Can someone explain Ratio Decompression?

[rjack321]

Interesting. Is it taught that way or is that simply a personal decision (250 is far beyond my knowledge base) ?

I'm not sure honestly, I have never bothered to take that class :shocked2:. I think 35/25 is "officially" added 260ft+. Some of us use it slightly shallower for rock bottom and a little deco padding but its atypical. I only did once and a little playing with vplanner illustrated adequately to me that it was giving us roughly a 15-20% deco time advantage in the 20mins @ 230-240ft range. I.e. 10-12mins on 32/25 was lowered the subsequent 50% and O2 time deco by ~7mins vs just doing those deep stops on 15/55 IIRC.

 

[Blackwood]

I'm not sure honestly, I have never bothered to take that class :shocked2:. I think 35/25 is "officially" added 260ft+. Some of us use it slightly shallower for rock bottom and a little deco padding but its atypical. I only did once and a little playing with vplanner illustrated adequately to me that it was giving us roughly a 15-20% deco time advantage in the 20mins @ 230-240ft range. I.e. 10-12mins on 32/25 was lowered the subsequent 50% and O2 time deco by ~7mins vs just doing those deep stops on 15/55 IIRC.

Yah, I suppose any time you increase the O2 you're going to get some sort of decompression advantage.

At some point, it's left to 'do what you're going to do.' My understanding of RD as it was taught to me (my only 'formal' Ratio Deco training was through 5thd-x with AG and JT, and I can't speak for other flavors including GUE or UTD) is that the obligations are a function of the specific gases called for by the system. You can add/pad to taste, but the baseline for decompression is the minimum gas set required by the depth (and exposure).

 

[rjack321]

Yah, I suppose any time you increase the O2 you're going to get some sort of decompression advantage.

Actually for short BTs its more a function of dropping the He %age earlier.

At some point, it's left to 'do what you're going to do.'

yup

My understanding of RD as it was taught to me (my only 'formal' Ratio Deco training was through 5thd-x with AG and JT, and I can't speak for other flavors including GUE or UTD) is that the obligations are a function of the specific gases called for by the system. You can add/pad to taste, but the baseline for decompression is the minimum gas set required by the depth (and exposure).

Well, 21/35, 18/45 and 15/55 are all "0% EAD gases". But it is true that you can't use them entirely interchangably. E.g. even if you're blown out, using 15/55 at 140ft with only 50% for deco is not a great idea.

 

[rjack321]

Thanks. Let me absorb this a bit and I'll come back with more questions.

No problem, sometimes users like myself wing around the "rules" or examples like its obvious. Good questions bring out the whys & hows.

 

[Slamfire]

.....

 

[Slamfire]

I really don't understand the theory, or I understand just enough to make assumptions, which we all know the results of....

It's not so much theory itself as it is application of theory...

By limiting the number of gases to one or two combinations per depth range, one is able to isolate a major variable within decompression algorithms and is thus patterns which may exist are more apparent.

What Blackwood said. There is no direct theory or science behind ratio deco. This is one of the first things Andrew Georgitsis mentions in his class -- you can't scientifically prove it by just referring to it alone. It is just a pattern that is identified when you limit yourself to certain conditions (gases, depths, etc.). You test that arithmetic pattern against Buhlmann GF (DecoPlanner) and it comes within acceptable parameters.

Because it is just a pattern, not an algorithm itself, there's nothing preventing you from developing other patterns that'll fit within other algorithms or your own personally preferred gases (not necessarily standard gases). Our own Doppler wrote this article with the help of Ross Hemingway (creator of V-Planner) about using similar methodologies that complied with the VPM algorithm. This article was written many years ago and Doppler tells me that he's refined the methodology since.

 

[Cave Diver]

This article was written many years ago and Doppler tells me that he's refined the methodology since.

Hopefully he'll post an update of it.

 

[Slamfire]

Hopefully he'll post an update of it.

I'll let him address that directly. I don't want to be guilty of spilling the beans .

 

[rjack321]

Because it is just a pattern, not an algorithm itself, there's nothing preventing you from developing other patterns that'll fit within other algorithms or your own personally preferred gases (not necessarily standard gases).

Hmmm where did you come up with this concept?

Algorithm, n. A step-by-step problem-solving procedure, especially an established, recursive computational procedure for solving a problem in a finite number of steps.

The problem is "how much deco do I need, where should I do it, and on what". RD is an algorithm with a finite number of steps, which solves the practical problem in a recursive manner, just like Buhlmann's more complicated mathematical algorithm or VPM. RD is strictly an applied algorithm though and ultimately based on relationships apparent in GF Buhlmann minus the helium penalty. It is possible to use it for "non-standard" gases with a bit of understanding of first principles. E.g. I think I could come up with a total time & schedule for pretty much any plausible depth/time/gas combo off the top of my head from first principles and that it would fall somewhere within the nominal to +5 range with VPM (which is about the size of the state of Delaware so its not very a difficult task).

 

[Trace Malinowski]

I cut tables with V-Planner. I usually cut my bottom time short by a couple of minutes because I am an easy bend. I usually deco for significantly longer than my tables call for, because I like being in the water, and I stay at 15 feet till the O2 is gone. How can Ratio Deco help me?

P.S. I'm not trolling, I really don't understand the theory, or I understand just enough to make assumptions, which we all know the results of....

If you are an "easy bend" you may want to make further inquiries about how ratio decompression can help you. A diving medicine expert like Saturation on the boards may have some recommendations. He has given lectures regarding GUE's "ratio deco" vs. VPM, RGBM and Buhlman modeling and spoken at Beneath the Sea about research that shows that divers are either deco weenies, have normal susceptibility to decompression stress or sickness, or are tanks and hard to hurt and that divers can construct profiles based upon their susceptibility.

While all decompression models are based upon ratios, the term "ratio decompression" is most associated with the concept of "deco on the fly" as provided by previous posters.