Padi Advanced OW - Deep stops??

[Diver0001]

You should really read and understand the deep stops thread. It is really very interesting between the bickering. The best bits were on RBW though.

A large part of it was about the relationship between the tested NAVY (
VVal-18 Thalmann Algorithm and BVM(3) ) A1 and A2 profiles and VPM7. Vpm7 was chosen because as you make VPM more and more conservative (in VPM terms) eventually it caught up with the proposed deep stop profile. This led to howls from Ross who was saying that the two tested profiles had twice as much deco as necessary, and compared to a typical profile they did.

So which is more conservative? The algorithm which produces more or less deco for the same bottom time?

This main line in the deep stops thread is really a different discussion than we are discussing on this thread because the study did not investigate deep stops. It investigated deep ascent lines from deco dives. Sadly divers use the term "deep stops" to mean both things, which creates confusion.

You've mentioned the NEDU study a couple of times in a way that makes me think you didn't really understand what they did there. The object of the study was to push a bubble model, namely the Thalmann model, into significant conservatism (which one would assume should result in safer dives as compared to "less conservative" profiles) in order to test certain assumptions about bubble models in general. The most important of these assumptions is that the deep ascent line (I will not say deep stops) results in LESS need for subsequent shallow decompression time.

The "Buhlmann" math didn't seem to add up but people like Dr. Wienke were insisting that this was the case, so NEDU tested it empirically and proved beyond any shadow of a doubt that this assumption is incorrect. Other bubble models showed the same tendency as Thalmann, namely, an innate bias to over-saturating "slow tissues" in order to keep "fast tissues" clean during ascent. The science they did was outstanding and very convincing if you read the report carefully and intelligently.

Not everyone is ready to accept that what NEDU showed is really the truth; among others, Ross. Ross has to make a paradigm shift in his thinking and he's struggling to get his mind around what this study means for VPM and/or if VPM can even be "fixed" to address this bias. I'm sure, although I haven't heard him on this topic, that Dr. Wienke will be fighting these conclusions as well because the entire claim to fame of RGBM is that deep stops make ascents SAFER AND FASTER -- both of these things -- but what NEDU shows is that the current crop of bubble models are incorrectly calibrated when we take into account what ACTUALLY happens physiologically, to a diver during such an ascent.

In other words, in order to remain safe, bubble models will need to make ascents SLOWER -- a lot slower -- in order to clean up this bias they have to overload slow tissues. I'm not a deco scientist but from what I'm reading, it seems plausible that the algorithms could be calibrated by using different bubble parameters for different tissue speeds (currently they do not), but the fact will remain that the algorithms, once properly calibrated, will by necessity, result in slower ascents, not faster ones.

All of this is very interesting to technical divers but to someone reading this thread and wondering about the utility of "deep stops" in the context of a no-stop dive I'm sure it comes across as a bunch of incomprehensible hocus-pocus. That's why I think it's good to keep sharp about defining when we are talking about a "deep stop" (where this thread started) and a "deep ascent line" from a decompression dive. They are two different animals that require two different discussions. They shouldn't be confused.

R..

 

[CuzzA]

The Shearwater Petrel2 and Perdix have a 'OC Rec' (recreational) mode which simplifies many features, including GF options. It's much more suitable for typical novice-intermediate recreational divers, as comprehensive deco algorithm knowledge isn't a prerequisite for use. It still gives flexible options to change conservatism and even decide between one of several safety stop options.

Yes. And the Shearwater products are great too. I wasn't trying to knock the Shearwater products, only draw some comparisons for the OP's question about the Icon.

 

[CuzzA]

For the OP or anyone who wants to watch a great presentation on decompression controversies, watch this. It definitely will clear a lot of things up.

 

[scubadada]

Hi @KenGordon

This thread is about no deco, recreational deep stops, not deco, technical deep stops. Looks like we were discussing different topics. I was discussing the former, you the latter. The thread had gone off track several times earlier due to confusion about the use of the term, "deep stops". The OP was mainly concerned about the feature in many dive computers to optionally include a deep stop in the ascent strategy for a no stop dive

Good diving, Craig

 

[dmaziuk]

Nobody knows the incidence of DCS. To know that we'd need to know how many dives are done (obviously very hard) and how many DCS incidents (not so obviously hard).

