A Question for Nitrox Mavens

[Mossman]

NOAA's chart for oxygen pressure time limits shows 180 min as the daily limit for a 1.4 exposure. Even at six dives a day, which is more than most liveaboards offer, a diver on 32% would have to spend 30 minutes per dive at 1.4. That's 30 min at 111', just do-able with NDC limits on this chart http://www.ndc.noaa.gov/pdfs/nitrox32.pdf. On 36%, 30 min at 95' is and that's easily do-able according to this table: Nitrox 36% Water Proof Dive Table (NT21) which gives the 36% NDC limit as 40 min.

At only 5 dives a day, say off a liveaboard, one would have to spend 36 min per dive at depth, only do-able on 36% without requiring decompression stop(s) according to the above tables, and most liveaboards I've been on that offer nitrox usually only pump 32%. That's why people don't get into trouble on liveaboards.

If I went to Bonaire and dove the six dives per day with an AL80 with an SAC of .5, I should be able to do 30 min at 111' and have 500 psi to spare, assuming a good fill and not counting ascent/descent time. Because ascent/descent time is part of the dive, one would need an SAC better than .5 and/or a larger tank. However, my NDC calculations were for single dives. Obviously residual nitrogen would restrict the NDC time of subsequent dives, so that 30 min at 111' isn't possible for 6 dives in a day without incurring deco obligations. Exactly how much deco, and how much air it would take to accomplish it, I leave to someone else to calculate.

Short answer: Nothing to worry about unless exceeding 6 dives a day and doing some deco, or using custom mixes of > 36% and still doing a lot of diving, so not at all a likely concern for the casual nitrox diver.

 

[Mossman]

As far as getting a safety factor by diving nitrox but using air tables, Halemano's second quote is the key one. If you are diving air within the NDL's, the odds of getting DCS are, I believe, something like a hundredth of a percent. If you dive nitrox to gain a safety advantage, then you are indeed getting a safety advantage, but the amount you have reduced those odds is close to immeasurable. It would be like driving 54 mph in a 55 mph speed zone on a clear road in order to gain a safety advantage.

As far as the concern with cumulative O2 loading--pulmonary oxygen toxicity--if you play with the numbers there you will see that it takes a tremendous amount of exposure to get anywhere close to the limits. Even divers using nitrox for 5 dives a day on a liveaboard are not likely to get close.

I started to play with the numbers in my previous post, and definitely agree with you. As for your first point, that is the correct answer statistics-wise. However, statistics belie the clear fact that there should be some physiologic advantage to absorbing less nitrogen (and offgassing with higher ppO2). It's my belief, though unfounded by any studies or statistics, that the margin created by diving nitrox to less than nitrox NDLs should at least mitigate other potentially DCS contributing factors such as dehydration or post-dive alcohol consumption. That's why I almost always dive nitrox on vacation even if I don't really need it.

 

[Thalassamania]

While we did rarely bump up against oxtox limits, our primary purpose for diving EAN on g air tables was the fact that we were making many, many dives a day and felt that the models really did not accommodate that with the margin of safety that we'd have liked. Also, consider that we were often four day's hard steam from helo evac, and (I know that it's controversial) everyone said that they felt better, had more energy, needed less sleep, etc.

 

[boulderjohn]

While we did rarely bump up against oxtox limits, our primary purpose for diving EAN on g air tables was the fact that we were making many, many dives a day and felt that the models really did not accommodate that with the margin of safety that we'd have liked. Also, consider that we were often four day's hard steam from helo evac, and (I know that it's controversial) everyone said that they felt better, had more energy, needed less sleep, etc.

Yes, it is controversial, but I will flat out tell you that when I am doing a lot of diving with nitrox to the nitrox limits, not the air tables, I feel better, have more energy, need less sleep, etc. I supposed it could be a coincidence or a placebo effect, but I sure feel that way.

 

[Splitlip]

That's very debatable. Statistically, there's no demonstrated benefit.

I don't know. But when I do the math, ???

 

[Sat Diver]

Hi, TMHeimer!

How "old" is your PADI Manual? Mine is from the 1980's.

joewr...clearing the mind and this space...

Hi Joewr,
I had no idea that PADI was advocating the use of Nitrox in the 1980's.
We were diving Heliox and Trimix in the 80's and were told that recreational dive institutions were dead against the practice of anything but air.

 

[MauiScubaSteve]

Perhaps Sat Diver (and joewr) have me on ignore :shocked2:

Perhaps someone could quote my post (with quotes included)

 

[Splitlip]

Yes, it is controversial, but I will flat out tell you that when I am doing a lot of diving with nitrox to the nitrox limits, not the air tables, I feel better, have more energy, need less sleep, etc. I supposed it could be a coincidence or a placebo effect, but I sure feel that way.

I am one of the believers. Not a hint of doubt in my mind.

 

[knowone]

Out of interest, considering the "tables" are reborn with more conservative numbers
every ten years or so with economics also a contributing factor (reprinting) I'm wondering about the vintage of the tables you were or still use
Thalassamania, thank you?

 

[TSandM]

To answer the original question: PADI does not teach that you are safer using EAN within air NDLs. In fact, question 2 in Section 1's Knowledge Review from the Nitrox class reads as follows:

How does using enriched air well within air no decompression limits affect safety?

The book answer is:

Using enriched air within air no decompression limits has no significant effect on safety, provided enriched air procedures are followed.