Ratio Deco

[boulderjohn]

As I suggested earlier, if a ratio deco plan creates a profile that matches an established, reliable algorithm, I have no problem with it, and I would have no more problem following it on a dive than I would following that algorithm.

The fundamental problem with planning a dive using either ratio deco or desktop software is what I described earlier--it is based upon the assumption that the humans who created the plan will be able to follow it as planned. Here is an example of a DCS case that hit a buddy team of friends of mine. They went to their planned depth for the planned time, ascended to their planned first deep stop, and then followed the planned ascent profile from there to the surface. They used bottom timers and a computer in gauge mode to guide them. They thought they had done everything perfectly, and they were surprised to get bent. Because they had used a computer in gauge mode, they had a computer log of the dive, and it showed that they had not done the dive they thought they had done. They had strayed below their planned maximum depth a number of times, something I see happening frequently on decompression dives. They had taken longer than planned to reach their first deep stop, and they had not realized that they were beginning their decompression profile from there several minutes later than they should have been. They had somehow miscounted their minutes on their final stop, leading them to surface a little early.

I am willing to bet a lot of people have made errors like that during their dives and were either lucky enough not to get bent or they did get bent but did not have a computer log to show them where they had messed up. There is enough wiggle room in most program to allow for those errors, as long as there are not too many.

 

[TSandM]

There seem to be quite a few different things being discussed in this thread.

The first question is that of min deco and surface intervals. Min deco was generated by plugging depths and times into DecoPlanner until it generated a min deco profile (1's from half max depth). I was taught the "same deco if over an hour SI, double the deco if under" approach, but GUE doesn't teach that any more. I have used that approach for 9 years, and a great many of my friends use it, and I am not aware of any DCS cases from any recreational dives using the strategy, even for those of us who travel and do multiple dive/multiple day trips. The plural of anecdote is not data, but there are an awful lot of anecdotes.

Using RD for technical diving raises two different issues. One, is the profile generated by the process a valid, safe profile? And two, can divers be depended upon to remember and execute the profile generated by the process when they are underwater?

I believe there has been quite a bit of research suggesting that the very deep stops called for by the current version of RD are not beneficial and may raise bubble grades. Nobody I dive with does them. And nobody does the S-curve (and GUE teaches "linear pragmatic" these days, as well). There has even been some discussion about whether the deeper first stops generated by bubble model programs are beneficial. So, to me, what this all brings up is that a lot of the programs that are generating profiles for technical diving involve a great many assumptions and very little actual testing. Whether you use Buhlmann or VPM or RD, you are using a system that hasn't had a huge amount of empiric validation, and as Rick Murchison said years ago, you are experimenting on your own body, and ought to keep careful notes. Where the system you are using is not in concert with the little data we DO have about decompression profiles in technical diving, it ought to be regarded with caution.

The question of whether divers are capable of making an accurate mental record of their dive, and remembering a profile (or generating it on the fly) may well be dependent on the individual divers, their experience, their stress level or level of attention during the dive, etc. That is why I have always allowed the VPlanner software on my X1 to run, although I plan and execute the few tech dives I do using a modified RD. If I am having a major brain fart underwater, I think it would be nice if the computer gave me a wake-up call.

 

[boulderjohn]

That is why I have always allowed the VPlanner software on my X1 to run, although I plan and execute the few tech dives I do using a modified RD. If I am having a major brain fart underwater, I think it would be nice if the computer gave me a wake-up call.

A couple of years ago I joined a group of 3 divers on a 300+ foot decompression dive in Cozumel. It was planned carefully using V-Planner, and they all had Liquivision computers with the same algorithm as backups. I of course followed their plan. As frequently happens, at maximum planned depth, everyone had moments when they got a little deeper than planned and had to scoot back up to the planned depth. Nothing serious, but it happened to everyone, including me. We then followed the plan to the surface--almost. When I say "almost," I mean they sometimes adjusted slightly to match their computer backups, and they did not end the final stop until the computers had cleared.

So it led me to wonder--were they following the written V-Planner schedule with a computer backup, or were they following the computer with a wrtten V-Planer backup? They would say the former, but it seems to me it was the latter. I would suggest that the slight differences in the computer profile and their original plan were the result of the computer dealing with the variations in the dive, including going deeper than planned, and they took what they thought was the safe route and followed the computer.

So if you see a difference, however slight, between what you think you are doing as per your plan and what your computer is telling you, which do you follow?

