Is there data on how close to NDL undeserved hits occur?

[Wibble]

On a more positive note…

People do get DCS when diving. Let’s put aside DCS through poor diving — pushing limits, uncontrolled ascents, ignoring safety stops or deco stops.

Undeserved DCS is rare, see @scubadada's post #17 above. The vast majority of divers I know have never had a bend. Those that have had a bend often discover they have a PFO (Patent Forumen Oviale, a.k.a. hole in the heart, which is surprisingly common, circa 25% of the population).

Most divers think about DCS and do their own risk assessment. This may be to avoid deep NDL diving, technical (decompression) diving, not to mention the more challenging diving in wrecks, overheads, etc.

However, many progress to technical, deep, multi-gas, long decompression, rebreather, cave and the myriad other challenging and interesting diving…. They very rarely get bent despite diving far beyond recreational limits where a direct ascent to the surface would mean death or life-changing injury.

Their main mitigation is training, practice, planning, preparation, rigorous kit maintenance, high standards and properly thought through fallbacks (reserve gas, bailouts, redundant kit…). Undeserved DCS doesn’t really factor in these risk assessments when compared with all the other risks!

Can’t wait for the diving season to get started!

 

[Duke Dive Medicine]

I am curious to know if an analysis has ever been made on how close to NDL does the average diver get to then suffers DCS on an "undeserved" hit dive? If so what is the range and what is the shape of the curve?

Edit: Let us assume there was no rapid ascent contributing to the DCS, just a normal recreational dive following all usual training guidance.

Given the nearly infinite variety of ways to get close to the NDL with a dive computer, this would be impossible for all practical purposes. Anecdotally, I've heard the words, "But my computer said it was ok!" from a lot of bent divers.

Best regards,
DDM

 

[Germie]

What is the NDL for example for 30m? There is already a difference between tables from cmas (15 minutes) and some others (20 minutes). So which of these tables is right?
Then, the safestop, is this done or not? Is the stop made longer?
So I think there are a lot of things that you don't know. So it is hard to make statistics from this.

 

[Wibble]

What is the NDL for example for 30m? There is already a difference between tables from cmas (15 minutes) and some others (20 minutes). So which of these tables is right?
Then, the safestop, is this done or not? Is the stop made longer?
So I think there are a lot of things that you don't know. So it is hard to make statistics from this.

Plus Nitrox 32% (30 min) or air (20 mins).

 

[boulderjohn]

Given the nearly infinite variety of ways to get close to the NDL with a dive computer, this would be impossible for all practical purposes. Anecdotally, I've heard the words, "But my computer said it was ok!" from a lot of bent divers.

All the posts above, including the one I am quoting here, do a great job of detailing the difficulty of getting the information. Some posts have also pointed out that the percentage of dives that lead to DCS is so very, very tiny that there is really no point to try to undertake the kind of massive study with massive costs that would be needed even to try.

I just want to add one more point in relation to DDM's last sentence. A recent European study on DCS (which I can't locate) found that the overwhelming majority of cases of DCS occurred with divers who were diving within their limits. That should be no more of a surprise than learning that the overwhelming majority of people who die in Germany are German citizens. It turns that that in diving today, it is pretty unusual for divers to exceed those limits, so of course most people who get DCS were diving within limits.

As for those people DDM is talking about, I was one of them. I got DCS hours after a 2-tank dive that was so very far within limits that I thought it was impossible for me to be bent, a belief that sent me down the river of denial, and I am glad DDM diverted me to the hospital instead. I can't estimate how many hundreds of dives I have done that were closer to the edge, wherever that edge may be.

That led me to suspect whatever is in play can be described with a crude analogy--swallowing food. Humans are the only animals with a strange throat design that requires our throat muscles to learn complex behavior to enable us to eat. Even though we are normally pretty careful about it, something we eat can "go down the wrong pipe." I suspect DCS in some cases is the offgassing equivalent of going down the wrong pipe. (BTW, I learned this after a very long surgery. As a result of the anesthetic, my throat muscles got confused and had to relearn the proper sequence of swallowing.)

 

[tursiops]

In my limited experience PFOs and fast ascents are the two biggest causes of DCS.

Concur.

 

[dmaziuk]

I just want to add one more point in relation to DDM's last sentence. A recent European study on DCS (which I can't locate) found that the overwhelming majority of cases of DCS occurred with divers who were diving within their limits. That should be no more of a surprise than learning that the overwhelming majority of people who die in Germany are German citizens. It turns that that in diving today, it is pretty unusual for divers to exceed those limits, so of course most people who get DCS were diving within limits.

Along the similar vein, sort of, ISTR a study (DAN EU?) that concluded that most cases of recreational DCS happen in "the middle tissue compartments". (It may or may not have been the same study John's taking about.) Well, since the slow compartments never really come into play on recreational schedules, and fast ones are taken care of by sticking to safe ascent rate, of course that only leaves the "middle" ones.

 

[scubadada]

@boulderjohn @dmaziuk

DAN Europe article from 2017, a interesting read. PDF can be downloaded from the link

Frontiers | Dive Risk Factors, Gas Bubble Formation, and Decompression Illness in Recreational SCUBA Diving: Analysis of DAN Europe DSL Data Base

Introduction: The popularity of SCUBA diving is steadily increasing together with the number of dives and correlated diseases per year. The rules that govern...

www.frontiersin.org

 

[L13]

I am curious to know if an analysis has ever been made on how close to NDL does the average diver get to then suffers DCS on an "undeserved" hit dive? If so what is the range and what is the shape of the curve?

Edit: Let us assume there was no rapid ascent contributing to the DCS, just a normal recreational dive following all usual training guidance.

The thing that bothers me about this question is the implicit idea that there is such a thing as "an NDL" for a given dive. Every algorithm gives a different NDL for a dive. Every different conservative setting gives a different NDL. "How close a given dive was to NDL" is almost independent of the dive (I could produce an arbitrary algorithm that produces any NDL from any dive), where as DCS risk is totally dependent on the details of the dive and diver.

Example:
Dive to 60ft for 15 min. on a computer with GF's.
GF high = 95% --> 40 min NDL remaining
GF high = 85% --> 29 min NDL remaining
GF high = 65% --> 11 min NDL remaining
GF high = 50% --> ~0 min NDL remaining

Was this dive at NDL, or 40 min from NDL? In either case, the DCS risk is the same and ~0 if safe ascent rates are used, especially if a safety stop was used.

EDIT: I just realized made essentially the same post earlier in this thread!
(I need to pay more attention to where I am on the SB)

 

[Wibble]

And of course there's plenty of other algorithms; DSAT, RGBM, etc. All of those have various conservatism factors. Then the tables.

Not to mention using nitrox.