Where does the 60ft/minute ascent rule come from?

[boulderjohn]

There is a amazing lack of clarity in the way in which this sort of information is taught. It has often led me to believe that many Instructors don't really understand it themselves.

I wonder about that as well, and when I saw this comment I had a bit of a quirky thought that I would like to toss out.

I have been mystified by the many posts in various threads by instructors who say that they can't teach decompression theory without teaching tables as well. It mystifies me because I have never taught decompression theory in conjunction with tables and don't understand why it is a necessity. I always taught decompression theory as a concept right near the beginning of the first class. Later in the class I show them how tables guide us through the decompression process.

I am now wondering if teaching decompression theory along with tables actually interferes with teaching decompression theory. When you teach both of them together, it is a lot for students to learn all at once, and I wonder if they aren't really focusing on the mechanical process of following the table guidelines to the point that they are missing the theory that lies behind them.

I further wonder, in the context of the remark above, if the people who say they can't teach decompression theory without tables understand it well enough to teach it by itself. If they have been doing the two together long enough, they may have forgotten key concepts themselves. (BTW, decompression theory, at least to this level of understanding, is required for both the DM and Instructor certifications in PADI, and I assume other agencies as well.)

 

[rjack321]

There is a amazing lack of clarity in the way in which this sort of information is taught. It has often led me to believe that many Instructors don't really understand it themselves.

There's a bit of a problem (with teaching/learning) once you start considering that you're <always> bubbling on ascent/deco/afterwards. So this:

But even with the most complex curve, you have a shape where you get the fastest outgassing just before you get bubble formation, in fact bubble formation slows outgassing because it reduces the gas tension(s).

Should really be more like:

But even with the most complex curve, you will always have some measurable doppler bubbling. Despite even the slowest ascent you'll always have a shape where you get the fastest dissolved outgassing just as you're gettting intolerable bubble formation and size increases due to the reduced ambient pressure. Unfortunately excessive bubble formation & size slows the net outgassing since it reduces the gas tension(s) so once you cross the line (M-value) between tolerated dissolved tissue tensions and intolerable tensions, symptoms tend to increase in likelihood and frequency. So not only do you need to control the dissolved offgassing in your ascent but you need to control the increases in bubble size, these factors are working against one another. Dissolved offgassing is maximized shallow where bubble formation & size is also maximized. Bubble size is minimal at depth and bubbles efficiently redissolve at depth as well, but dissolved offgassing is minimal at depth as well. Unfortunately variation between individuals is very high and day to day variation within one individual can shift the line between no symptoms, subclinical symptoms, and clinical DCS considerably.

Roughly, there's VPM added into your dissolved theory.

 

[Thalassamania]

John, you're right on the money. I teach decompression in three stages. First comes theory moving from derivation from first principles of the molecular model for an ideal gas up through tissue half time, surfacing ratios and M-value (sounds kinda highfalutin, but you can drop all the big worlds and use naive physics to get through even to a ten year old). My second stage is dive computers, because they follow cleanly from the theory section, no big deal. Then we tackle tables on the basis of being nothing more than time slices of what the computer would be telling you. Could you eliminate the last step? Sure. But I think it provides both a useful tool and good reinforcement of concepts.

Rjack: Thanks for the expansion.

 

[rjack321]

John, you're right on the money. I teach decompression in three stages. First comes theory moving from derivation from first principles of the molecular model for an ideal gas up through tissue half time, surfacing ratios and M-value (sounds kinda highfalutin, but you can drop all the big worlds and use naive physics to get through even to a ten year old). My second stage is dive computers, because they follow cleanly from the theory section, no big deal. Then we tackle tables on the basis of being nothing more than time slices of what the computer would be telling you. Could you eliminate the last step? Sure. But I think it provides both a useful tool and good reinforcement of concepts.

Rjack: Thanks for the expansion.

You're welcome although re-reading it now I'm not sure its as succinct as it could be.

Do you teach bubble mechanics? At what point in the curriculum? When I was initially taught deco "theory" we probably spent more time on the surface tensions vs. sizes of bubbles at different depths as we did on dissolved gases.

 

[boulderjohn]

Do you teach bubble mechanics? At what point in the curriculum? When I was initially taught deco "theory" we probably spent more time on the surface tensions vs. sizes of bubbles at different depths as we did on dissolved gases.

I do not teach it at the OW level except to say, when done with the explanation of the dissolved gas theory, that things are actually more complicated than that and there are existing bubbles that need to be dealt with. The problem is that there are conflicting theories about how to deal with them. If I were to go into much more deeply than that, I would have to compare the different theories in order to be accurate, and that would take a lot of time teaching something that I don't think they need to know at this point in their diving careers.

 

[mbacarella]

You're obviously going to struggle with the "art" parts of deco.

Ha!

30ft/min with a 10ft/min slowing (either as one 15ft stop or sliding through the ~30ft to 0 ft zone) is pragmatic and it works. Many of us start slowing down to ~10ft/min at roughly 1/2 our depth.

Gas and conditions permitting, the more you can shape your ascent like an exponential curve the better, always remembering you <could> surface directly. This is a function of the pressure changing most rapidly the closer you get to 1 ATA to have a similar pressure change over time that you did at (e.g.) 2.5 ATA you have to slow. (among other reasons).

I'm confused. You're say exponential but it seems like you're describing a logarithmic curve?

So what I'm gathering from this thread is that not only don't people put too much faith in the models, but that there isn't even a compelling heuristic.

Maybe there's space for someone to solicit dive profiles from a few hundred thousand divers via, oh, popular scuba forums, and performing a regression analysis to find the characteristics of a safe dive? The survey would have to ask many, many questions but with enough data something could come of it. Even if it doesn't lead to a better model, it'd probably at least help come up with a better heuristic.

Yes, my feeling is there isn't nearly enough data.

 

[Kern]

DAN Divers Alert Network : Medical Research : PDE

V-Planner Live dive database

 

[ucfdiver]

I'm confused. You're say exponential but it seems like you're describing a logarithmic curve?

Either you're trying hart to pick a fight, or you missed something in high school algebra.

 

[rjack321]

I'm confused. You're say exponential but it seems like you're describing a logarithmic curve?

I'll WAG base e will work pretty well.

 

[Thalassamania]

Logarithmic curve:

Exponential Curve:

Any questions?