Diving Nitrox to increase safety AND bottom time!

[vladimir]

Everyone does that kind of tea-leaf-reading of their "symptomology" after they've been nitrox certified. I fondly recall writing stuff exactly like you just wrote. If I had a penny for every minute I've spend arguing over nitrox's effects on narcosis or fatigue or whatever in the past decade I'd be rich. There's just no way to eliminate placebo effect or simply random chance. To take an example, I've done back-to-back dives at the same site over two days, under the same conditions (light, silt, current, cold, etc) and one dive got completely hammered by narcosis and one dive had no issues at all -- both dives on 32%. And to top it off my dive buddy had the reverse experience, she was narc'd out of her mind on the dive I was clear on, and vice versa. You throw random variables like that at this problem and whatever pattern you think you found in the first few dives you've done on nitrox is likely very wrong.

Thank you, well put. The key point, I think, is that other factors will impact your sense of post-dive fatigue or well-being far more than nitrox, so it is impossible to make an objective determination.

It would be easy and cheap to do a double-blind test. Get a hundred people to do identical dives to 60 feet for 55 minutes in the same quarry. Dispense numbered tanks, half with 32%, half with air. The diver wouldn't know what he was diving, the tank jockey wouldn't know. (Divers would be instructed to assume they had 32% for MOD purposes, and to assume they had air for NDL purposes, in case they had to deviate from the dive plan.) Let each diver rate his fatigue afterwards. They got ~900 divers in to Gilboa Quarry to set a silly record, you'd think they could recruit a 100 to do some actual science.

I'll bet on the null hypothesis.

 

[DaleC]

The study would be limited because doing one NDL dive is the least likely scenario to manifest a difference between the two gasses. Try a week of 4 dives/day and I bet there might be a difference.
Also, divers vary in so many ways that trying to equate their reactions would be meaningless. Who would feel less fatigued, the 20 year old fit diver on air or the 50 year old unfit diver on EAN? What would it mean.

 

[aquaregia]

Have both divers do crappy ascent profiles. Roll 60' for 0:55 on 21% and 40% and do a one-minute ascent with no safety stop. I really think "feel better" argument is avoiding a subclinical at the edge of the models.

 

[vladimir]

The study would be limited because doing one NDL dive is the least likely scenario to manifest a difference between the two gasses. Try a week of 4 dives/day and I bet there might be a difference.

I agree that repetitive dives would be a better test—that's why I always dive nitrox. Yet there are some in here who can feel the benefits on every dive. I am skeptical.

Also, divers vary in so many ways that trying to equate their reactions would be meaningless. Who would feel less fatigued, the 20 year old fit diver on air or the 50 year old unfit diver on EAN? What would it mean.

That's why you have a hundred divers. Each diver makes a subjective judgement of his own fatigue. You just need to establish a statistically significant correlation between those who dived nitrox and those who were "less fatigued" to reject the null hypothesis. Then you have evidence (as opposed to proof).

 

[DaleC]

I would be somewhat skeptical of single exposure effects too (unless they were consistently pushing the envelope perhaps).

When I began diving I was always very bushed after a two (cold water) dive day and had real problems staying awake for the 1.5 hour drive home afterward. Around the time I began using EAN I noticed that I was less tired... but was that due to the EAN or the fact that I had gained sufficient buoyancy control that I was not ascending rapidly at the end of my dives any more and was less psychologically stressed in general than when I was new. I don't know.

 

[lamont]

I would be somewhat skeptical of single exposure effects too (unless they were consistently pushing the envelope perhaps).

When I began diving I was always very bushed after a two (cold water) dive day and had real problems staying awake for the 1.5 hour drive home afterward. Around the time I began using EAN I noticed that I was less tired... but was that due to the EAN or the fact that I had gained sufficient buoyancy control that I was not ascending rapidly at the end of my dives any more and was less psychologically stressed in general than when I was new. I don't know.

^^^ exactly what happened to me.

I can vividly recall nearly nodding off on one drive home.

That was practically 7 years ago now. Is it the 32% I usually dive, or did I get better?

