Deep stop rule.

[kr2y5]

I've been reviewing some Fundies materials, and I came across a "deep stop" rule of 65% ATA for no-deco (and even more for deco) dives. The rule seems to be easily generating stops below the off-gassing ceiling on shorter dives, which seems undesirable... just wondering how this is dealt with, and whether this rule is actually used in practice.

 

[rivers]

I've been reviewing some Fundies materials, and I came across a "deep stop" rule of 65% ATA for no-deco (and even more for deco) dives. The rule seems to be easily generating stops below the off-gassing ceiling on shorter dives, which seems undesirable... just wondering how this is dealt with, and whether this rule is actually used in practice.

It's 65% of max ATA or 50% max depth (general rule of thumb). For example, complete a min deco dive to 30m, your first stop is at 15m

 

[Imla]

Personally I have just considered the "stops" for ND-dives as more of a "slowed ascent" than full on stops. I don't think a 30sec stop at 15m (Or whatever depth you start at...) will cause a major shift in risk considering it is a ND-dive after all.

 

[kr2y5]

Ah, these are the 30-second stops, so just a way of enforcing 10 feet/min. ascent rate... thanks, that makes sense. /thread

 

[kdupreez]

That off gassing ceiling is heavily dependent in which algorithm and conservancy factors you apply..

GUE min deco is a very pragmatic approach on what we know what works well in practice and also try and accommodate the best of bubble and dissolved gas worlds..

Everything wil be explained on depth in your fundies class and as well as why the min deco curve looks the way it does..

I would not break my brain too much until then about something that's pretty much an empirical science based on law of averages where personal physiology plays a big big role...

If you really are interested, get the book by mark powell on decompression for divers.. Super awesome book in plain English for us mortals.

In class you wil very quickly learn that it's pretty much "measuring with a micrometer, and then cutting with an axe" when it comes to deco black magic...

 

[DaleC]

I would add that during our recent fundies course min deco and deep stops was explained in a pretty easy to understand way. Even if you don't agree with the modeling perse, the what and why and practical application is laid out so that it all makes sense.

I'm no expert but I understood the stops do reenforce a controlled ascent rate but they are also part of VPM modeling that chooses to slow bubble growth instead of the Haldane model of bend and mend (creating large gradients to drive offgassing) which PADI style ascents do (go shallow and hold). I'm not arguing which is more right but during the course all that stuff is laid out for you. What you probably don't get over the internet is the real world assessment of how closely to follow the protocol and when to flex it.

I wouldn't worry about studying for it. Instead, practice some of the math for tank comparisons volumes conversions from cuft to psi, and ppo2 calculations etc... but even then not to hard. More like getting familiar with how to move back and forth through the formulas. Fundies was curious in the way that the math actually got simpler as we progressed (unlike highschool).

 

[kr2y5]

What you probably don't get over the internet is the real world assessment of how closely to follow the protocol and when to flex it.

That's pretty much all my question was about! I have much faith in the Internet

it's pretty much "measuring with a micrometer, and then cutting with an axe" when it comes to deco black magic...

If the bottom line is +/- 10 feet practically doesn't matter, I'm perfectly satisfied with that answer...

That off gassing ceiling is heavily dependent in which algorithm and conservancy factors you apply.. (...)

I understood the stops do reenforce a controlled ascent rate but they are also part of VPM modeling that chooses to slow bubble growth instead of the Haldane model

...but this confuses me a bit, so let me drill into it, in case I can learn something from the conversation. I always thought that whether a tissue is still on-gassing, or already off-gassing, depends only on the tissue, how saturated it is, and the depth, which is strictly a function of dive profile. It has been my understanding that algorithms differ in how far they allow you to go past that point where off-gassing begins, but that they agree on where that point lies. If my understanding is wrong, I'm glad I asked that question, so that I can clear this up...

get the book by mark powell on decompression for divers

I've read that book, and the above is what I vaguely remember... maybe I missed something?

