Riding GF99 instead of mandatory/safety stops

[L13]

Saying what you are doing is one thing; suggesting why based on total delusion is another.
You really should not mislead people on matters of safety.
You are saying that reputable, fact-based, experience-tested, expert consensus-agreed decompression models provide no useful information, a "crap shoot."
You are wrong, and you are misleading readers by claiming this.

Thank you for calling out the nonsense!

 

[Wallowa]

Well, actually, there is: it's consensus-agreed that gas is driven by the pressure differential, that log(2) approximates its kinetics well enough for practical purposes, that overall gas kinetics for the entire body is approximated by taking discrete "slices", and that at certain values of delta-P the clinical DCS becomes very likely.

Most of the "differing algorithms" differ in the details of what values of delta-P are "too risky" and exact details of how they compute them. (And what bubble models actually do, all their nuclei crushing pressure voodoo notwithstanding, is compute M-values differently.)

Not doubting that some of the parameters are accurately described but with the use of "well enough for practical purposes" ,"become very likely", "differ in details of how to compute them" and "compute M-values differently", it is reasonable to recognize that there are still a lot of unknowns that could injury and 'close' may not be good enough. The function and physiological or physical responses of nuclei for example in respect to DCS is not voodoo but another exemplar of how poorly we can describe and verify the role of a significant factor in decompression modeling. Another example is to downplay the damage that micro-bubbles independent of nuclei cause in cells. We should look at everything without prejudice or being dismissive. There is still a lot to learn.

Does ‘dive computer science' construct valid means to lessen DCS? Of course it does. However, what is accepted today as optimally safe computer-generated profiles will change as more questions are asked and answered. Again, not static. That has been, and will continue hopefully, how better computers and algorithms will be developed to lessen symptomatic and asymptomatic diver injuries. Never 100% as each dive differs from the last, human physiology continuously varies, environmental states are always in flux and equipment induced factors are largely unknown and changing. Homeostasis is a myth; change is the rule, moment to moment the parameters shift.

Just my thoughts, so don’t take umbrage. “No divers were injured in the writing of my views”.. Those reading the forum threads are capable of making their own informed decisions.....if they are "mislead" by opinions, that is on them.

 

[rjack321]

And of course Rjack’s addendum was inaccurate as well….use of oxygen on safety stop is a proven manner to wash out inert gases….pretty certain ‘tech” divers use this to avail themselves of the ‘oxygen window’ benefit with 100% oxygen….but Jack knows that.

Show me one study that actually put recreational <NDL divers on O2 at a safety stop for 3 to 5 minutes and showed a reduced incidence of DCS. Heck even a doppler study or with rats, rabbitss or beagles. It's not proven to be relevant - in part because the rates of DCS for recreational divers are already extremely small and measuring the difference (if it even exists) between 1 in 7,000 dives and 1 in 8,000 dives is ridiculously expensive and not worth anyone paying for. And the actual time on O2 at a safety stop is similarly tiny.

As pointed out to you MANY times before, whatever value 100% offers during a 3 minute safety stop is functionally reproducible by doing a 6 minute safety stop. Although neither is going to substantively change your overall gas load in the controlling tissues and thus the net rate of DCS is going to remain within the margin of error anyway.

 

[dmaziuk]

Not doubting that some of the parameters are accurately described but with the use of "well enough for practical purposes" ,"become very likely", "differ in details of how to compute them" and "compute M-values differently", it is reasonable to recognize that there are still a lot of unknowns that could injury and 'close' may not be good enough.

This is all good and well but it's the textbook case of perfect being the enemy of the good. Until they figure out exactly what causes DCS in people, we'll have to live with models that offer statistical approximation of the outcome. There's one thing about these kinds of models that's been proven over and over again all over the place: Keep It Simple Stupid.

Of the two models that get the divers out of the water equally "not bent", the simpler one wins every time.

 

[old frogman]

You are mixing up doing multi-level dives with changes in on-gassing due to reduced pressure gradients as you multi-level your way up - with changing your GFs which is altering your allowable offgassing as you ascend.

You can just leave the GFs alone. There is no demonstrated need to alter them throughout the dive.

PS the VR3 is absolutely ancient technology

Agreed, you do not have to change the GF during the dive. However, why does the shearwater computer have a facility to change the GF during the dive?

The VR3 computer may be considered superseded by OLED screen dive computers. However, the VGM algorithm was released in the mid 2000s and appears to be more sophisticated than a Bhulmann algorithm with GF (Bhulmann algorithm was released in early 1980s and the GF was released in mid 1990s). In fact, Bhulmann with GF is an attempt to replicate bubble model decompression theory in deep diving application.

