two factors yield the 'dose.' The normal working ?O2 partial pressure limit is 1.6ATA (bar), which is achieved on air at a depth of 218.4 feet (67.2) and on Nitrox 32 at 132 feet (40.6m). The NOAA recommended maximum time at a 1.6 ?O2 is 45 minutes. The risk factors are the same regardless of whether you are breathing nitrox or air.

Comic for the Nitrox article

The old Navy dive table no-decompression time for Nitrox 32 was 20 minutes. So with an O2 dosage time limit of 45 minutes (more than double the no-deco bottom time), there is very little likelihood of any oxygen toxicity problems. If you were to go deeper and hit a PO2 of 1.8, you would simply have a reduced exposure time of 30 minutes. Many people who don't understand the applicable physics and working physiology seem to assume that if you drop below 132 feet (40.6m) on Nitrox 32, that you'll suddenly experience spontaneous combustion. It's just not true. It's all about depth and time that produce the oxygen 'dose.' It's not just depth.

One reason that Nitrox 32 evolved into the most widely used mix was that it matched up perfectly to the 130-foot (40m) maximum depth limit recommended for most sport divers. And it also equaled an oxygen exposure at 1.6 ATA (bar) that was also in widespread acceptance. It's worth noting that the NOAA oxygen dose tables have been around for nearly five decades, and I'm not aware of a single case of O2 toxicity when divers have stayed within those limits ... regardless of the breathing mix. The protocols work.

[Editor's Note: If you don't dive deeper than 130 feet (40m) and never use a nitrox mix greater than 32 percent O2, a single tank diver would have nothing to worry about regarding oxygen toxicity!]

So stop worrying needlessly about an artificial O2 threat. Stay within no-decompression limits, and you automatically will not violate the O2 dose limits ... with a safety factor of over 50%.

Now, about those divers who are either "unaware or unknowledgeable of the dangers of oxygen," either they were poorly trained or forgot what they were taught, just as beginning divers forget things. That's why they are required to sign off on the nitrox O2 content and maximum depth limits and why an operator should reinforce the depth limits in the briefing. If that's done, you can't do much more to take care of an oblivious or suicidal diver other than to ground him after his first idiotic dive.

- Bret Gilliam

How Did Nitrox Get Such a Bad Rap?

In the late '80s, those divers who started to advocate the use of nitrox faced fierce opposition from their peers. Why was that?

During World War II, combat divers used bubble- free oxygen rebreathers for covert operations. Using pure oxygen, they were trained not to go deeper than 30 feet (9m). Many did not return. Long periods of breathing oxygen at that pressure made many susceptible to oxygen toxicity, and they drowned. In fact, they should not have gone deeper than 20 feet (6m) -- and oxygen for diving got a bad name.

Nitrox is not pure oxygen. It only has elevated levels of oxygen. That said, when nitrox was first introduced, the only way to make it was to fill a tank partially with pure oxygen and then top it off with air -- the 'partial pressure' method. Handling pure oxygen can be hazardous, since it supports combustion. Cavalier handling, ill-thought-out pipework with constrictions and dangerously tight turns, neoprene O-rings and equipment polluted with hydrocarbons (oil) caused some spectacular fires. People learned the hard way. Quite frankly, many divers did not know what they were doing.

Tanks needed to be oxygen-clean, and in oxygen service, O-rings had to be made of Viton. One training agency, ANDI, demanded that regulators should be oxygen-clean to be safe. (Remember those yellow and green nitrox regulators?)

Then somebody learned that freighters preserved perishable cargos by removing the oxygen from the air in ships' holds, keeping the cargos in inert nitrogen. Using the same technique, it was simple to remove some nitrogen from the air, leaving the air with an elevated level of oxygen. The nitrox membrane system was born.

Nitrox could be delivered directly to a tank with an enhanced but safe level of oxygen. From then on, divers could use ordinary scuba equipment, so it was nitrox on tap.

Nonetheless, some people still had ingrained in their minds that divers died using oxygen, and oxygen also caused fires. For them, nitrox was tainted. It remained a devil gas.

Modern Rebreathers and Oxygen

Today, closed-circuit rebreathers mix nitrox or a trimix of helium, oxygen, and nitrogen on the fly. To do this, divers go in equipped with, among other things, a supply of pure oxygen in a high-pressure cylinder. Despite many early rebreather casualties from lack of knowledge of things like CO2 poisoning, there were few oxygen toxicity deaths and few fires. To our knowledge, no mainstream rebreather manufacturer has been successfully sued for making a dangerous product.

- John Bantin

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