 

[vladimir]

Have both divers do crappy ascent profiles. Roll 60' for 0:55 on 21% and 40% and do a one-minute ascent with no safety stop. I really think "feel better" argument is avoiding a subclinical at the edge of the models.

Yeah, my experiment was designed with some ethics in mind: I'm trying not to bend anybody. But your point, and Dale's, is well taken: the best test would push the subjects as close to DCS as possible.

 

[TjRjn]

Try slower ascents. The headaches and fatigue will go away. I've been nitrox certified for a dozen years and have never felt a difference. I'm usually refreshed after every dive! As far as safety goes, the incedent rate of DCS is so low now that any reduction using nitrox would be miniscule. There are two reasons to dive nitrox. All the "feel good" reasons are placebos but some people insist on believing anything they hear.

I always do a safety stop at 5 metres and monitor my computer so I'm not sure that's the issue. The couple of times I've had headaches (which has lasted all of the next day) have been on deep dives 26 & 30 metres. It may be dehydration although I try and drink plenty before and after. I also find looking up (and around) tends to make me feel nauseous and stays with me for quite a while. Not sure what that is all about.

 

[beaverdivers]

Against most training agency standards.... I don't know why don't you look mine up.

Since there are no dives required for this certification, what standard is being violated?

I just said that I usually role this into another class, so I can physically show them how to use a few different types of O2 analyzers. Then we go over EAD again in real life between dives. Dive with the table, dive with the computer etc... Works well in an advanced class. And is by no means a training violation.

Zen,

Exactly, dives are not required ( or needed to be required ). The skill is the same as the #1 rule of diving = Breathe.

 

[doctormike]

No, I'm saying that anyone who argues that EAN does not provide safety against DCS has to explain why one would breath it should they fear an impending hit. What's the protocol for a rapid ascent or missed deco stop? Amongst other things, breath the richest mix possible - why? I say we do it because we hope EAN will provide more safety from DCS than just breathing air. It's not complicated. You can do it after the dive as an acute measure or or during the dive as a preventative measure.

I think that we aren't understanding each other.

My understanding of this discussion was whether or not diving a rich mix but within air tables reduced your risk of undeserved hits. That is - if you are diving nitrox and using air tables, does that make you less likely to get DCS when compared to diving air for the same profile. All I was saying is that we have no evidence of that, even though it seems like it might be true. It also might be possible that within air tables, there is no measurable difference in the DCS rate, you just don't know. In other words, some functions are non-linear, and only apply within certain boundaries. Dropping the speed limit from 55 to 35 would save some lives. Dropping it to 15 would save more. Dropping it from 4 to 3 would probably not save any lives.

So all I'm saying is that maybe we shouldn't present as a fact that diving nitrox using air tables reduces the rate of undeserved hits (the original point that I commented on). I was NOT suggesting that reducing nitrogen loading in general doesn't decreases the risk of DCS on any given profile (which is the physiological theory, backed up by data, on which diving is based).

So then the question is, are we willing to do ANYTHING to reduce a theoretical risk when the chances are vanishingly small? Of course not, we wouldn't dive at all if that was the case. So the task before us is to decided which changes to our gear and practices are worthwhile and which are not. At some point, you reach a limit in which you just aren't getting any more safety for your incremental change. For example - many people carry a completely redundant gas supply - a pony bottle - in case of failure of some component of the rig. But what if you have a double failure - LP hose rupture and a non-functional pony regulator. Well, you can eliminate that miniscule but potentially deadly problem by carrying a second pony bottle. But I don't know anyone who does that, because the risk is vanishingly small.

So I guess all I'm saying is that if you are making a decision to dive air tables but breathe nitrox, you should at least have some idea about how much it actually improves your safety, since there is a small but non-zero downside of nitrox, and the only way to make any decision like this is to weigh the risks and the benefits.

Unlike some, I don't have the disposable income to purchase a CCR for those moments.

I wasn't suggesting CCR, I was just saying that because I thought you were saying something about an in-dive change in the mix to optimize N loading, which is one of the benefits of a rebreather... I probably misinterpreted the comment...