What is Ratio Deco?

[tc]

The latest UTD newsletter had a lot of info on Ratio Deco-

UTD Journal Volume 2, Issue 11, November 2014

 

[boulderjohn]

The preliminary results seem very positive. It was stressed that the dive profiles for the ratio deco divers were an ascent strategy incorporating S curves vs the Other divers using Buhlman with GF's.

It is interesting that they incorporated S-curves. My understanding is that GUE has done away with them completely, especially since the research on which the concept was based has been discredited. When I was with UTD, I did the S-curves, but I argued vehemently against them, for the reason that I did not accept the research on which it was based. When I took my RD class, Andrew acknowledged that it looked like the research had been discredited, but they were going to continue with them for a couple other reasons that I don't recall.

As I understand it, then, the research is based on the UTD version of RD, not the GUE version.

 

[Diver0001]

Sure but I am interested in knowing what real divers think of it. Has anyone ever tried this and then decided, "No thank. I need to go back to my dive table or computer."

I've had it explained to me but I don't have any real experience using it. I find some of the concepts useful but tend to assess my ascent more based on set of general practices I've learned over the years on the one hand from doing a lot of similar dives and on the other hand listening to people who make sense to me like Uncle Pug and Steve Lewis.

I personally don't feel inclined to want to use ratio deco (or any other non-verified algorithm) for calculating the shallow stops on a non trivial dive. I just don't trust myself enough to do the mental arithmetic better than a computer can do it.

As for how it works, John explained it pretty well. Basically it's just using some rules of thumb to decide how much time you need to spend in certain depth zones and how to spread out the necessary stops within that zone in some kind of logical way. It's not rocket science but it's not water-tight either.

It would appear that for a certain bandwidth of dives (certain gasses, certain depth and time limits) it works much of the time for most people. Speaking only for myself, I prefer to use a computer for calculating required stops and applying certain ascent strategies on the way from the bottom to the shallow stops. In other words, I don't ride the computer but I pay attention to the ceiling it calculated.

My least favorite way of planning technical dives is using tables. With the kind of diving we do it usually leads to much longer deco times than are really necessary. If I had to choose between ratio deco and using tables on every dive, I'd be inclined to give ratio deco a fair shake. As it is, there are simply better options, imo, since the advent of good technical computers.

R..

 

[manni-yunk]

I use ratio deco to plan a dive - and - I use V planner. I compare the 2 profiles and make my tables for that dive based on the comparison of the 2 profiles (whichever is more concervative). I also determine if Ratio deco was in fact "acceptable" to me based on the comparison to V Planner. Then - as extra redundancy - if my computer craps out - AND....somehow I drop my tables on ascent.....I can still calculate the rest of my ascent profile in my head.

It really isnt rocket science - its actually very simple.

I have planned and executed dives completely on ratio deco (wiht computer set as bottom timer) as training excercises but I dont do it with any regularity.

 

[decompression]

It wasn't so much that he was arguing that narcosis might impair judgment enough to have you make computational errors, although that is certainly an argument one could make. The context for that particular statement was a response to someone who was very much pro RD. That person said that one of the great things about RD is that making the computations while diving was great because it gave him something to think about during the dive. Chatterton responded that when he is exploring a wreck, he has plenty to think about, and when he is going in and out of rooms and changing decks, his mind is too occupied with that exploration to be computing average depths and planning an ascent profile. He was happy to use a computer, with another computer as a backup, his buddy's computer as a second backup, his buddy's backup computer as a third backup, and the tables they had cut prior to the dive as a fourth backup. He thought that was enough.

The only times I have dived with him he was teaching a student (not me) in a TDI decompression procedures class, and that class does not allow helium, so narcosis would have been something he would have had to consider.

That totally makes sense.


Sent from my iPad using Tapatalk

---------- Post added November 12th, 2014 at 06:39 PM ----------

I started my deep diving on cut profiles and paid very close attention to the oxygen windows. I saw the development of deep stops become acceptable. RD isn't an algorithm but using an algorithm and tweaking it to take advantage of the oxygen windows and bubble mechanics. I still enjoy comparing V planner profiles (and others, ANDI gap, pasto deco) just to see the differences and if I can figure out why and what is the pro or con.


Sent from my iPad using Tapatalk

 

[Doppler]

Someone mentioned I outlined a version of "deco on the fly" in one of my books... And for the record have been teaching it to students for more than 15 years as an option when the rottweilers hit the fan while executing staged deco dives.

There are a few provisos, but following some pretty simple guidelines, it is a better option that floating in the water scratching your head wondering "WTF do I do now?".

Very basically the premise is/are this/these.

That there is a ratio within dive exposures (between bottom time and ascent time) was recognized by Haldane. In other words, and in a sense, ratio deco has been around since the beginning of the 20th century. This was and has been elaborated on by deco theorists since that time... in particular Workman, Lewis (no relation), Lambertsen, and Buhlmann... this is evidenced by the published work on the topic.

That we can optimize inert gas washout by using gases containing less inert gas or different blends of gases than those used on the bottom is commonly accepted (thank you to Dr. Hamilton et al).

That using "standard" gases removes a variable from the decompression equation is commonly accepted.

And finally, that decompression algorithms are mathematics attempting to model gas behavior in vivo... and this is alchemy rather than something strongly supported by science, is commonly accepted.

