RGBM & Fast Decent

[CelticRavenVA]

I was curious if a rapid decent (Less then 1 min) to a depth of say 100 feet, had any bearing on NDL's in the RGBM model?

 

[MechDiver]

The sooner you get to depth the sooner you begin ongassing, but otherwise I don't think so.

You control the dive profile, hence loading, by the descent/ascent values you plug into the software. Tables assume a pretty much instant arrival at depth, while a dive computer would give you credit for less offgassing during a slow descent.

I don't believe descent speed would have any bearing unless going very deep.

MD

 

[rcohn]

Fast descent in bubble theory aids in crushing micro-nuclei reducing the number of nuclei that can form bubbles on ascent. Slow ascent allows off-gassing, minimizing growth of existing nuclei.



This is also why bubble mechanics predicts that the deepest dive should be done first. Deepest first, crush the micro-nuclei. Shallow first; expand the nuclei so that fewer will be crushed on the subsequent deep dive leading to a larger fraction of gas in the free phase.



I anticipate that Dr. Deco wil tell us this remains theoretical and that muscle motion creates so many micro-nuclei that any crushing action from depth is insignificant.



Ralph

 

[IndigoBlue]

I was curious if a rapid decent (Less then 1 min) to a depth of say 100 feet, had any bearing on NDL's in the RGBM model?

75 fpm is his (BRW's) stated descent limit. If you rocket down faster, then you would be spending more time at MOD than he has calibrated, and you might void his result on ascent. Better follow the recipe if you plan to use his cookbook.

 

[Diver0001]

....snip.... Tables assume a pretty much instant arrival at depth,

I'm not sure about that. I recall reading somewhere that the DSAT tables were based on a 60ft per min descent rate. It's relevant if you consider the way bottom-time is estabilished....namely from the surface to the beginning of an uninterrupted ascent. If you descend faster than than the model then you're taking on N2 faster than the model predicts. The difference is probably minor and for normal non-stop diving within the 40mtr limit a quick descent could probably be adquately compensated with a longer saftety stop. I hope Dr. Deco checks in with an opinion on how much longer you should stop for.

....snip.... while a dive computer would give you credit for less offgassing during a slow descent.

Dive computers obviously work differently than the tables. They sample depth at fixed intervals (often a matter of a couple of seconds or less) and adjust NDL's on the fly. Maybe you're saying the same thing I am but I'm not sure why you're saying that the computer would give you credit for a slow descent. I'm not aware of any computers that monitor descent rate and use it in the calculations.

R..

 

[CelticRavenVA]

Who is BRW? Sorry.... I am just learning about dive med.

75 fpm is his (BRW's) stated descent limit. If you rocket down faster, then you would be spending more time at MOD than he has calibrated, and you might void his result on ascent. Better follow the recipe if you plan to use his cookbook.

 

[Diver0001]

Who is BRW? Sorry.... I am just learning about dive med.

That would be Dr. B. Wienke, aka Mr. RGBM. He basically invented the idea. His handle on scubaboard is BRW. Together with Dr. R. Powell (aka Dr. Deco) whose work resulted in (a.o.) the PADI RDP, they make this forum one of the most valuable resources available to sport divers..

R..

 

[MechDiver]

I'm not sure about that. I recall reading somewhere that the DSAT tables were based on a 60ft per min descent rate. It's relevant if you consider the way bottom-time is estabilished....namely from the surface to the beginning of an uninterrupted ascent. If you descend faster than than the model then you're taking on N2 faster than the model predicts. The difference is probably minor and for normal non-stop diving within the 40mtr limit a quick descent could probably be adquately compensated with a longer saftety stop. I hope Dr. Deco checks in with an opinion on how much longer you should stop for.

Dive computers obviously work differently than the tables. They sample depth at fixed intervals (often a matter of a couple of seconds or less) and adjust NDL's on the fly. Maybe you're saying the same thing I am but I'm not sure why you're saying that the computer would give you credit for a slow descent. I'm not aware of any computers that monitor descent rate and use it in the calculations.

R..

You may be correct about the tables, not sure now that you said that about the 60fpm rate.

My comment on the computer. No, the computer wouldn't give you a direct credit for a slow descent, but you would get an indirect credit because you would not be ongassing as fast with a 40fpm descent as you would with a 100fpm descent. Difference wouldn't be much I guess, depending on the computer, depth and bt involved.

MD

 

[ArcticDiver]

You may be correct about the tables, not sure now that you said that about the 60fpm rate.

My comment on the computer. No, the computer wouldn't give you a direct credit for a slow descent, but you would get an indirect credit because you would not be ongassing as fast with a 40fpm descent as you would with a 100fpm descent. Difference wouldn't be much I guess, depending on the computer, depth and bt involved.

MD

Credit as compared to what?

Context: Since a computer calculates based on its' sampling and built in algorhythm it seems that rate of descent is immaterial. The computer will sample the various depths and continuously update its' Limits Solution. Or, so it seems to me.

 

[CelticRavenVA]

But isn't that the same as saying it doesn't impact negetivly for fast ascent (which Suunto does), I guess if it is bad to decend that fast, why wouldn't it track for decent as well as acent.

The other piece I am not quite following is what are the negative implications of fast decent??

Credit as compared to what?

Context: Since a computer calculates based on its' sampling and built in algorhythm it seems that rate of descent is immaterial. The computer will sample the various depths and continuously update its' Limits Solution. Or, so it seems to me.