Diving after DCS

[Duke Dive Medicine]

I can't speak to the proprietary algorithms, but anything with gradient factors will have a fairly consistent tissue supersaturation, because that's at the heart of the algorithm. Conceptually, I believe even the proprietary ones will act similarly, as that's the entire point (limiting the tissue loading).

Consider a dive to the air NDL for 60 ft at a medium conservatism of GF x/85 -- 41 mins -- and you do this dive on nitrox. Which nitrox? For the vast majority of divers, that's simply the one the boat crew hands you. Could be 32%, 36%, 37% (because they overshoot the 36% mix) or even 27%. If you lucked out and got the 36% mix, you're effectively 54 mins shy of your actual NDL. On the other hand, if you get the 27% mix, your margin is only 16 mins. As I said, the margin varies with whatever mix you happen to receive.

Thanks for this. If you were to make a specific computer conservatism recommendation to a diver with a prior incident of (likely) PFO-associated DCS, what would it look like?

Best regards,
DDM

 

[inquis]

Hardly a recommendation, but if I had reason to suspect that I had a PFO, I would back off to something very conservative, perhaps GF x/60. Times would be about half that of someone with "Medium" conservatism of x/85. If that allowed me to do the dives I wanted, cool. If I wanted to stay longer (at the cost of increased risk), slowly increase, paying careful attention to how I felt across multiple dives. Hopefully, there would be minor evidence of going too high (e.g., lethargy/sleepiness, itchy skin, or minor discoloration) before anything serious occurred.

 

[davehicks]

I can't speak to the proprietary algorithms, but anything with gradient factors will have a fairly consistent tissue supersaturation, because that's at the heart of the algorithm. Conceptually, I believe even the proprietary ones will act similarly, as that's the entire point (limiting the tissue loading).

Consider a dive to the air NDL for 60 ft at a medium conservatism of GF x/85 -- 41 mins -- and you do this dive on nitrox. Which nitrox? For the vast majority of divers, that's simply the one the boat crew hands you. Could be 32%, 36%, 37% (because they overshoot the 36% mix) or even 27%. If you lucked out and got the 36% mix, you're effectively 54 mins shy of your actual NDL. On the other hand, if you get the 27% mix, your margin is only 16 mins. As I said, the margin varies with whatever mix you happen to receive.


No argument there, but reducing the variability where you can seems sensible to me.

Your description highlights to flaw in your argument. It takes way more than one sentence to describe and won't make any sense to many people. Most divers do not have a Shearwater or another GF based computer. These features are not available to everyone. My classic Suunto computers have a Conservative Yes/No option. What does that do?

Technically informed divers can figure this out on their own. Everyone else needs clear and effective advice they can follow easily.

 

[Duke Dive Medicine]

Hardly a recommendation, but if I had reason to suspect that I had a PFO, I would back off to something very conservative, perhaps GF x/60. Times would be about half that of someone with "Medium" conservatism of x/85. If that allowed me to do the dives I wanted, cool. If I wanted to stay longer (at the cost of increased risk), slowly increase, paying careful attention to how I felt across multiple dives. Hopefully, there would be minor evidence of going too high (e.g., lethargy/sleepiness, itchy skin, or minor discoloration) before anything serious occurred.

If we were working with someone with your level of knowledge, we'd collaborate to come up with a recommendation that would minimize risk of another DCS event, mindful that (again) there's little in the way of empiric evidence for different conservatism settings and gradient factors. I'm kind of reiterating what @davehicks said above, but most divers don't have the background or training to do what you're talking about and computer capabilities vary quite a bit, thus the nitrox on air setting recommendation.

Best regards,
DDM

 

[Maolli]

The OP reported that her husband already had a PFO test.

She said her husband got the basic test which was inconclusive and was hesitant to incur the costs of the more accurate one.

 

[inquis]

Everyone else needs clear and effective advice they can follow easily.

These approaches seem equally clear:
1. Pretend you're diving air when you're not -- definitely results in a gas-dependent / variable safety margin. This also deactivates any MOD warnings, but surely they won't lose track of their depth, right?
2. Use the computer's safety features as designed -- most likely results in a consistent safety margin

I don't know that Suunto's conservatism is inconsistent, but I also have no reason to think that it is. On the other hand, I absolutely do know approach #1 does NOT yield a consistent margin. Even as a "non-technically informed diver", why would I pick the approach that is known to be inconsistent?

I also don't think understanding the flaw in #1 is beyond "most divers" with a nitrox certification. It relies on having less inert gas than the computer thinks. Sometimes that is a small reduction, other times it's a larger reduction. Do you honestly believe people can't understand the margin is tied to the about of reduction?

 

[davehicks]

These approaches seem equally clear:
1. Pretend you're diving air when you're not -- definitely results in a gas-dependent / variable safety margin. This also deactivates any MOD warnings, but surely they won't lose track of their depth, right?
2. Use the computer's safety features as designed -- most likely results in a consistent safety margin

I don't know that Suunto's conservatism is inconsistent, but I also have no reason to think that it is. On the other hand, I absolutely do know approach #1 does NOT yield a consistent margin. Even as a "non-technically informed diver", why would I pick the approach that is known to be inconsistent?

I also don't think understanding the flaw in #1 is beyond "most divers" with a nitrox certification. It relies on having less inert gas than the computer thinks. Sometimes that is a small reduction, other times it's a larger reduction. Do you honestly believe people can't understand the margin is tied to the about of reduction?

After decades of user-centric product design and development I can tell you with confidence that simple solutions win every time. It is also imperative that nerds-like-us take a step back and try to imagine other mindsets. It is not something that comes easily to some.

 

[tursiops]

After decades of user-centric product design and development I can tell you with confidence that simple solutions win every time. It is also imperative that nerds-like-us take a step back and try to imagine other mindsets. It is not something that comes easily to some.

Yes, which is why I suggested just using a air dive table. Cheaper than a computer, more conservative, runs for ever, waterproof.
Easier to learn than a computer, especially a really cheap single-button computer.

 

[johndiver999]

TLDR; Setting AIR is a simple nudge. The reality when dealing with humans is that as soon as there is even a tiny amount of friction you lose half the audience.

The reason that "lying" to your computer by setting Air on a Nitrox mix works is that it's easy. Practically any diver can do this without additional explanation. Setting conservative options works differently on every single computer and most people don't know how to do it. If you are willing to figure out how to manage conservative options on your computer and stick with them over time, then do that. But don't assume everyone is just like you.

I get your point about wanting "accurate" data, but the O2 exposure for recreational open circuit divers has absolutely ZERO value. O2 exposure is just not relevant to this audience so that level of accuracy is not helpful. And the false N2 reading is the entire point.

Yup, exactly.

 

[inquis]

most divers don't have the background or training to do what you're talking about and computer capabilities vary quite a bit

Not knowing how much conservatism is needed plagues the "pretend it's air" approach as well. Perhaps on 36% they're unlikely to have an issue, but on 28% they might. The risk clearly increases, which was the main point I was trying to make.