Which ascent profile to use when?

[PaulChristenson]

DESCRIPTION: A collection of technical topics in diving are discussed in a fourteen part series, with each topic self-contained and strategically developed in relationship to diving. Topics include energy and matter interactions, thermodynamics, pressure and density, mechanics, gas kinetics, free and dissolved phase transfer, nucleation and cavitation, bubbles and surfactants, oxygen dose, gas mixtures, buoyancy, gauges and tanks, compressors and regulators, maladies and drugs, statistics, risks and probability, binominal distributions, waves, transports, currents, geology, oceanography, geophysics, solar energy and radiation. A suite of application problems (with solutions) is provided for the fastidious reader.

"Those who are interested in diving beyond recreational limits will want to add this comprehensive reference to their shelves."
—Immersed Magazine

"Technical Diving in Depth is on our must read list and is a great reference tool that can be used by all"
—Tim O'Leary, NAUI Director, Technical Training Operations


ABOUT THE AUTHOR(S): Bruce Wienke is an Instructor Trainer/Technical Instructor with the National Association of Underwater Instructors, has served on the Board of Directors (Vice Chairman for Technical Diving, Technical and Decompression Review Board Member), is a Master Instructor with the Professional Association of Diving Instructors (PADI), and Institute Director with the YMCA, and is an Instructor Trainer/Technical Instructor with Scuba Diving International (SDI/TDI).

All you ever wanted to know...
http://www.scubadivingbooks.com/index.htm

 

[Doc Intrepid]

Double-digit ascent times would seem to be desirable. Why wouldn't you guys apply the Marroni results and increase your ascent times to neurological tissue half times? What am I missing?

The Marroni paper makes me wonder if ascent rates you use are too slow....

Couple points to note:
1. The Marroni dives are conducted on air (or N2 = 79%)
2. The Marroni dives are recreational profiles.
3. The Marroni studies are 'small-N' studies.

I have nothing but respect for researchers in this field who conduct well-designed experiments controlled for internal and external validity. Despite the challenges inherent to testing with humans, more research can always be fruitful and may indeed lead to safer dives for all divers.

That said, note that the authors of the study confirm that the incidence of DCI has historically remained remarkably invariant over the preceding 40 years, globally, despite significant variance among such variables as ascent rates and the introduction of mixes such as nitrox. Clearly there is a toleration for variance within the phenomenon, or one would expect to see a more linear corellation between changes in sport diving habits and associated changes in DCI incidence over a 40 year period.

Remember also that dive planning must encompass physical environmental parameters. Dynamic environments, such as 70 miles offshore in the north Atlantic, include currents, waves, and most notably, rapidly changing weather conditions. Dynamic environments may force deco decisions that would not be made in more static environments (such as caves). Weather may force divers out of the water before optimum deco can be accomplished. Planning dives with 'extra-long' deco (for insurance) may not be prudent, as depending on which mix of gas is being breathed at which ATA, too long may be as problematic as too short. There is no "optimum" way. There are only tools in the toolbox, which the diver selects according to the task at hand.

My dive profiles and ascent rates are planned in conformance with the shape of the deco curve, taking into consideration my environment, the gasses and depths, and always the weather. They have worked well for the team of divers I dive with, and I'll likely continue to use them accordingly.

Here are some other resources you may wish to peruse. There are also papers on the same website, for $5 each, however, I suspect you may be better off with one of the broad-view books first.
http://www.rgbmdiving.com/site/bestpub.htm

Best,

Doc

 

[limeyx]

You are also correct that deep stops control bubble formation. Distributing some of your stop time deeper than 15' is a prudent way to control bubbles deeper in your profile.

The "essentials" profiles work well IMHO, despite their apparent contradiction with the Marroni paper.

Using how you feel post dive is a better measure of success than any theoretical "best way".

This is an interesting discussion, and I'm definitely going to bring it up with Gideon when we take our next class with him.

We've been doing the same 10 feet/min from 1/2 depth that GUE teaches in DIR-F ( and is taught in essentials), but have been adding a 6 minute ascent from 20 feet (if we know we are close to the boat etc and not drifting).

I wonder how practical it would be to get our own doppler measurer ?

 

[Charlie99]

The "essentials" profiles work well IMHO, despite their apparent contradiction with the Marroni paper.

No contradiction really. Note that the slowing of ascent in the Essential profile starts at 50% of max depth. The ascent in the Marroni paper that came out so poorly was a 10fpm ascent starting right at the max depth. Big difference. (The profile taught in many DIR-F classes, 10fpm ascent from 80% max ata is much closer to the "bad" profile in the DAN Europe study, since the ascent is slowed at a depth before offgassing starts. A simple run of bubble model programs such as VPM or RGBM will easily show that.)