 

[MXGratefulDiver]

A couple of years ago I joined a group of 3 divers on a 300+ foot decompression dive in Cozumel. It was planned carefully using V-Planner, and they all had Liquivision computers with the same algorithm as backups. I of course followed their plan. As frequently happens, at maximum planned depth, everyone had moments when they got a little deeper than planned and had to scoot back up to the planned depth. Nothing serious, but it happened to everyone, including me. We then followed the plan to the surface--almost. When I say "almost," I mean they sometimes adjusted slightly to match their computer backups, and they did not end the final stop until the computers had cleared.

So it led me to wonder--were they following the written V-Planner schedule with a computer backup, or were they following the computer with a wrtten V-Planer backup? They would say the former, but it seems to me it was the latter. I would suggest that the slight differences in the computer profile and their original plan were the result of the computer dealing with the variations in the dive, including going deeper than planned, and they took what they thought was the safe route and followed the computer.

So if you see a difference, however slight, between what you think you are doing as per your plan and what your computer is telling you, which do you follow?

... the more conservative one. Short of a suit flood or other emergency, there's no downside to spending a few extra minutes on deco ...

... Bob (Grateful Diver)

 

[LiteHedded]

Well this thread is all over the place now...

 

[decompression]

Ya, I think the debate of the virtues and failings of both mindeco and ratio deco will continue for a long time to come. In the end, decompression strategies are just tools to help you get back safe. Pick the one that will work best for you from the best of your knowledge.


Sent from my iPhone using Tapatalk

 

[Adobo]

Ya, I think the debate of the virtues and failings of both mindeco and ratio deco will continue for a long time to come. In the end, decompression strategies are just tools to help you get back safe. Pick the one that will work best for you from the best of your knowledge.

Sent from my iPhone using Tapatalk

If the data from the study and the video referred to in the article become publicly available at some point, please share. I really am genuinely interested in their findings.

 

[DaleC]

The first question is that of min deco and surface intervals... I was taught the "same deco if over an hour SI, double the deco if under" approach, but GUE doesn't teach that any more...

Something about that UTD card piqued my interest.

It says that for an SI of <1hr you double up the min deco on your three shallow stops and more than 1hr you do the same profile. However, I believe in our fundies class we were taught that for 1-2hr SI's you do 50% of the ndl for the next dive (same stops) and for SI's of more than 2hr's you do 75%.

I'm curious what the effect of each strategy would be? I don't have planning software and don't really know the relationship between reduced BT vs extended stops ie. which is better, double the stops or half the BT?

 

[WhiteSands]

Something about that UTD card piqued my interest.

It says that for an SI of <1hr you double up the min deco on your three shallow stops and more than 1hr you do the same profile. However, I believe in our fundies class we were taught that for 1-2hr SI's you do 50% of the ndl for the next dive (same stops) and for SI's of more than 2hr's you do 75%.

What GUE teaches now (in fundies) is very conservative. I learned the same thing basically, but I was also told not to do more than 3-4 dives a day, otherwise the shortcut that you mentioned above, will no longer keep you safe.

Correct me if I'm wrong, but I believe the shortcut is made to approximate standard Nitrox 32 tables. If you plot out all dives at various depths to the NDL limit, do a 1-2 hr SI, then calculate the next dive's NDL as a percentage of the original NDL (when clean) at that depth, you will see the percentage as roughly 50%.

And then do the same but with a 2hr SI, and the percentages will all fall close to 75%.

For me, I just found it more straight forward to do my dive planning with PADI tables.

I'm curious what the effect of each strategy would be? I don't have planning software and don't really know the relationship between reduced BT vs extended stops ie. which is better, double the stops or half the BT?

It seems intuitive to me that halving bottom times will be vastly better. Halving of bottom times could translate to anywhere from 10-15 mins less intake of Nitrogen or more, compared to adding a minute here and a minute there of off-gassing at the stops.

 

[Slamfire]

What was different about your friend's dive profiles? Did he follow a ratio deco guideline instead of a traditional algorithm to determine his deco obligation (and was ratio deco showing something different than what buhlmann or VPM was saying)? Or did he implement S curves as opposed to a more linear type of distribution of his stops? Did he do both?

Because if he did neither, I don't know that we can necessarily attribute the bend to ratio deco. Am I making any sense?

That's the greatest point of defense for RD. Nothing will ever be able to attribute a bend to it. AG makes it very clear at the beginning of his course. He says, "If you want to be 100% sure that you will avoid dcs, then don't dive at all." While the statement in itself is true, it should not be used as an excuse to justify faulty logic in a deco strategy.

I was not in either of the two dives in question. I do know that my friend has only been formally instructed in RD and nothing else. UTD does not teach anything else. I do know that his deco dives were done relying on rd as his primary (only?) strategy. If there was somebody that would have been implementing ratio deco just like AG personally teaches it, this would be him.