Ongoing discussion of Ratio Deco

[rjack321]

grazie any chance you coudl fix up the formatting on that table? It pretty hard to read on my screen.

 

[rjack321]

What he is describing here is basically the same concept as "Diving EAN32 using Air tables". It gives you a conservative deco profile for using a non-standard mix.

basically EAD.

However, if you start mucking with your gases and diving "best mix" you may end up blowing CNS limits easier. Something which is virtually impossible with standard gases and RD.

 

[grazie42]

There may still be mistakes in there aranoid: , please let me know if you find any...

 

[rjack321]

Thanks, much easier to read!

I'm just looking at the T1 depths but I can see how RD "breaks down" at 120ft. That depth in particular is very awkward to plan for inbetween 30/30 and 21/35. If knew I had a dive right there I would take 25/25 and 50% or O2 depending on the exit and RB requirements. Oops, where's that koolaid again I'm running low...

BTW, in my playing with decoplanner, I had a more limited set of depths and times and that's where RD matched closer (within a minute as I recall).

Anyway...
I still don't see VPM or Buhlmann as "correct". Dive one of those profiles and tell me how you feel afterwards. And no you can't go fudging in extra time or moving time here or there. Just dive a straight VPM or 30/85 Buhlmann profile as spit out by the software and report back.

If you feel like poo, that will be evidence enough that they are "wrong". Maybe they are a reasonable standard to establish total deco time. Seems like that's being done worldwide with success. But they are still a crappy way to decide on a deco schedule.

I have not heard a good argument for using decoplanner or VPM to establish total time and then shaping it (though the same thought processes used for RD I guess??). RD is like one stop shopping that's free too! Time + schedule all in one, UW on the fly. Why bother with anything else?

 

[amascuba]

Thanks, much easier to read!

I'm just looking at the T1 depths but I can see how RD "breaks down" at 120ft. That depth in particular is very awkward to plan for inbetween 30/30 and 21/35. If knew I had a dive right there I would take 25/25 and 50% or O2 depending on the exit and RB requirements. Oops, where's that koolaid again I'm running low...

On DIRX, I explained that my RD time for the 120ft profile would give me 20min deco, because it's easier to have numbers that are divisible my 5 when calculating the deco in my head. (Basically rounding up to the nearest time divisible by 5) My profile would be 20/2 = 10 and 10/5 = 2 from there I would have worked out the s-curve. Doing the profile like that actually makes the profile very similar to the other models.

 

[rjack321]

Hmmm, hadn't thought of making the stops logical! I guess if I'm bringing along a deco bottle I'd just as well do a dive worth using it.

30 mins at 120 on 21/35 with a deco of 15 (or even 20) mins is just plain weird.

 

[FishTaco]

I have not heard a good argument for using decoplanner or VPM to establish total time and then shaping it (though the same thought processes used for RD I guess??). RD is like one stop shopping that's free too! Time + schedule all in one, UW on the fly. Why bother with anything else?

Here are some possible reasons to use Deco Planner or VPM:
1) Planning repetitive dives (also repetitive dives over several days)
2) A validation of a profile outside a divers range of experience i.e. sanity check
3) If a dive team wants to experiment with different gradient factors
4) A study tool to help understand the mechanics of decompression
5) Planning dives with non standard gasses

Dive safe,
Chris

 

[grazie42]

Originally Posted by rjack321
Anyway...
I still don't see VPM or Buhlmann as "correct". Dive one of those profiles and tell me how you feel afterwards. And no you can't go fudging in extra time or moving time here or there. Just dive a straight VPM or 30/85 Buhlmann profile as spit out by the software and report back.

If you feel like poo, that will be evidence enough that they are "wrong". Maybe they are a reasonable standard to establish total deco time. Seems like that's being done worldwide with success. But they are still a crappy way to decide on a deco schedule.

I agree with you that there is no "correct", it´s about best guesses and how much alteration you need to do to make those guesses work for you...

First off, for the shallow profiles in the tables, exposure is so limited that I think you could plan deco accoring to a winning lottery-ticket and still come out in reasonable shape.

I actually have dived the first 3 (35-45m) profiles according to V-planner +2 and gotten out feeling just fine. Like I said, I think you´d be hard pressed to hurt yourself in that range. For deeper dives I add things, subtract things, change stop times etc and because I´m "used" to it, I do it for the shallower stuff as well. I do feel better after "playing" with schedules but the sceptic in me wonders how much of that is placebo and ego telling me to feel good (of course what works, works).

I have been thinking about running a few shallow dives according to the "nominal"-profile of V-planner (because I´m thinking of using that as my "get out of dodge"-plan), but I´m chicken and it definetly won´t happen until the water gets "warm"...

Then there´s the fact that whenever I´m doing something "new" I take extra-care to be properly hydrated, to stay warm, not excert myself etc which propably has far more impact on the effectiveness of decompression than a minute here or there...getting a "fair" comparison is just about impossible...

 

[globaldiver]

I cannot see any disadvantage to consult different sources in the process of planning a decompression dive. From information given from these sources (including RD), and your personal experience of your body's needs in regards to decompression, it will hopefully help you to make an intelligent and sound decision of how to form your deco schedule. One can never use too much safety in Scuba diving, so why only rely on one source for something so important as planning a decompression dive?

Even if one decides to, in the end, use RD as the deco model for a particular dive, you can make a more information based decision by first consulting different models/programs, compared to solely relying on RD just because it is easy to use or "free"...

Anders

 

[Phil K.]

Actually it can sometimes...... some call it 'lucky shot'. Not sure if that applies here, this is all 'black magic ' to me....

This unfortunate statement from the author of this thread clearly shows how Meng-Tze has been poorly served by a supposedly DIR forum. The last thing that we should be doing here is giving the appearance of managing a diver's risk of DCS based on methods that amouint to "black magic" or rely on a "lucky shot." Wrong wrong wrong attitude.

RD has it's place in a dive planning protocol, but it's value is in it's utility, assuming a dive plan that falls within its constraints, i.e. it's ease of application, not in it's own independently proven scientific validity, because all the anecdotal evidence aside, it has no proven validitity except to the extent that under its constraints it approxoimates other algorithims that have been subjected to rigorous testing and peer review. If you attack the validity of all other algorithms, you also undermine RD's own validity. It is important to harmonize the best available DCS predictive tools, which aren't coming out of Monterey; but Durham, NC and a few other laboratories and universities around the world; and making prudent judgements based on the requirements of a proposed dive and the specific situation. RD cannot stand alone if all other approaches are relegated to junk science. Without that connection, there's little wonder that the average reasonable person would think of decompression planning as "black magic."