This may be news, but there is this agency that starts with a "D" and goes on to "AN"... For example,

"We estimated the annual per-capita DCS incidence rates for 2000-2007 based on insurance claims submitted by members of the Divers Alert Network (DAN), Durham, N.C., with dive accident insurance. The overall per-capita DCS claims rate (DCR) was 20.5 per 10,000 member-years"

Per-capita claims rates for decompression sickness among insured Divers Alert Network members. - PubMed - NCBI

 

[DevonDiver]

I think it's possible to assume that actual DCS rates will be an order higher than DAN's estimation.

Their figures are restricted to those holding DAN's dive accident insurance - a demographic likely to seek expert diagnoses and treatment for DCS without hesitation. Divers without specific dive insurance are likely to much less likely to either seek, or receive, treatment for DCS - especially where a bend might be less serious symptomatically, or less obviously diagnosed, type 1 pain-only bends.

The issue of 'diver denial' has long been recognized as a barrier to getting effective and timely DCS treatment. That denial is likely to be much more prevalent amongst those without insurance to pay costly treatment bills, or those whose diagnosis isn't conducted by, or in liaison with, specialist diving medical practitioners. I'd guess that this issue also skewed DCS statistics based on treatments and/or claims.

 

[KenGordon]

This may be news, but there is this agency that starts with a "D" and goes on to "AN"... For example,

Per-capita claims rates for decompression sickness among insured Divers Alert Network members. - PubMed - NCBI

How many dives did those divers do? Per capita only tells us the rate per person and not the rate per dive. We could also take all the DCI treatments in the world and divide by the number of certifications. That would be another almost useless number. The BSAC numbers are for all of BSAC members and all of the U.K., but nobody knows how many dives are done so again we do not know the rate per dive. This might seem like splitting hairs, but if the number goes down in a year is that because we all got better at diving, the weather was bad or BSAC lost a lot of members?

Other attempts include the uploading of dive profiles, but again there is no linkage for most DCI events to the profiles.

 

[KenGordon]

Hi @KenGordon

This thread is about no deco, recreational deep stops, not deco, technical deep stops. Looks like we were discussing different topics. I was discussing the former, you the latter. The thread had gone off track several times earlier due to confusion about the use of the term, "deep stops". The OP was mainly concerned about the feature in many dive computers to optionally include a deep stop in the ascent strategy for a no stop dive

Good diving, Craig

What the OP is concerned with is only part of the story. In the stuff said to and by the OP there are things I disagree with and which I have an opinion on. The main one is the use of NDL times as a worthwhile indicator of how a computer will perform.

Also, all dives are deco dives, it is just that some have ascent rates slow enough to avoid stops below the surface. There is no line where suddenly a dive is a technical. If you want to make that line be compulsory stops then apparently all the Suunto users will suddenly be technical divers while the brand X buddies are still recreational divers. My reference to the NEDU study is not about technical diving but the comparison of algorithms use by the navy with comercial ones to argue against a single point you made.

It really is pertinent to the the OP, some of these algorithms will give lots of stops once past the NDL. That is a foreseable contingency, like needing an octopus, and should not be ignored. If you do not have the gas for those stops then some degree of concern will be due. But of course it depends on the dives.

I suggest that anyone wanting to research a computer gets in the water with one, or sees what their buddies dive and how they behave.

 

[KenGordon]

This main line in the deep stops thread is really a different discussion than we are discussing on this thread because the study did not investigate deep stops. It investigated deep ascent lines from deco dives. Sadly divers use the term "deep stops" to mean both things, which creates confusion.

You've mentioned the NEDU study a couple of times in a way that makes me think you didn't really understand what they did there. The object of the study was to push a bubble model, namely the Thalmann model, into significant conservatism (which one would assume should result in safer dives as compared to "less conservative" profiles) in order to test certain assumptions about bubble models in general. The most important of these assumptions is that the deep ascent line (I will not say deep stops) results in LESS need for subsequent shallow decompression time.