Were the Essentials profile applied to the Dan Europe study, the ascent from 82' would have slowed at 40', the same point the ascent is slowed in the dir-diver.com ascent. The point where the Essentials profile DOES deviate from Dan Europe findings is that they found either a deep stop, or a 6m/20' or both would provide even less doppler bubbling.

 

[Charlie99]

We've been doing the same 10 feet/min from 1/2 depth that GUE teaches in DIR-F ( and is taught in essentials), but have been adding a 6 minute ascent from 20 feet (if we know we are close to the boat etc and not drifting).

In my long, long post above, I recommended that we look for the SIMILARITIES in the various profiles known to work. Adding that 6 minutes shallow makes the overall ascent more like the shape of required deco from a long, deep dive. That "shape of deco" curve can also be seen in VPM by cranking up the conservatism very high.

That 6 minutes also makes the ascent much more like the DIR-DIVER.COM ascent.

In even simpler terms, that added 6 minutes is similar to a PADI safety stop. Continued offgassing of fast tissues, while at a shallow enough depth that the ongassing of slower tissues isn't relevant.

 

[Charlie99]

Couple points to note:
1. The Marroni dives are conducted on air (or N2 = 79%)
2. The Marroni dives are recreational profiles.
3. The Marroni studies are 'small-N' studies.

4. The Marroni studies didn't include the ascent that I wanted to have tested.

On a more serious vein, note that none of the 8 tested profile match up cleanly with common ascents, other than PADI 30fpm or 60fpm, followed by safety stop. I have no idea how they chose 15m/50' as the deep stop point for a 25m/82' dive, but that is a bit deeper than I would expect to be optimal. If I happen to acquire a chamber, a few researchers and a bunch of test subjects, then I'll get to do the tests I want, but until then I'll just have to accept what limited data is available.

p.s. 1a. Most recreational dives are conducted on air.

 

[TSandM]

I just want to say that this kind of thread is why ScubaBoard is such a gem -- intelligent, educated people discussing a wide-ranging set of sources regarding a controversial issue. Thanks, you guys! I print threads like this out and keep them.

 

[Charlie99]

Double-digit ascent times would seem to be desirable. Why wouldn't you guys apply the Marroni results and increase your ascent times to neurological tissue half times? What am I missing?

Boredom and impatience, and diminishing returns on safety. How safe is safe enough? Why not hang yet another 5 minutes? At some point you have to draw a line and get on the boat. That's why I plan my ascent in terms of a time and gas budget. I pick my total time of ascent, and then work backwards from there (See the long, long post above for details)

2- Something I've picked up is that bubble theory requires controlling off-gassing rather deep, and that dissolved gas theory requires
controlling it shallow. I'm guessing that this is the theoretical difference between Profile 3 and the other two.

Ignoring that those profiles are probably rather arbitrary as opposed to truly being model based, your understanding of how dissolved gas theory affects an ascent is not exactly true. http://www.gap-software.com/staticfiles/deepstops.pdf is an interesting article by Erik Baker that explains the gradient factor method of generating deep stops. It is described using only a dissolved gas model. One way to describe the ascent is that it trades off the overpressure gradients of the various compartments, both fast and slow.

Take an ascent that goes straight to the either surface or to the first required deco stop, and analyze it using a dissolved gas model. It is readily apparent that that the fastest compartment is taken right to the maximum limit. The gradient factor adjustment of the dissolved gas ascent simply trades off a dramatic reduction in max overpressure of fast compartments for a small increase in the maximum overpressure in the slow compartments. 100% dissolved gas calculations. Very similar to the "shape of deco" curve generated by VPM or RGBM. Similarities like this probably mean we are on the right track.

Unlike Bruce Wienke, Erik Baker writes in a clear, concise fashion. In the same way that Newtonian mechanics adequately describes much of our real life experience, and in the same way that a good understanding of Newtonian principles is a necessary foundation for study of relativistic phenomena, it is best to have a good understanding of dissolved gas, aka Haldanian, aka neo-Haldanian, aka Buhlmann models before attempting to understand dual phase, aka bubble models, such as VPM and RGBM.

A good starting point is Erik Baker's Understanding M-Values article.

 

[PaulChristenson]

Nevermind...

 

[Doc Intrepid]

p.s. 1a. Most recreational dives are conducted on air.

I failed to make my point clearly enough...

Big Tuna specifically inquired:

Double-digit ascent times would seem to be desirable. Why wouldn't you guys apply the Marroni results and increase your ascent times to neurological tissue half times? What am I missing?

The Marroni paper makes me wonder if ascent rates you use are too slow....

My response is (in part) that I'm diving helium mixes, not air - as was used by Marroni et. al.

Which impacts my ascent rates.