Ongoing discussion of Ratio Deco

[FishTaco]

In my T1 class, it was obvious but unspoken that Buhlmann was the "legal dept. approved" way of deco. And roughly a RD profile was the "real way".
Then again, I was taught by the best koolaid heretic out there

Hi rjack321,

I got a very different impression then you did, on how to approach decompression in my T1 course. But,then again, I was taught by JJ.

Take care,
Chris

 

[TSandM]

I think the RD approach is great because it can be used "on the fly" - but why do that when most dives are planned?

One of the things I've asked a number of times is how you plan deco for shore based or terrain-following dives. Preplanning a square profile, like diving a wreck, is easy to do with a program. But if you are shore diving and there are variables in how deep and how long that you may not know ahead of time, a tool like DOTF seems like a very useful thing. You can agree on basic parameters ahead of time for total average depth and time, but adjust according to what you find by having a tool you can use in your head underwater.

I don't do this yet, but I've read a lot about it, and that's what the highest utility seems to be to me.

 

[rjack321]

RD uses "80% ATA (75% depth)" as an approximation of the commonly calcuated start of decompression zone where the leading compartment has a gradient to off-gas. Just the same as running a VPM-B profile (deco zone starts ~2 ATA less than bottom) or running a (GF Low=5 to 10) Buhlmann profile. /QUOTE]

Why not simply adjust the low GF value in Decoplanner to incorporate the deep stops and the high GF to get the total deco time in line? This allows for extension to profiles not quite at the "RD sweet spot" and that use an accepted GUE approach (the science behind Decoplanner)...GF 5/90 or thereabouts yields very GUE-RD profiles.

I think the RD approach is great because it can be used "on the fly" - but why do that when most dives are planned? Decoplanner is a very good tool. Why limit the power of a tool that has a GF adjustment. After all, the low GF applies only to the deepest stop, and the high GF to the shallowest - all the other stops are linear interpolations of GF setting by depth (deepest to shallowest).

Stow the Napalm, just use the flamethrower - Good thread.

Well that's the problem with deco planner. Its shapes are all "wrong". They definately aren't a linear interpolation. More like an exponential interpolation.

Adjusting the GFs to get a deep stop down around where people like/chose to do them is fine. But the rest of the profile still needs major tweaking IMO to get more deco done deeper. I for one, do not want to be pushing gradients the way Buhlmann would have you do it. Too much risk of making a bubble and then trying (possibly unsuccessfully) to get it to offgas.

 

[rjack321]

I know I’m a little off topic here…
But, I wouldn’t shake my head at Haldane’s contributions in developing the dissolved gas model. Remember, it was Haldane who established the concept of “tissue” compartments, and “ascent limiting criteria”. Then other scientists built upon this work, Workman established M-values, not to mention contributions by Schreiner, Buhlmann, Hamilton, etc. that built upon this work.

BTW, does anyone know who actually developed ratio deco? Was it AG?

Take care,
Chris

Yeah you're right that was the wrong smiley. I have the highest respect for Haldane. Weren't ascent rates even more wack before he proved he could keep goats alive with something a bit slower?

Scientists of the 19th century accomplished as much or more than most scientists of the 20th with far fewer resources. Quite commendable

 

[rjack321]

Hi rjack321,

I got a very different impression then you did, on how to approach decompression in my T1 course. But,then again, I was taught by JJ.

Take care,
Chris

I'm sorry

I think I was Mr. RD's last GUE student ever. He had to re-fax my info in proving that the original paperwork had arrived prior to his "termination date". So those scant few months before I may have been inadventantly exposed to some of the "irreconcilable differences" expressed in the divorce

 

[FishTaco]

One of the things I've asked a number of times is how you plan deco for shore based or terrain-following dives. Preplanning a square profile, like diving a wreck, is easy to do with a program. But if you are shore diving and there are variables in how deep and how long that you may not know ahead of time, a tool like DOTF seems like a very useful thing. You can agree on basic parameters ahead of time for total average depth and time, but adjust according to what you find by having a tool you can use in your head underwater.

I don't do this yet, but I've read a lot about it, and that's what the highest utility seems to be to me.

Hi Lynne,

It's just as easy to use average depth in combination with tables written down in your wet notes. But, be careful when playing with avg. depth.

Chris

 

[rjack321]

An average going deep-shallow is fine.

Average going shallow-deep then up is not cool.
http://www.dirdiver.com/en/knowledge/average_depth.html

Simple, and everyone I know does multi-level dives the former way.

Caves have a remote chance of going down-up so when you return its not a good profile to strictly average. Just go use something close to the max for your DOTF calcs instead.

