Reducing DCI risk by staying immersed on surface after a dive

[Seaweed Doc]

@tursiops & inquis:
Is the 1.05 bar pressure on my tissue at about 50 cm average depth of the submerged body not relevant?
Intuitively, I would not consider the position of the airway opening to be more important.

But it could also be that it is just a theoretical construct and has no clinical relevance for my dives.

That's correct. It's lung position, not mouth/nose position, that matters.

That said, I think you'd be better off spending any extra time at depths up to 20-30' (up to 9 m or so) than at the surface. As others have noted, extending the safety stop at 15'/5 m isn't a bad idea. I routinely use a 5 minute safety stop when coming up from dives of 100'/30 m or greater.

Back in the dark ages when I learned to dive, it was said that you couldn't get DCS at depths shallower than 30'/9 m. We know a little bit better now, but even so I suspect there's a depth not much shallower than 30'/9 m that, if you were fully saturated, you could still safely ascend. On the other hand, the difference in partial pressure (and thus off gassing rate) would be greater in the shallows.

Without good evidence, I can tell you what I do. I tend to start my safety stop at 20'/6 m because my computer starts the safety stop countdown at that point. But then I slowly work my way up to 10'/3 m. Generally I'm at 20'/6m or so for the first minute, at about 15'/5 m during minute number 2, then at 10'/3 m for the final minute. This keeps my computer "happy" and seems to make sense to me.

 

[boulderjohn]

Back in the dark ages when I learned to dive, it was said that you couldn't get DCS at depths shallower than 30'/9 m. We know a little bit better now, but even so I suspect there's a depth not much shallower than 30'/9 m that, if you were fully saturated, you could still safely ascend.

About two years ago we had a thread that ended pretty conclusively with a link to a study of saturation divers. It indicated that all tissues saturated to depths shallower then 20 feet could surface safely. That means you can stay indefinitely at a safety stop depth. The tissues that are still supersaturated can drop to safe levels, and the tissues that are still on-gassing cannot reach unsafe levels.

 

[lowwall]

Back in the dark ages when I learned to dive, it was said that you couldn't get DCS at depths shallower than 30'/9 m. We know a little bit better now, but even so I suspect there's a depth not much shallower than 30'/9 m that, if you were fully saturated, you could still safely ascend. On the other hand, the difference in partial pressure (and thus off gassing rate) would be greater in the shallows.

I think this is still true with the one caveat that it assumes you are limited to one or two tanks. The shallowest depth a fully saturated diver (generally considered to be a diver with at least 24 hours at that depth) has gotten DCS is 23'. At least according to this study: https://www.researchgate.net/public...n_humans_DCS_risk_and_evidence_of_a_threshold

(edit: @boulderjohn and I seem to be on the same wavelength )

 

[dmaziuk]

... I suspect there's a depth not much shallower than 30'/9 m that, if you were fully saturated, you could still safely ascend.

It is called the no-limit depth and is determined by simply looking at M0 values for every tissue compartment in your model and finding the depth at which none of your compartments can ever be saturated to a greater than that partial pressure of inert gas. I.e. it depends on the model and on the breathing mix.

The slowest compartments have the lowest M0 values, and it takes 6 half-times to "practically saturate" one. E.g. DSAT's slowest one is 480 minutes -- times 6 makes 48 hours to saturate.

Haldane thought it was 10 msw, Workman adjusted that to N2%: 7.8 msw, subsequent models may have it a bit shallower but they're talking 6hr+ compartments that we would never saturate on a single Al80, so... 8-ish msw sounds good enough for my dives.

 

[Seaweed Doc]

I think this is still true with the one caveat that it assumes you are limited to one or two tanks. The shallowest depth a fully saturated diver (generally considered to be a diver with at least 24 hours at that depth) has gotten DCS is 23'. At least according to this study: https://www.researchgate.net/public...n_humans_DCS_risk_and_evidence_of_a_threshold

(edit: @boulderjohn and I seem to be on the same wavelength )

Thanks for the link. I tried doing a quick search on scholar.google.com, but couldn't find anything.

T

 

[Seaweed Doc]

It is called the no-limit depth and is determined by simply looking at M0 values for every tissue compartment in your model and finding the depth at which none of your compartments can ever be saturated to a greater than that partial pressure of inert gas. I.e. it depends on the model and on the breathing mix.

The slowest compartments have the lowest M0 values, and it takes 6 half-times to "practically saturate" one. E.g. DSAT's slowest one is 480 minutes -- times 6 makes 48 hours to saturate.

Haldane thought it was 10 msw, Workman adjusted that to N2%: 7.8 msw, subsequent models may have it a bit shallower but they're talking 6hr+ compartments that we would never saturate on a single Al80, so... 8-ish msw sounds good enough for my dives.

Yeah, the tricky part is how saturated you got when you were (hypothetically) down at 130'. With the larger pp difference at that depth, I suspect even slow tissues load faster than at 30'. Or following multiple dives.

Regardless, I have no qualms starting a safety stop at 20'/6 m even after repetitive dives. And it seems at least one study supports that.

 

[dmaziuk]

Yeah, the tricky part is how saturated you got when you were (hypothetically) down at 130'.

Apples and oranges. If on-gassed to over M0 at 130', then your computer told you you're in deco and the 20' stop is mandatory. Then you'll be off-gassing there.

 

[DrWilliam]

Remain horizontal during SS and all the way to surface. Use GF99 and SurGF to optimize off gassing at SS.

 

[Seaweed Doc]

Apples and oranges. If on-gassed to over M0 at 130', then your computer told you you're in deco and the 20' stop is mandatory. Then you'll be off-gassing there.

I wasn't thinking of going into deco, but I think I see your point and my error.

Slow tissues load SO slowly that it's not the deeper dive followed by safety stop at 30' that gets you, it's the buildup of time at shallower depths. On the deep dive, it's the fast tissues that get you.