What is Ratio Deco?

[boulderjohn]

Although they seem to be used together these days, Ratio Deco and Minimum Deco are really not the same thing. Ratio Deco was created for decompression diving, and Min deco for NDL diving. They work differently.

Many tech divers plan dives using dive planning software. They anticipate what they bottom times will be, feed their anticipated plan into a software program, and generate special tables for that dive telling them what to do. So what happens if you don't follow the anticipated plan? Well, if you do it enough, you can see the patterns and plan on the fly, coming up with something like what you would have gotten if you had planned the dive the way you actually did it. Some of the people who noticed that came up with a mathematical construct that enabled them to create such plans on the fly with reasonable accuracy. They called it Ratio Deco, and it has been a key part of GUE training. Some time after that, there was an acrimonious split in which a GUE trainer, Andrew Georgitsis, left and formed UTD, taking his version of GUE training with him.

Although both agencies use it, they are not the same. I was trained on it by Andrew while I was with UTD. My understanding is that the math is not the same as the GUE version, but I am not sure. There are other differences, some of them significant. With GUE, Ratio Deco is used as originally designed. With UTD, it is the primary planning device for all dives, with no software used. With GUE, a RD plan should match the software plan on which it is based. With UTD, it should not match a software plan, because it is believed to be superior to software plans. With GUE, the belief is that the system was created to match profiles created at sea level, it has not been tested at altitude, and has no known validity there. With UTD, altitude is not believed to have a significant effect on decompression needs, so no adjustment is necessary.

My comparisons are based on my experience taking the ratio Deco class from Andrew Georgitsis and from an email exchange on the topic with GUE director Jarrod Jablonski. My information is a few years old and may need updating.

 

[OtherHalf]

I love the concept of ratio deco, and seems like it was a necessary thing with the quality of DCs when it was developed. Is it still relevant with the popularity of high quality, reliable tech computers on the market, like say the petrel? I think knowing whats happening in the computer is value added, but also seems like additional stress loading and like it adds a mental point of failure.

Does ratio deco have a future with the drastic improvement in dive computers over the last few years?

 

[LiteHedded]

the way it was taught to me by someone with the agency since the days they were developing it is that if the schedule is different than ratio deco, ratio deco is wrong.
because it was developed based on trends in the algorithm. therefore if the algorithm is different, RD is incorrect.

 

[PfcAJ]

I love the concept of ratio deco, and seems like it was a necessary thing with the quality of DCs when it was developed. Is it still relevant with the popularity of high quality, reliable tech computers on the market, like say the petrel? I think knowing whats happening in the computer is value added, but also seems like additional stress loading and like it adds a mental point of failure.

Does ratio deco have a future with the drastic improvement in dive computers over the last few years?

It does when your computer craps the bed when you're 1000miles from the nearest dealer. Or when your luggage is broken into and your expensive bits are stolen.

I can RD with a watch and an analog depth gauge.

 

[decompression]

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.

 

[boulderjohn]

the way it was taught to me by someone with the agency since the days they were developing it is that if the schedule is different than ratio deco, ratio deco is wrong.
because it was developed based on trends in the algorithm. therefore if the algorithm is different, RD is incorrect.

That is what Jarrod told me. When people question the validity of a ratio deco profile and the degree to which it has been tested, the reality is that it should be as reliable as the program on which it is based, which is, I believe, DecoPlanner. If DecoPlanner works, then RD works within the parameters within which the RD math works, as others have noted. DecoPlanner was originally a Buhlmann-based program, although I believe it ow incorporates VPM.

---------- Post added November 12th, 2014 at 03:12 PM ----------

It does when your computer craps the bed when you're 1000miles from the nearest dealer. Or when your luggage is broken into and your expensive bits are stolen.

I can RD with a watch and an analog depth gauge.

I used to use RD exclusively when I was with UTD. I don't use it any more. It is not because I don't think it creates a respectable decompression profile when used with standard gases within its known parameters. It does. I would happily dive such a profile without second thoughts.

My original concern with it was the altitude issue. Our small group of divers usually ended up taking about 9-10 weekend trips per year to a reasonably high altitude, bringing a differing group of about a half dozen divers each time. Over a few years, we had 6 DCS incidents, including a helicopter evacuation. That seemed like a high level of incidence to me, but the UTD leadership explained that it could not be related to altitude because there was always something else causing the problem. They did not always know what else caused the problem, but they knew it was not altitude, because altitude is not a factor in decompression.

In two of the cases something else was definitely involved. Both members of the team got bent, and one of them was using a computer on gauge mode as a bottom timer. They downloaded the profile of the the dive, and it showed that they had made three separate miscalculations during the dive. These were very bright young men, much more capable of doing mental arithmetic than the average Joe. If they had not made those errors, they would have done much more decompression before surfacing than they did. (One of the errors was simply miscounting the number of minutes they had done on the last stop.)

Based on that, I decided that although computers can make errors, my feeble brain is much more likely to do so, especially while undergoing the complexities of a dive. Others may be more confident than I. In a thread debating this topic a number of years ago, John Chatterton talked about exploring a new wreck, not knowing exactly where you are going to go as you do so. He did not trust his ability to come up with a decompression plan under those circumstances, and he has a pretty fair level of experience. I do have redundant instruments and redundant plans. I do have the training that allows me to work out a new plan in the very worst of situations, but that is not my normal mode any more.

 

[LiteHedded]

i'm not a fan of doing math underwater

 

[PfcAJ]

Not adjusting for altitude is bananas.

 

[decompression]

That's understandable John. I know John Chatterton and he is definitely old school, in so much as he still is a die hard narcossis management believer. I can understand his point of view.

 

[boulderjohn]

That's understandable John. I know John Chatterton and he is definitely old school, in so much as he still is a die hard narcossis management believer. I can understand his point of view.

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.