Nitrox and repetitive dives, what mix first?

[mongodives]

In the situation you describe, I would have done my deep dive on air then my second dive on EAN. I wouldn't have planned on going to the PO2 1.4 MOD of EAN 36.

 

[Kern]

However ,follow the rule of thumb ,deeper dives first followed by shallower.

Why?

I also use the right mix for each dive, which is always 32% at 111' or above.

Put another way, there basically isn't a recreational dive I can't do with EAN 32. Makes the logistics, math, etc. ridiculously easy. :cool2:

John, while it may be the preferred mix for you're diving philosophy, it is it's not the ideal mix for all depths down to 111' if minimising N₂ absorption is you goal.

. I wouldn't have planned on going to the PO2 1.4 MOD of EAN 36.

Why?

 

[Thalassamania]

You should make the dive with the deepest Equivalent Nitrogen Depth first.

 

[Kern]

You should make the dive with the deepest Equivalent Nitrogen Depth first.

And the reason for this is?

 

[rstofer]

You should make the dive with the deepest Equivalent Nitrogen Depth first.

That's an interesting way to put it! It's a lot like my idea of coming up with the lowest pressure group.

If we ignore the Nitrox bit, we were all taught to make the deepest dive first because subsequent dives would still allow nitrogen absorbed on the deeper dives to leave the body. Tissue compartments, and all that stuff.

Nothing changes just because we move to Nitrox, we still want nitrogen to leave the body in a controlled manner.

From this point of view, you would want to use the highest %O2 possible on the first (deeper) dive consistent with not busting the MOD. If you don't take on much nitrogen on the deep dive, there isn't a lot to give up. On subsequent shallower dives, you won't be taking up as much nitrogen anyway so a lesser %O2 is acceptable.

I still think the right way to verify this is to come up with a series of dives, SITs and mixes. Then work through the tables and see what the ending pressure group is at each dive.

EAN36, 90', 30 min -> 'N'
1 hour SIT -> 'D'
EAN32, 60', 60 min -> 'W'

EAN32, 90', 30 min -> 'P' (higher than with EAN36)
1 hour SIT -> 'E' (still higher)
EAN36, 60', 60min -> 'U' (lower and shows the flushing action of high %O2 on last dive)

These examples lead to some contradictory results. Clearly, using the higher %O2 for the first (deeper) dive results in a lower pressure group after that dive. However, after the 2d dive we are at a higher pressure group because EAN32 has more nitrogen and it's a long dive.

So, I guess it depends on where you want to be at the end of the day. I don't know if I care what my pressure group is while I'm driving home.

Best case would be to use EAN36 for both dives:

EAN36, 90', 30 min -> 'N'
1 hour SIT -> 'D'
EAN36, 60', 60 min -> 'T'

But I don't know if it's enough of an improvement to care. Still, an interesting diversion...

Richard

 

[Blackwood]

John, while it may be the preferred mix for you're diving philosophy, it is it's not the ideal mix for all depths down to 111' if minimising N₂ absorption is you[r] only goal.

Fixed.

And the reply: in which case gas containing no nitrogen is the ideal mix. Relative to this discussion, that would be pure oxygen. Of course, there are other concerns (not the least of which is PO₂, but also including those goals John listed: logistics, math, etc.).

 

[Bubbletrubble]

The "Equivalent Nitrogen Depth" mentioned by Thalassamania is essentially what is at issue here. With the exception of managing the OTU clock or making "efficient use" of bottom time, this entire discussion could theoretically be reduced to a debate over the safety of reverse dive profiles. Conventional wisdom is that reverse dive profiles increase the risk of decompression sickness (DCS).

A very nice discussion of reverse profiles can be found in the Rubicon Foundation online archives:
Rubicon Research Repository: Item 123456789/4244
This AAUS conference occurred in 2000, but I don't believe that any paradigm-shifting research has been published since then to contradict the consensus opinion: There is no evidence to suggest that reverse profiles, in the context of no-stop recreational diving, are more likely to cause DCS than traditional forward profiles.

[rant]
I cringe every time I see/hear someone stating decompression dogma as if it's incontrovertible truth. Some of what we know about DCS is based on animal experiments. In reality, embarrassingly little is known about DCS (etiological mechanism) in the human body. Definitive studies simply haven't been done yet. Mathematical models of decompression are just that -- mathematical models. And mathematical models shouldn't be used as "evidence" for supporting/contradicting elements of decompression theory. That's putting the cart before the horse.
[/rant]

Now that I've warned you against tacit acceptance of conventional wisdom, consider the following...
As a dive community, many more forward dive profiles have been conducted safely (without DCS occurrence) than reverse profiles.
In my opinion, this is the most compelling reason to plan forward dive profiles. Extending that reasoning, adopt a dive profile with the deepest equivalent nitrogen depth first (as put forth by Thal). I don't want to be anyone's guinea pig or goat. My recommendations to the OP are simple: Do the best you can with the info that's out there. Learn how your computer's deco algorithm stacks up against other major deco algorithms in use nowadays. Understand how and when to add in more conservatism. We still don't know to what extent hydration status, exhaustion, cardiovascular fitness/exercise tolerance, deep stops, water temperature, age, gender, and personal physiology contribute to DCS.

Diving is a risky sport. When it comes to managing that risk with deco algorithms, it's better to run with the herd...

 

[rstofer]

A very nice discussion of reverse profiles can be found in the Rubicon Foundation online archives:
Rubicon Research Repository: Item 123456789/4244
This AAUS conference occurred in 2000, but I don't believe that any paradigm-shifting research has been published since then to contradict the consensus opinion: There is no evidence to suggest that reverse profiles, in the context of no-stop recreational diving, are more likely to cause DCS than traditional forward profiles.

That a nice paper! Obviously, I haven't read it all. But I did read page 271 (page 273 of the pdf file) where Tom S. Neuman, University of California, San Diego states:

I would like to put a word of caution in before I say reverse dive profiles don't seem to be a big problem. First of all, the points that Richard made, I think, are extremely important. Little things going wrong at the end of a deep dive lead to real bad things happening to divers. That doesn't fit into any mathematical model. If you've got a big gas load from a deep dive and all of a sudden you find yourself out of air, it's a whole lot better if you're out of air at 20 feet than if you're out of air at 100 feet. This is a very real operational issue for the diver or from the point of view of the diving supervisor who takes care of people who are hurt. A lot of things can go wrong on a deep dive that tend to be more dramatic than when they go wrong on a shallow dive. For that reason, personally, I would rather have a smaller gas load when I'm making my deep dive than a larger load."

While Mr. Neuman isn't throwing reverse profiles under a bus, it doesn't sound like he plans to dive them either.

Richard

 

[Cyber_Ghost]

An extremely interesting paper, still have finish it, but meanwhile it's going great.

 

[Kern]

Fixed.

And the reply: in which case gas containing no nitrogen is the ideal mix. Relative to this discussion, that would be pure oxygen. Of course, there are other concerns (not the least of which is PO₂, but also including those goals John listed: logistics, math, etc.).

I would imagine, that for most of us, limits such as ppO₂, MOD etc. are all a part of using the ideal mix. The maths seems pretty straightforward, but as you say, the logistics in many places is a problem. (I'm a bit spoiled with access to a membrane system).

I can only see O₂ being an ideal mix down to 6mts, so anytime I'd like to go deeper some physiologically inert gas is required.