Should you be diving NITROX if you can not answer these questions?

[The Kraken]

I usually breathe a mix with a PPO₂ of 0.21 during surface intervals to help off-gassing. Am I being too conservative?

the K

 

[scubadiver888]

The Kraken has a very valid point. I recently took the basic EANx course and left with the impression that going beyond 1.6 was pretty much guaranteed to cause oxygen toxicity and convulsions.

In 2000, DAN had a workshop and the results are posted here.

Pages 88 to 98 are people far more qualified then myself discussing this very topic. I'd recommend anyone seriously interested in this topic to read that as a minimum. Page 89 has THE best explanation.

Personally, I'd recommend you read the whole article.

 

[TSandM]

What's frightening about Nitrox is the page of studies in my Rec Triox workbook . . . The variability between individuals and with the same individual on different days is enormous. It makes it quite difficult to propose a ppO2 above 1.0 that could be considered to be "safe". For a long time, 1.4 was felt to be pretty benign, but I know of two incidents where people seized on 1.4, and one died. It is extremely difficult to manage a seizing person underwater, and rarely successful even if the divers involved have extensive training. I don't want to risk it, myself.

There is a difference, too, in the motivations of recreational and technical divers. For a tech diver, reducing the ppO2 means doing more deco -- which in the majority of cases, is quite possible. Although hanging in cold water isn't fun, the people who do it are trained to do it, and the risk of more deco is substantially less than the risk of toxing. Most of the people I know who do staged decompression diving are very respectful of that risk.

But recreational divers have a motive to push ppO2s . . . more bottom time. Since they don't intend to go into deco (as a general rule) they are constrained by that pesky NDL. So it becomes attractive to use the hottest mix that will permit keeping ppO2s to whatever the diver has decided is safe FOR HIM. One hopes that's the agency's chosen standard, because the recreational diver is least likely to have a buddy who can recognize oxygen toxicity and manage a seizing diver to the surface.

 

[MXGratefulDiver]

I'm sure I'll get stomped on here, but here goes anyway . . . .

PPO's are "RECOMMENDED".

We seem to be in an era of "less is better".

If I recall correctly, when I took my nitrox course it was posited that a PPO of 1.6 was the level which was recommended that a diver not exceede because beyond that level the possibility of oxygen toxicity was more POSSIBLY likely to occur.

For some reason, the 1.6 PPO seems to be interpreted nowadays as the level at which O2 toxicity WILL occur if the diver exceeds it.

I agree with prudence, caution, safety and so forth, but there comes a time when being safe becomes being paranoid.

I can understand not diving beyond 1.5 or 1.4, but when we start getting to PPO's of 1.2, or 1.1, or 1.0???

What's next???

Let's not dive beyond 0.5, that way we'd be really safe.

I have no compunction about diving a PPO of 1.6, although I prefer, for safety's sake, 1.5 or 1.4.

But, in my opinion, going to such reduced percentages of O2 effectively negate the rudimentary reason for diving nitrox.

Just my opinion.

the K

I tend to agree with you ... and like you, I was taught (in my original TDI nitrox class) that if you stayed at a PPO2 of 1.6 or lower, your risks of ox-tox were very low.

However, a comment on the highlighted sentence above.

NAUI's position, as expressed in their Nitrox manual, is that a diver should plan for a working PPO2 of 1.4 and a "contingency" PPO2 of 1.6. So they're not saying "don't dive at 1.6" at all ... they're saying you should reserve it for unforeseen circumstances.

I think that's a prudent call for recreational divers.

... Bob (Grateful Diver)

 

[siikik]

They could have been certified ages ago. NOAA Nitrox I (32%) was chosen to hit its 1.6 MOD at 130 fsw to match up with the limits of rec/NDL diving. Since then the MOD limits have tightened up to 1.4 and with some agencies recommending no more than 1.2

So you've narrowed it down to basic stupidity because they won't update their skills... when I was first certified (not Nitrox) my tables were very different. If I were to use them today, I'd blow every NDL there was on current tables.