I suspect that many divers including recreational divers are using OLED screen computers because they are easy to read in all light conditions. The algorithm is probably a secondary consideration in many divers' mind.

In addition, I understand that the Shearwater computers are available with alternate algorithms such as, VPM (bubble theory model) and DCIEM (ultra conservative Haldane theory model).

Finally, decompression is a crap game. Crap is a game of chance. Chance is taking a risk. The US Navy has published risk rates for using their tables in conservative mode, designed mode and overextended mode. The fact that decompression is a crap game is supported by the presence of recompression chambers on naval diving vessels and civilian industrial/construction diving vessels.

Risk management 101 - You cannot eliminate risk, you can only reduce risk to a level the is as low as reasonably practicable, tolerable and acceptable.

 

[tursiops]

Finally, decompression is a crap game. Crap is a game of chance. Chance is taking a risk. The US Navy has published risk rates for using their tables in conservative mode, designed mode and overextended mode. The fact that decompression is a crap game is supported by the presence of recompression chambers on naval diving vessels and civilian industrial/construction diving vessels.

Risk management 101 - You cannot eliminate risk, you can only reduce risk to a level the is as low as reasonably practicable, tolerable and acceptable.

To be fair, when one is taking a risk that does not mean that all possible outcomes are equally likely. Even with Craps, all outcomes are not equally likely. Referring to decompression models as no bette than a crap shoot is wildly in accurate: the most likely outcome of following a modern decompression computer in recreational diving is maybe one chance in (say) 10000 that you will get DCS. Those chambers on Navy and commercial diving ships ae there because the risks are higher for that kind of diving.
Saying deco models are a crap shoot is just ignorant of the facts.

 

[boulderjohn]

However, the VGM algorithm was released in the mid 2000s and appears to be more sophisticated than a Bhulmann algorithm with GF (Bhulmann algorithm was released in early 1980s and the GF was released in mid 1990s). In fact, Bhulmann with GF is an attempt to replicate bubble model decompression theory in deep diving application.

The VPM algorithm was all the rage for many years, and, yes, GFs were added to Bulmann to replicate bubble models. However, that all changed quite a while ago, and the main reason is that the VPM algorithm is now believed (based on a NEDU study) to create first stops that are too deep. Expert opinions now favor initial stops that are deeper than pure Buhlmann but not as deep as VPM.

In addition, I understand that the Shearwater computers are available with alternate algorithms such as, VPM (bubble theory model) and DCIEM (ultra conservative Haldane theory model).

About a dozen years ago, a survey of tech divers on ScubaBoard showed overwhelming support for VPM. A popular computer then was the Liquivision, X-1, which could be run with a VPM program. Today a poll would likely find very little support for VPM in the tech community. Yes, you can get VPM on a Shearwater, but I don't know anyone who has it. Every tech diver I know uses Shearwater with Buhlmann GFs nowhere close to a replication of VPM.

 

[old frogman]

To be fair, when one is taking a risk that does not mean that all possible outcomes are equally likely. Even with Craps, all outcomes are not equally likely. Referring to decompression models as no bette than a crap shoot is wildly in accurate: the most likely outcome of following a modern decompression computer in recreational diving is maybe one chance in (say) 10000 that you will get DCS. Those chambers on Navy and commercial diving ships ae there because the risks are higher for that kind of diving.
Saying deco models are a crap shoot is just ignorant of the facts.

Crap like decompression can be managed conservatively or aggressively depending on how much you want to bet.

 

[old frogman]

The VPM algorithm was all the rage for many years, and, yes, GFs were added to Bulmann to replicate bubble models. However, that all changed quite a while ago, and the main reason is that the VPM algorithm is now believed (based on a NEDU study) to create first stops that are too deep. Expert opinions now favor initial stops that are deeper than pure Buhlmann but not as deep as VPM.

About a dozen years ago, a survey of tech divers on ScubaBoard showed overwhelming support for VPM. A popular computer then was the Liquivision, X-1, which could be run with a VPM program. Today a poll would likely find very little support for VPM in the tech community. Yes, you can get VPM on a Shearwater, but I don't know anyone who has it. Every tech diver I know uses Shearwater with Buhlmann GFs nowhere close to a replication of VPM.

The famous 2008 NEDU test was designed for evaluating VPM for Navy diving not technical diving, so we do not know if any form of conservative adjustments were made as technical divers generally do. The outcome was that it scared a whole bunch of divers into moving into other algorithms e.g. Bhulmann GF. Also, I believe it was diving with just air and to a depth of 170 ft. Form a technical diving perspective it is questionable/controversial.



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