Deep Stops Increases DCS

[manni-yunk]

Hello,

This is a fair question, and to be honest, my playing a "look he made a fundamental error" game feels a bit childish for my own liking. However, you have to understand that these debates with Ross about David's study have played out over years now, and have involved hundreds of time consuming posts on multiple forums. We have generally afforded Ross the courtesy of debating him as though he is someone with a legitimate viewpoint that has to be worked through and properly examined. On this basis genuine progress has been made, and I do think the community is better educated about the issues as a result.

However, you (meaning me) do get a bit worn down by it all. Ross frequently becomes overtly abusive, and some deep flaws in his arguments are extremely difficult to tease out in a clear understandable manner because his confident pronouncements and copious production of complicated looking diagrams are often sufficient to sow the seeds of doubt in readers who are struggling to get their heads around it.

In this milieu, when he produces one of his trademark diagrams that is clearly wrong, then the temptation to point that out is obviously strong. This is particularly so when it is obvious that this diagram is a prelude some some new tangent he is going to go off on in an attempt to discredit a presentation on deep stops I recently gave using similar diagrams to illustrate some basic principles.

The fact that he made a major error in a very basic definition (of supersaturation) is undeniable. You would not know it now though because he changes the diagram on his website which means it updates on this forum without a change in the edit time / date stamp of the post itself. Luckily I kept a copy of the original before it changed (see my post 854).

You ask "did he try to deny it" and the answer is unequivocally yes. His original diagram appeared in post 823. In post 824 I pointed out the error. He changed his diagram on his website to remove the error and it updated on this forum without a change in time and date of his post. Then, in post 829 he quoted my post about the error and wrote "Really? David seems to think it is OK now". Anyone reading the thread, seeing no evidence of the error on his diagram, and looking at the sequence of posts and their dates / times would therefore be very confused about what I was making a fuss about and would have thought his denial was legitimate.

I put it to you that this was not an example of a discussant allowing persuasion to change their mind and legitimately acknowledging that change (which I completely agree is laudable). Indeed, I believe it was disingenuous in the extreme, and we have seen this sort of thing before in these debates. Hence I gave in to the temptation to point it all out.

Simon M

Thank you for the reply. I understand your point about the situation and how it played out over the years.Although I dont see his "changing" or updating of the diagram as a denial in any way - I see it quite the opposite. That being said, understand the history of the debate. I agree that the technical diving community is much more educated on the topic overall as a result in the debate occurring in multiple forums so there is value to these conversations and thank you for that.

To be honest, Im really struggling with the whole topic. Unfortunately I do see and understand Ross's points about what was studied and how the profiles that were used are not directly indicative of an actual real life VPM profile, therefore any conclusion drawn from these dives may not be the same as an actual VPM dive.....

On the other hand, I see the science pointing towards the elimination the mindset of deep stops.

One of the problems I see is that not everyone is working from the same exact definition of a "Deep Stops". What I mean by that is I talk to people that dive my boat and they tell me they recently read all of these threads and switched from VPM and went back to buhlman and are using GF to get shallower faster. They seem excited as their overall run time is about the same but they are excited that the science has pointed to a safer way to decompress. Then when I ask what GF they are running....their profiles are almost identical to what I can see from VPM +2 or +3 in terms of where their stops are beginning. I wish the industry would agree to a standard set of defined terms. Then, whether you still subscribe to deep stops or not....at least we are all talking the same language.

I recognize that many of the experts on this thread have attempted to define many of these terms, but it doesnt seem to have become "sticky" among divers yet....

All that being said....Ive recently (recently being 3 months ago) adjusted my GF to a 60/75 for the deeper dives and so far so good! Up until 2 years ago I was diving VPM +3...and until late last year I was diving GF 30/70.

 

[shoredivr]

This seems categorically incorrect. Im not defending Ross here, Ive never met him and dont know him at all - but - from a motive standpoint - I cant find any motive for him to fight one way or the other. Whether you choose to use a GF software, or a VPM based software for your dive planning - he has both options and from my own experience.....its the one that most people use. So he doesnt care what profile you use....he gets paid either way.

The earliest references I've found to VPlanner being used by divers is in 2005.
The NEDU study was presented in 2008 IIRC.
That's at least three years of "allegiance" to bubble models before they were challenged by NEDU's study.

Looks like Multideco hit the market in 2011 or thereabouts.

Way more customers using VPlanner. Certainly I have noticed this on dive boats here.