And waaaayyyy back... I remember the PPO theoretical limit was 2.0 so it's obvious things have changed.



Ken

 

[scubadiver888]

What's frightening about Nitrox is the page of studies in my Rec Triox workbook . . . The variability between individuals and with the same individual on different days is enormous. It makes it quite difficult to propose a ppO2 above 1.0 that could be considered to be "safe". For a long time, 1.4 was felt to be pretty benign, but I know of two incidents where people seized on 1.4, and one died. It is extremely difficult to manage a seizing person underwater, and rarely successful even if the divers involved have extensive training. I don't want to risk it, myself.

There is a difference, too, in the motivations of recreational and technical divers. For a tech diver, reducing the ppO2 means doing more deco -- which in the majority of cases, is quite possible. Although hanging in cold water isn't fun, the people who do it are trained to do it, and the risk of more deco is substantially less than the risk of toxing. Most of the people I know who do staged decompression diving are very respectful of that risk.

But recreational divers have a motive to push ppO2s . . . more bottom time. Since they don't intend to go into deco (as a general rule) they are constrained by that pesky NDL. So it becomes attractive to use the hottest mix that will permit keeping ppO2s to whatever the diver has decided is safe FOR HIM. One hopes that's the agency's chosen standard, because the recreational diver is least likely to have a buddy who can recognize oxygen toxicity and manage a seizing diver to the surface.

I have noticed recently, PADI is pushing all divers should go to Rescue Diver (on the PADI website and at all the LDS in my area). This is one stop short of Master Diver (currently their highest non-professional level) or Dive Master (first level of dive professional). Cynical people will assume it just another case of Put Another Dollar In but I think they also want to have more people with rescue knowledge as recreational divers push the limits more and more.

 

[drbill]

I solve the problem by not diving Nitrox at all, although I may use it for off-gassing in the future. At the usual depths I was diving in the past only normoxic and hypoxic mixes would have been possible. Now that I'm back in the largely recreational depth range, my profiles don't suggest it is all that beneficial... and it would restrict me from going deeper IF I happened to see a subject heading down, down, down.

 

[Anti-Hero]

By any definition I've seen, if it contains gases other than Nitrogen and Oxygen in more than trace amounts, it's not nitrox.

Agreed, but remove the helium constituent and you have 25% nitrox which is very "diveable" down to 150 fsw. I wouldn't do it, but it's certainly done (on air) often.

In any case, I get narced at 100 feet on air (at least in the conditions in which I usually dive), and 150 on 25/25 is like 100 on air. Personally, I'd rather a little more helium and a little less oxygen, since adding the former doesn't hurt (wallet aside) and subtracting the latter is a more conservative (tox).

I'm very susceptible to narcosis myself, it sucks but at least we have ways to manage it. 100' END isn't too bad for me, but any deeper would be pushing the redzone of comfort.

To me, toxicity and narcosis are the biggest concerns from a gas selection standpoint. Deco is what you live with after addressing the primary considerations.

Agreed

 

[hobodiver]

What's frightening about Nitrox is the page of studies in my Rec Triox workbook . . . The variability between individuals and with the same individual on different days is enormous. It makes it quite difficult to propose a ppO2 above 1.0 that could be considered to be "safe". For a long time, 1.4 was felt to be pretty benign, but I know of two incidents where people seized on 1.4, and one died. It is extremely difficult to manage a seizing person underwater, and rarely successful even if the divers involved have extensive training. I don't want to risk it, myself.

That is the part that I was trying to point out.

 

[Van Isle]

FWIW, the DAN report cited above (which in part is a roundtable of PADI, SSI, TDI, NOAA, USN, IANTD, and NAUI hyperbaric medicine experts, amongst others) cites the number of recreational divers taking an ox-tox hit ("seizure") at 1.6 ppO2 or less from single exposures in recorded history as zero.