Nitrox discussion (Split from "Reasons not to use Enriched air" thread in Basic)

[Blackwood]

I do not know if you are a GUE-ie but you are starting to sound like one, with all these senseless rules.

One man's senseless is another man's sense.

Like in that other thread where I'd said I'd choose oxygen to deco from a 90 foot dive and you said that if you only bring one deco gas it will always be 50% (actually... that's less of a senseless rule as it is a baseless one).

Not going solo during the highest PO₂ portion of the dive makes sense to me. And while we're at it, so does avoiding charters on which the captain dictates deco procedures.

To me, unless there is a compelling reason to do so (no, not agreeing on a deco schedule doesn't suffice), breaking up the dive team is right up there with violating the gas plan (and I'd submit that the former necessarily results in the later, but that's how I dive and I know you won't agree).

Getting along with other techdivers is a skill that involves minding your own business as much as possible.

I get along with most everyone I've met, regardless of their recreational predilections. I'd no more rag on another team for solo deco as I would suffer it within my own. (I will, however, put in my two cents on a discussion forum )

Besides, in choosing a team mate, I'm personally much more concerned with how they act underwater than above.

 

[Catt99]

The myth of the magical properties of 1.4 ATAs as a limit is the main cause for my concern. Hopefully you have now got a better picture of that from some of the technical replies, such as Thal.

Thanks for responding. Yes, Thal's summary was concise -- the thrust of it was pretty much why I didn't want to get into a discussion whether planning dives to 1.6 ATA was "safe" or not -- to me it is a point along the line of relative safety as opposed to safe or not safe (and the point moves with the diver in question, dive conditions, etc.)

I was mostly concerned with your unqualified statements as to "general rules" and what depths were appropriate for various standard mixes (which were also described by you as "ideal" for those depths). I don't agree with your opinions and I'm still trying to figure out how you arrived at them.

Mainly, I don't understand how you arrived at these opinions regarding depth break-points on when "ideal" moves from EANx32 to EANx36. If driven with safety in mind, i.e., a concern for PO2, then I don't understand your methodology. Why, in formulating your opinion of appropriate depths for standard mixes, is 1.4 ATAs the limiting factor on EANx36, and 1.6 ATAs the limiting factor on EANx32? Or is there factor other than PO2 that you're considering when coming up with your suggestions? And yes, lively thread

 

[Thalassamania]

There is no "cliff" that you're going to fall off suddenly. There are times and dives when using EAN36 down to 1.6 or even 1.8 is the thing to do, there are other times and dives when for the same depth EAN32 or air will be a better choice. EANxx is just a tool, but it is doubled edged, and if you're careless it can cut you both ways.

 

[Catt99]

There is no "cliff" that you're going to fall off suddenly. There are times and dives when using EAN36 down to 1.6 or even 1.8 is the thing to do, there are other times and dives when for the same depth EAN32 or air will be a better choice. EANxx is just a tool, but it is doubled edged, and if you're careless it can cut you both ways.

Hi Thal - can you expand on this a little, please? As a novice diver I'm not all that interested in pushing limits on my own susceptibility to to ox tox -- if not a cliff, it certainly feels like a crumbling and steep slope that I should be cautious about trying to peer over. If I knew my planned dive depth would put me at 1.6 or 1.8 on EANx36 I'd be much more inclined to either change my plan or use a less rich mix. When and why would you (or someone) choose to use a mix on a dive that resulted in that pp02 exposure?

 

[SteveAD]

Hi Thal - can you expand on this a little, please? As a novice diver I'm not all that interested in pushing limits on my own susceptibility to to ox tox -- if not a cliff, it certainly feels like a crumbling and steep slope that I should be cautious about trying to peer over. If I knew my planned dive depth would put me at 1.6 or 1.8 on EANx36 I'd be much more inclined to either change my plan or use a less rich mix. When and why would you (or someone) choose to use a mix on a dive that resulted in that pp02 exposure?

The cliff reference goes back to an earlier post about EAN being a bad choice for wall dives with no bottom, where the diver could "fall" below the MOD for their mix. With decent bouyancy control no-one is going to "fall" off the wall.

Earlier posts also referenced the apropriateness of elevated PPPo2 at different times during the dive. A PPo2 of 1.4 during the "active" (don't want to say working or nearass will scold me ) and a PPo2 of 1.6 to 1.8 during a resting deco stop.

 

[Thalassamania]

Actually my cliff comment was intended as an analogy and was not referring to an actual cliff or wall. You are quire right when I comes to deco, but it goes beyond that. If I have an intense diving schedule, in warm clear water, with dives that require very little physical effort (e.g., fish behavior watches) I may well go as high as a _ppO₂ of 1.8 on some parts of some of the dives. But, if I have to do a few dives with heavy work in bad currents, with low visibility I'll hedge my bet and try to keep the _ppO₂ in the range 1.4 (or lower) so that I have some margin. My real point is that rules for an exact _ppO₂ out of the context of the dive(s) I find to be more posturing, or lack of operational knowledge, than anything else. It's like depth ... 130 is ok, but if you hit 131 ... you're a'gonna die!

 

[SteveAD]

Actually my cliff comment was intended as an analogy and was not referring to an actual cliff or wall. You are quire right when I comes to deco, but it goes beyond that. If I have an intense diving schedule, in warm clear water, with dives that require very little physical effort (e.g., fish behavior watches) I may well go as high as a _ppO₂ of 1.8 on some parts of some of the dives. But, if I have to do a few dives with heavy work in bad currents, with low visibility I'll hedge my bet and try to keep the _ppO₂ in the range 1.4 (or lower) so that I have some margin. My real point is that rules for an exact _ppO₂ out of the context of the dive(s) I find to be more posturing, or lack of operational knowledge, than anything else. It's like depth ... 130 is ok, but if you hit 131 ... you're a'gonna die!

OK. My bad.

Disclaimer: My last post, above, was in no way answering for Thal. It was entirely my own answer.:14:

 

[Thalassamania]

Steve, yours was a perfectly reasonable interpretation. I only corrected it because I saw where I'd not been clear. Thanks for pointing it out.

 

[nereas]

No it isn't...

According to who ?! ...

Wrong. It also places them further to the safe side of the curve for immediate acute O2 hits. Something you dont seem to realise even exists.

...

[edited by almitywife]

An NDL diver who does not exceed 1.6 ATAs ppO2 is never, ever going to exceed ox tox limits. It is simply not possible. He/she will get DCS first, long before CNS toxicity. Do the math.

 

[nereas]

...

Earlier posts also referenced the apropriateness of elevated PPPo2 at different times during the dive. A PPo2 of 1.4 during the "active" (don't want to say working or nearass will scold me ) and a PPo2 of 1.6 to 1.8 during a resting deco stop.

DWayne will probably barf now, since somebody mentioned 1.8 ATAs. I am waiting for him to post about it.

As for me, 1.6 ATAs is my own absolute ppO2 limit.