*edit* Obviously I'm speculating, because nothing really explains the obfuscation and belligerence that Ross has been posting in the RBW thread and here, towards the NEDU study, Dr Mitchell, Dr Doolette and UWSoujourner. For years.

 

[lowviz]

...//... I wish the industry would agree to a standard set of defined terms. Then, whether you still subscribe to deep stops or not....at least we are all talking the same language. ...//...

This appears to be the crux of the matter. One usually defines the problem before one begins to work against it.

I have been following this thread with great interest. I dive recreational depths but am no stranger to deco. In my limited comprehension of this topic, it appears (to me) that the off-gassing ceiling would be everyone's first sensible deep stop.

Could this be a place to start?

Is it remotely possible that all the experts would agree on the placement of the OGC for any given dive? If so, then someone please define it. Where is the OGC? If there is no agreement as to this depth, I would be insanely interested as to who disagrees and why...

 

[KevinNM]

In my limited comprehension of this topic, it appears (to me) that the off-gassing ceiling would be everyone's first sensible deep stop.

I don't think there is an off-gassing ceiling, as off-gassing continues after you reach the surface. Do you mean on-gassing?

 

[lowviz]

No, off-gassing. Say you spent X minutes at Y depth. How far do you have to ascend to get to the deepest point where the net result on your body is off-gassing?

 

[boulderjohn]

I fully agree that one of the problems we have is that different people are using the term "deep stops" differently.There were several posts in which that is specifically stated. I feel like I am in the scuba equivalent of that old Crocodile Dundee movie when Dundee and his girlfriend are the victims of an attempted mugging:


I feel like people should be saying, "No, that's not a deep stop. Now, THIS is a deep stop."

So, I am going to start a new thread about defining it in some way so that can have a common language.

 

[UWSojourner]

I think the quest for an exact definition of "deep stop" will come up empty. If you worked through the RBW thread, you'll see that at some point I started using the more relative terms "deeper stops" and "shallower stops", or "deeper stop profile" and "shallower stop profile". It was just my conclusion that a bright line test would likely fail no matter what line was drawn. And, of course, a bright line once drawn is worth a few hundred posts arguing about the line. In contrast, the terms "deeper" and "shallower" are much more easy to establish when comparing profiles generated by various models.

For example, the NEDU profile A2 generated by the navy bubble model had a 1st stop at 70ft, the LANL profile 120ft, the VPM profile 100ft. All of these are "deep stop (bubble) models", but with various 1st stops. Dr. Doolette highlighted the issue when discussing gas-content vs. bubble models here. His list of all the unknown parameters in bubble model development that have to be used somehow (or disregarded via simplifying assumptions) is pretty long. Each bubble model's approach to answering these unknowns produce different output, but all within the banner of "bubble model".

So perhaps the answer is in abandoning the absolute terminology in favor of more relative terms that are certainly easier for *most* to agree on when comparing specific profiles.

 

[cerich]

No, off-gassing. Say you spent X minutes at Y depth. How far do you have to ascend to get to the deepest point where the net result on your body is off-gassing?

just gonna point out upon assent you start to off gas, stops ideally are to stop you where(depth) the off gassing is most rapid without getting out of control (too fast) and bending you.

 

[emmbee]

If you think papers are invalid do the formal review and rebutal.

Sorry if I'm late. I just had a look at the 2010 paper.

The instantaneous decompression stress for "Haldane models" (I quote), is given as (broadly)

rho = (ptissue-pambient)/pambient

So, according to the author, stress while compressing is negative...

The whole dive decompression stress is the sum (integral) over the whole dive of this instantaneous decompression stress. Right-o.

It follows that, again according to the author, compression counts as credit during decompression. That may be true, but "Haldan[ian] models" simply do not do this.

So the "Deep stops clobber traditional M-values." comparison is trivially completely unsubstantiated: it simply isn't how M-values work. To have any validity, rho should have been floored at 0.

I hasten to add that this problem may not have been apparent to a reviewer unfamiliar with decompression theory.

Minor point. Ireland is a small country. There's quite a few PADI divers. There's the local CMAS chapter in the ROI (CFT, Irish Underwater Council, IUC, in English) - we use Buhlmann. There's BSAC in NI. Irish Diving Federation? Never heard of them.

Cheers,

Matthieu

 

[lowviz]

just gonna point out upon assent you start to off gas, ...//...

Only if I'm saturated.

If I go up a foot on a typical dive, I'm still on-gassing...