 

[grazie42]

Originally Posted by ScubaMilo
I'm not the only person saying D-plan is DIR it's actually in the GUE standards as the program of choice for verifying deco planning. Actually I did not say D-plan was not DIR what I said was using it in the manner you suggested would not be considered a DIR aproach to planning deco using this program.

I disagree with the Idea of giving one program credit for deep stops and not the other.

I think some of my post may have been misunderstood.
I hope this makes it a little clearer.

Safe diving,
Milo

I understand your point about D-plan. My counter-argument would be that it´s sold freely to anyone who will pay for it. Whatever GUE standards say on the matter, and I respect that its important to DIR-divers, that is a separate issue from the software...IMHO

The only program I´ve given "credit" for deepstops is RD, because it is "integrated" into the model, on V-plan, D-plan etc it is user-configurable like a whole host of other features. My stance on this is that if you start to deviate from the profiles that the sw produces, what changes are ok, and which ones aren´t? We´d still be arguing this on threads like this one 5/10 years from now...I agree and understand that there are limitations to a comparison like this but whenever you make a comparison you have to make a decision about what level of detail and nuance you allow into it, this is mine...

There was something about the "tone" of your post that REALLY pissed me off last night, I´ll just put it down to the limitations of the medium and move on...sorry...

 

[grazie42]

An average going deep-shallow is fine.

Average going shallow-deep then up is not cool.
http://www.dirdiver.com/en/knowledge/average_depth.html

Simple, and everyone I know does multi-level dives the former way.

Caves have a remote chance of going down-up so when you return its not a good profile to strictly average. Just go use something close to the max for your DOTF calcs instead.

This is propably where the paper "breaks down" and you need actual interaction with an instructor...but I´ll ask my question anyway...in the profile described on that site (going from 30m to 20m to 40m) and in general, when do you stop "weighting" your average and start your ascent?

Let me give you an example:
21/35, 50% deco, using RD.
I spend 30mins @ 45m/150ft, then 10mins @ 30m/120feet, another 10mins@ 25m/75ft.

If I´d just spent 30mins @ 150ft, RD wants you to add deepstops on the ascent...
The above example is pretty obvious (I think), you simply calculate your avg. depth and move up to 21m/70ft and start your deco (you´ve already done your deepstops).

But what is the cut-off point? How many minutes do you need to spend at depth X for it do be included in the weighted avg. rather than an ascent? If you start an ascent from 45m/150ft and "quickly" move towards your deepstop but find something "cool" and unplanned at 30m/120ft, how many minutes do you need to spend there for you to start re-weighting your avg. instead of treating it as a prolonged deepstop?

I hope it´s possible to understand my question, even if I suspect the answer needs to be that "it depends"...

 

[rjack321]

Well think of it this way...
You do a 25 min dive at 150ft. First deep stop is 100ft

You start ascending and, gas permitting (deco and backgas), you see something really cool at 120ft. You spend 5 minutes looking at it. Total time now is 30 mins (you had an instant descent) You haven't started deepstops or deco. You're still "on the bottom phase" for this 5 mins.

What's your "average depth"?

5/6ths of your time was spent at 150ft. 1/6th of your time was spent at 120.

Scubamath weighted average says your average is 145. If you work the RD that suggests that you need 27.5 minutes of deco.

I personally don't work in "half-step increments" (i.e. 5ft) I average in strictly 10ft increments because 1) that's what I'm comfortable with and 2) that's where the time changes become relevant. I do have a Aladin Tec which does averages for me. I still think in 10ft increments though.

So what deco would I do? 29 mins minimum on the 50%. That 1 minute change is less than the amount I might pad deco for a host of factors including: remoteness of the dive site, water temp, workload, etc.

Now assume this really cool thing is up at 90ft. You've started your deep stops already but not really off-gassing much. First I might linger a couple minutes, but probably not 5. Second I would count about 1/2 of this 2-4 mins as bottom time and extend my deco a couple minutes to compensate.

Same story if the cool thing is up at say 50ft. One, don't linger much. Two extend shallow stops a couple minutes to compensate for the couple extra minutes "wasted" at 50ft.

But really there's a limit to modifying plans on the fly. I have the gas to accomplish my plan and some reserve. Every modification I make is chipping away at my reserves in one way or another. I'm willing to change the bottom phase of the dive by up to 5 minutes. But once onto the deep stops I don't have the experience to go mucking with the ascent portions of the dive by that much. I'd keep any on the fly modifications to 2-3 mins.

Clear as mud?