Ascent Rate at Depth

[100days-a-year]

Yoop,fast ascent rates were actually from the early days of mix.The premise was to get you off He soon as it was a fast gas.I heard this first in 87' or 88' before I knew any better.VPLAN supports 60 fpm as does GAP,ZPLAN and Proplanner.If you want you can do the math youself on critical volume,seed propagation and such .The work on VPM is years old ,but even the newest version gives me a -60 fpm to the 1st deep stop.Ricks analogy is good ,but in practice a linear relation between depth and ascent rates shows critical dangers when all the factors are considersed.Hence the limits mentioned.See if DrDeco,ScubaJorgen and some of the other resident calculus fans weigh in.Bottom line tho is getting bent is dependent on other factors as well.I stay with my buddy for his longer decos cause he 10 yrs older,rounder and doesn't have cardio-vascular obsession.

 

[Lost Yooper]

The older Haldane models were based on getting you out of deep water faster and allowing the pressure gradiant in the shallows to decompress you. This is how they became known as the "bend and mend" tables. By the time you got to the decompression stops, you were thoroughly bent and there was no room or fudge.
Using bubble models that incorporate deep stops and slow ascents, you are significantly off gassed by the time you get to the shallower stops. What does this mean in practical terms? It means that you may stand a better chance of living through a complete blowout of your deco (or part of your deco) if the need were to arise. This is especially true if you can get on 50/50 at 70' for a few minutes or O2 at 20' for a little while. It also means you feel mucho better post dive, as sub clinical DCS won't be nearly as noticeable (if at all).
Barring any preconditions, being physical fit, staying hydrated, and avoiding post-dive exertion, you can be totally cleared of any detectable bubbles in a relatively short amount of time (post dive) by using these decompression procedures (ie. avoiding deep air, using the right deco gasses, slow ascents, deep stops, and a very slow final ascent). The models that deal with this type of decompression attempt to avoid bubble formation deep which is obviously a good thing. Unfortunately, I have yet to see a model, dive computer, or table (other than that advocated by the WKPP) that requires you to do the most important step -- the very slow final ascent from 20'. This is so huge, that it's hard to fathom why it's not stressed by the agencies more. For those lurking, if you do no other step than this final slow ascent (which I recommend taking 2-3mins) from 15'-20', you will notice an improvement on how you feel post-dive and significantly reduce the probability of a DCS hit.

Take care.

Mike

PS. Rick, are you basically using Plye type stops?

PSS. Tony, you may be right about V-planner using 60fpm. I don't recall. I have mine set at 30fpm -- I don't recall what the defaults for the program were. Regardless, the difference in deco time is only two minutes or so.

 

[Rick Murchison]

Originally posted by Lost Yooper
PS. Rick, are you basically using Plye type stops?

Not exactly... In thinking about the dynamic of bubble formation and growth ages ago, right after doppler studies started hitting the news, I started crunching numbers on my own to "improve" on the Navy tables based on what was being seen - and I decided that (1) 60 FPM was not smart close to the surface, (2) the initial bubbling ratio was much lower than Haldane imagined, (3) equivalent delta P over P combined with a less than 1:1 sat ratio of disolved inert gas to ambient pressure would allow for very rapid ascents for the first few feet off the bottom of a deep dive, (4) stops much deeper than the Navy tables called for would be prudent.
Rolling all this together, at a minimum I added one minute stops at half depths until reaching the first deco stop called for in the Navy tables, added the three-five minute safety stop at 15 feet, and cut the ascent rate to 15 FPM max from 30 FSW - and to 10 FPM from 10 feet if on a deco dive. As I started using newer tables I retained those additions. I haven't done any substantial deco diving lately, but I have been running some of the newer planning programs and am impressed by what I see. (and gratified to see that my admittedly crude calculations and biologist/aviator gut-level reasoning came pretty close to what's now being produced by some real mathematicians and physicists with real training in the dynamics of decompression)
I wish I could claim some really solid theoretical math behind what I came up with, but frankly I just sorta backed into it.
Rick

 

[g2]

I'm impressed by the thought put in to all of your answers! I haven't had the need or desire to learn trimix (yet), but this has given me some more insight into that as well.

It sounds as if the concensus is slower is better, and deep stops are an excellent idea. I've been doing some more research (I wish I had the NOAA manual), and the best reference I've found is the opening editorial in the March 2001 edition of Alert Diver. They don't address deep ascent rates, per se, but they do suggest an approach to deep stops that agrees with Rick's description of what he does.

Originally posted by Lost Yooper
...Using bubble models that incorporate deep stops and slow ascents, you are significantly off gassed by the time you get to the shallower stops. What does this mean in practical terms? It means that you may stand a better chance of living through a complete blowout of your deco (or part of your deco) if the need were to arise.

That's good enough for me. And I don't plan on pushing the envelope so much that a faster ascent rate at depth would buy me any significant additional safety anyway, even if there was a valid reason for doing so.

BTW, I totally agree with extremely slow ascents for the last 20ft or so, after the safety stop. In calm water, I have sometimes stretched the final ascent to as much as 5 minutes or more, usually while watching other divers climb back on the boat.

Best regards, g2

 

[Vinga]

I used to amaze my students by having them time a walk up a staircase. The stairs were a twelve foot ascent, and I told the folks to take a full 24 seconds to get to the top. Wowee! Were they ever surprised at how slowly they had to go.
Try it yourself.
Cheers,
Melissa

 

[100days-a-year]

Yoop,I have a protocol for slowing myself in the last 20.We clip off fish, bugs,goodies and any gear that can be.We use a granny line with a 10# hard weight at each end of the boat 20' down.Clips also at 10'.Clips at 5' to start attaching harness too.I don't get sub-clinical due to using more conservative buddies profile,lotsa water,lotsa aerobic exercise and no caffiene,alcohol,or nicotine habits.Buddies with same profile but less emphasis on physical conditioning get headaches and blahs sometimes.Deco here is almost like setting in a hot tub in the summer.......very relaxing.

 

[Lost Yooper]

Sounds good, Tony. After doing a dive in 38F water, the deco is actually a warm up for us since the water can be 55-60F at 20'.

Mike

 

[VTWarrenG]

*shivers*

- Warren