The "Buhlmann" math didn't seem to add up but people like Dr. Wienke were insisting that this was the case, so NEDU tested it empirically and proved beyond any shadow of a doubt that this assumption is incorrect. Other bubble models showed the same tendency as Thalmann, namely, an innate bias to over-saturating "slow tissues" in order to keep "fast tissues" clean during ascent. The science they did was outstanding and very convincing if you read the report carefully and intelligently.

Not everyone is ready to accept that what NEDU showed is really the truth; among others, Ross. Ross has to make a paradigm shift in his thinking and he's struggling to get his mind around what this study means for VPM and/or if VPM can even be "fixed" to address this bias. I'm sure, although I haven't heard him on this topic, that Dr. Wienke will be fighting these conclusions as well because the entire claim to fame of RGBM is that deep stops make ascents SAFER AND FASTER -- both of these things -- but what NEDU shows is that the current crop of bubble models are incorrectly calibrated when we take into account what ACTUALLY happens physiologically, to a diver during such an ascent.

In other words, in order to remain safe, bubble models will need to make ascents SLOWER -- a lot slower -- in order to clean up this bias they have to overload slow tissues. I'm not a deco scientist but from what I'm reading, it seems plausible that the algorithms could be calibrated by using different bubble parameters for different tissue speeds (currently they do not), but the fact will remain that the algorithms, once properly calibrated, will by necessity, result in slower ascents, not faster ones.

All of this is very interesting to technical divers but to someone reading this thread and wondering about the utility of "deep stops" in the context of a no-stop dive I'm sure it comes across as a bunch of incomprehensible hocus-pocus. That's why I think it's good to keep sharp about defining when we are talking about a "deep stop" (where this thread started) and a "deep ascent line" from a decompression dive. They are two different animals that require two different discussions. They shouldn't be confused.

R..

I think you have your gas saturation and bubble models wrong the wrong way. See Thalmann algorithm - Wikipedia, the free encyclopedia, VVal-18 (Thalmann) is the gas saturation model (ie traditional ish shallow stops) whereas BVM(3) was the bubble model in the NEDU study resulting in deeper initial stops.

It was suggested that VPM-7 (every conservative VPM) was similar to the BVM(3) (Navy bubble) model by many more qualified than me on the deeps stops thread. So I am using that to dispute scubadadas claim that navy models are always less conservative than comercial models. In particular Ross (author of multideco) was claiming that the navy profiles had twice as much deco as required.

There is also interesting history in the relative conservatism of the navy models over time and how they got where they are.

I agree that whether you do your first stop at 30m or 9m after a 60m dive is unlikely to be practically interesting to the OP yet, but he has just decided which to do when it comes to it.

 

[dmaziuk]

How many dives did those divers do

One vacation/year diver: a week of 3-tank dives is 21, which we can round to 20.5 and claim 1/10,000 chance of DCS. 2 diving vacations of one week each and it'll round down to 1/100,000. It's not exactly rocket science.

You've read the deep stop threads, I'm sure you've see the links to the papers where similarly low DCS estimates were cited. You may have also noticed e.g. Dr. Mitchell's post #302

Klingmann et al 2003 reported 11 cases of IEDCS in 9 divers all of whom (100%) had a major right to left shunt. Cantais et al 2003 reported 34 cases of IEDCS of whom 24 (~71%) had a major shunt and another 4 (~12%) had a lesion that did not spontaneously shunt; a total of 83% with a right to left shunt. It is notable that the same authors found a major shunt in only 12% of control divers who had never suffered DCS. Ignatescu et al 2012 reported 30 cases of IEDCS who underwent PFO testing in which 24 (80%) were positive. Peter Wilmshurst told me recently that he thinks it is virtually 100% and that the few IEDCS victims who don't show a PFO just didn't have their test done properly!

I would think that when "9 out 11" DSC cases are attributed to individual physiology while "1 in 10,000" gets bent diving a computer/table, it's fair to say that
the factors that will get you bent are most likely your individual physiology and not the math model running inside your dive computer (behind your dive planner's numbers).

And I did say it.

Whether DAN (BSAC, whoever) numbers are representative is more of an issue, but not for the reasons Andy stated. The problem, again, is that the numbers are so low as to be comparable to the measurement & calculation errors. But that's also not the point: the magnitude is. One in umpteen-mumble vs. 80-100% is I think quite clear. Regardless of whether the "mumble" is 10^3 or 10^5.