Everything wil be explained on depth in your fundies class (...) I would not break my brain too much until then

during the course all that stuff is laid out for you. (...) I wouldn't worry about studying for it.

I see you are trying to build up suspense Sorry, guys, I can't help myself... I read when something interests me, and I don't allow questions pickle inside my head, for the fear that my brain might explode.

 

[DaleC]

I'm not quashing your curiosity, I'm like that myself.

I see you are looking at off gassing vs on gassing but my take was that there is another factor at play and that was the size of the bubbles being produced on ascent. Haldanian thought is that you want to create a large gradient differential to drive off gassing, so you ascend most of the way and then do your stops. VPM tries to both create a gradient and control the size of the bubble by moving to half depth (gradient differential) and then slowly ascending (controlling bubble growth).

Although this is the model chosen by GUE I think it could be partly a semantic exercise at simple rec depths. Realistically, you can bend the rules for rec dives if conditions warrant (just like you can on most PADI table dives that don't approach the NDL's) but you still want to cement those skills by practicing min deco. Besides a stand alone concept, min deco is setting the foundation for deco skills that will come into play later on (beginning with the end in mind).

Those are only my own thoughts and I could stand to be corrected if they are wrong. I'm no deco expert.

 

[kdupreez]

..but this confuses me a bit, so let me drill into it, in case I can learn something from the conversation. I always thought that whether a tissue is still on-gassing, or already off-gassing, depends only on the tissue, how saturated it is, and the depth, which is strictly a function of dive profile. It has been my understanding that algorithms differ in how far they allow you to go past that point where off-gassing begins, but that they agree on where that point lies. If my understanding is wrong, I'm glad I asked that question, so that I can clear this up.

Depending on the implementation of the algorithm, I'm fairly certain that they all somewhat vary in Half time, M and modifiers to those values thats used and I think for various tissues and that many manufacturers have slight variances in those and different modifiers on the algorithms and some are fixed gradient factors and some are adjusted, some take into account bubbles and some dont.. all of this affect ascent profiles.

Thats why you will never find 2 dive computers from different manufacturers match 100%... Some are heavily biased to pure Bühlmann models will chase you up shallow to spike gradient to maximize diffusion, some will stay longer deeper to account for bubble growth, some will stop at 70ft with 50 for longer than the shallow ones to maximize O2 window.. etc. etc. etc.

All this said, everything is based on what we "think" works best, there is no real way to figure out when off-gassing really starts in your body..its all theoretical..

GUE profiles are what we call "pragmatic" deco and was built and shaped based on empirical data from both thousands and thousands and thousands of dives between all our members overlayed what the various computer generated profiles seem to think works best AND whats practical and easy to apply, calculate, remember and perform during a dive.. we pretty much then create a linear ascent thats really very easy to remember and calculate.. And its like you said.. we ascent 30ft p/min to half max depth in ft, then stop 1min every 10ft to the surface... super easy.. and pretty much gaurentees that rule will fit into any recreational no-deco dive planned to 100ft on Nitrox32.. but you always only have to remember one ascent profile rule..

The golden key here is that this is a "standards" based ascent profile and dont matter what minor variance in dive profile or gas selection we each have, the entire TEAM will have the exact same deco schedule and ascent profile.. we favor the TEAM cohesiveness above all.. i.e. this leads to zero need for discussion and debate underwater between 3 divers making an ascent.. versus same 3 people diving 3 different dive computers thats all running different software and telling them different things... who is right? did the dive planning computer use the same profile that you based your gas planning on?? In GUE the entire team knows the EXACT ascent plan as soon as the dive is thumbed.. no surprises or debates on the way up.. the bottom phase and deco gas plan can be very very easily calculated in finite detail using such a standards based approach as well..

Will you shave off 3min here or add 2min there using a dive computer.. sure. probably.. or maybe not.. we just dont know.. so we choose to stick with the team and rather all be on the same page and keep things easy and simple

hope that makes sense.. If not.. it WILL make a lot of sense very soon..

 

[kr2y5]

Thanks, Koos, that makes sense.