Therefore, in mathematics, when we seek to describe the rate of change expressed over the passage of time (which can be accomplished using differential calculus for example), we arrive at a function best shown as a neatly formed curve. Follow that curve on our way to the surface, and chances are, we will get out of the water in a healthy state... at least when speaking about dissolved gas.

Too complex... well, yep but that's what our PDC does. To emulate it, we can simply look at the pattern of stops and adopt one of two options to form a template to substitute for that curve... one is to follow a sequence of prime numbers the other is to follow a sequence of Fibonacci numbers. Both seem to work as well as anything else.

As mentioned, very basic, and I am not going to elaborate here. However, I will say that understanding it is, in my opinion, essential if you are going to execute staged decompression dives...



Even when you opt to strap a fourth generation PDC to your wrist... which, for the record, is what I do.

 

[PfcAJ]

I personally don't feel inclined to want to use ratio deco (or any other non-verified algorithm) for calculating the shallow stops on a non trivial dive.


My least favorite way of planning technical dives is using tables. With the kind of diving we do it usually leads to much longer deco times than are really necessary. If I had to choose between ratio deco and using tables on every dive, I'd be inclined to give ratio deco a fair shake. As it is, there are simply better options, imo, since the advent of good technical computers.

R..

Those two statements don't make any sense. RD (the way GUE teaches it) is verified as much as VPM or Buhlmann is. If 20mins at depth results in 20mins on the 50% bottle and 20mins on 100% via a buhlmann table (which is a pretty good algorithm), then its the same with RD. You verify the 'ratio' with decompression planning software through the depth/time range of your planned dive.

There's not a whole lot of mental math needed for RD, and it really becomes easy of you do the same types of dives all the time. If you're doing different depths/BTs every time you go out, then I would suggest taking tables with you.

 

[MXGratefulDiver]

Sure but I am interested in knowing what real divers think of it. Has anyone ever tried this and then decided, "No thank. I need to go back to my dive table or computer."

Most of my technical training involved using ratio deco for my ascents. I used only a bottom timer for the first year or so of doing dives in the 130-200 foot range. Then I acquired a Liquivision X1. While I would still use ratio deco with my dive buddies, who were using BT's, I noticed that my X1 ... with a conservatism setting of +2 ... tracked pretty well with the ratio deco schedule we were using. So I started using the computer more as a "sanity check" than to actually determine my ascent schedule.

I don't do that much tech diving anymore, and now it's usually with people who are relying on dive computers. But I still find myself mentally counting the minutes at each stop and approximating a ratio deco schedule anyway ...

... Bob (Grateful Diver)

---------- Post added November 13th, 2014 at 07:24 AM ----------

It's a method that's only really reliable within a relatively short range of exposures and, as a certain mouthy Truk diving SB denizen can attest, a great way to bend yourself like a pretzel if you do the math wrong on ascent. It also requires standard gasses. Overall, it's good to know as a back-pocket resource and as a sanity check on plans, but it's not a universally applicable Jedi mind trick for deco.

Bad example ... he bent himself like a pretzel because he misapplied RD to the dive he was doing. Furthermore, RD assumes standardized back gasses, which he was not using at the time. This was all discussed at length in some rather contentious threads at the time, but I would in no way use that incident as an example of the shortcomings of RD, except to point out that it's easier to read a computer than it is to apply a theory and do the math properly.

... Bob (Grateful Diver)

 

[laurin]

One thing a lot of people seem confused about is that they seem to equate RD with just jumping in the water and running the deco on the fly and doing lots of math underwater. You could do it, it just isn't very smart or even fun. It's much easier to run the plan on the boat, and then just dive the plan, and adjust if necessary. The way RD works any small adjustments are a piece of cake, and at least for 1:1 and 1:2 ratios even gas plans behave very nicely and predictably once you know your consumption well enough. Personally using RD forces my to think about the plan way more before the dive than I see the guys using computers do, regardless of what they tend to say. It's generally the computer diver who blows the plan, if they had any in the first place...

//LN

 

[TSandM]

Ratio Deco is a curve-fitting exercise. In mathematics, if you have a very complex function that generates a curve or set of curves, it is sometimes possible to approximate the same curve -- through very defined regions -- with a much simpler function. The function that generates the ascent profile in Gap or DecoPlanner or V-planner is not one you can do in your head, at least if you're normal. The math of RD is much simpler, can be done in one's head, and closely approximates the output of V-planner +2, as Bob has mentioned, WITHIN CERTAIN PARAMETERS. It is dependent on some assumptions, those being the use of standard breathing gases and bottom times within what is typical for the depth. For the commonly dived 30 minutes at 150 feet (or 200) it works very well. It diverges more when you are looking at very, very long, relatively shallow dives.

Minimum deco is a way of using a set of mental tables, combined with a standard ascent profile, to manage recreational dives. As it was explained to me, it was derived from playing around with DecoPlanner, and seeing what combinations of depth and time generated a profile that required 1 minute stops from half maximal depth. Again, it is dependent on the use of 32%, and also requires divers who can control an ascent profile fairly precisely. As the majority of the diving world doesn't meet either criterion, Min Deco would not be a very workable approach to decompression for most recreational divers. (And I'm not being condescending -- I know how much work it took before I could reliably execute a precise min deco ascent, especially if I got at all task-loaded.)