Is DCS possible under 20fsw, or 100% impossible?

[boulderjohn]

No.

You would be diving beyond Non-Deco Limits and be fully saturated, i.e. Saturation Diving. You would probably need to decompress from 20'/6m.

Actually, according to the study, if you are fully saturated at that depth, you can still go to the surface.

You may be confused about the idea of saturating tissues. It is not like filling a bucket. A tissue is saturated when it is at equilibrium at its present depth. If you have been at your present elevation and out of the water for a few days, your tissues are totally saturated right now. When you descend on a dive, they are not saturated any more. Stay at 5 feet of depth for a few days, and they will be saturated at 5 feet of depth.

 

[Moerae]

Something that always bugs me in these threads.

People will talk about NDLs, how long it takes to hit them, if it's possible, etc.

NDL != DCS.

You can get bent without hitting your NDL. Tissue loading will happen even at 20 feet, and bolting to the surface could potentially create bubble formation. Especially if you start combining factors like a diver being cold, drinking the night before, etc etc. Algorithms do not account for all factors.

I say this because people should respect ascent speed limits, even (especially!) at shallow depths and not confuse NDL as a magic no DSC risk threshold.

 

[boulderjohn]

You can get bent without hitting your NDL. Tissue loading will happen even at 20 feet,

Perhaps you can explain why the study cited in post #12 is wrong.

 

[Esprise Me]

I know this is a little beyond the scope of OP's question, but I imagine flying right after a 20-foot dive might still cause problems.

 

[Moerae]

Perhaps you can explain why the study cited in post #12 is wrong.

"You can get bent without hitting your NDL."
That is a correct factual statement not in contradiction with the study.

"Tissue loading will happen even at 20 feet,"
That is a correct factual statement not in contradiction with the study.

I'm not sure how those two sentences made you think I was claiming the study was wrong.

But for the sake of discussion: Did the studies data come from divers ascending at recommended rates? Was anyone freezing their ass off or dehydrated? I didn't look at the data in detail, so I genuinely don't know the answers to those questions. If most divers in the study were ascending at recommended rates my warning not to blow those rates still holds as the study did not find that you "have no risk even if you ignore basic protocols".

Computer algorithms are based on studies, too, and people still get bent following them. It's rare, but the point is not to start diving like an idiot under 20ft just because one study said there was low risk.

To be clear, I do not in any way dispute the studies conclusion that diving below 20ft presents little to no DCS risk assuming the dive is done under the same parameters as the study. But that's a nuanced statement that is easy to oversimplify.

 

[boulderjohn]

About 100 years ago, John Haldane theorized that a saturated diver could ascent directly from 2.0 ATA. Later research put that 1.58 ATA, which is where thinking is now. That's about 20 feet salt water at sea level.

 

[Brett Hatch]

I think part of the problem here is the framing of the question. In diving medicine, there are few questions for which "100% impossible" is an epistemically honest answer.

The theoretical models we have for questions like this are probabilistic by nature, and take a variety of input variables. Those variables and their values have been chosen over time, based on available evidence from a relatively small number of studies. Millions of people make millions of dives every year, some get bent, most don't. Of the ones who do get bent, a good chunk of them are following standard operating procedures that we consider to be "safe." This is important for all divers to understand and accept.

In the end, I choose to dive with a non-zero risk of the bends, and try to dive in such a way to keep that risk small, using the imperfect information available. I also choose every day to drive my car, and cross the street, and drink beer, and tie my shoes. Even though I cannot say for certain that these are risk-free activities, the reward outweighs the risk, so I do them.

All that being said, diving shallower than 20 ft seems to have a very small probability of causing DCS. I am not aware of any cases of this, and would be surprised to hear of one, especially given the information in the study John posted in #12. That study mentions that the 1999 US Navy Divers Manual allows for a direct ascent from 20 ft, even for fully saturated divers. I looked for a similar figure in the current USN manual (2017), and couldn't find it. Not sure if their position has changed, or if I just couldn't find it buried in the 1000-page document.

 

[scubadada]

Actually, according to the study, if you are fully saturated at that depth, you can still go to the surface...

I'm with @boulderjohn. Even at a week, there is no deco obligation at a GF high of 95

 

[ChuckP]

I'm with @boulderjohn. Even at a week, there is no deco obligation at a GF high of 95

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And you shouldn't get any deco because it's all based on the math calculations that assume the 1.58ata correct??? I've been slacking for a year and need to re read a few books....

Like John said, the original theory was 2ata then I thought lowerd to 1.7 then lower again..... Mucho re reading to do

 

[dmaziuk]

And you shouldn't get any deco because it's all based on the math calculations that assume the 1.58ata correct??? I've been slacking for a year and need to re read a few books....

Like John said, the original theory was 2ata then I thought lowerd to 1.7 then lower again..... Mucho re reading to do

Haldane's number was for air as a whole. Subsequent studies dug into its composition and zeroed on nitrogen; at 78% N2, Haldane's 2:1 becomes 1.56.

Different models come with slightly different M-values because they come with (statistical) target incidence of DCS: e.g. a Navy model could be more "aggressive" because its target demographic is very fit young people doing one dive a day from a boat with on-board chamber, whereas a "vacay" model may add conservatism for aging overweight divers doing multiple dives over multiple days.

Either way, the No-Limit Depth is where you can go directly to the surface at any time, without exceeding the M-value in any compartment. (Assuming safe ascent speed, no PFO, perfectly spherical diver of uniform density, and all that, of course.) Working out the NLD for your favourite model and mix is fairly straightforward from the table of M-values, e.g. from Powell's book.