My Journey into UTD Ratio Deco

[rossh]

@Michael Guerrero is actually referring to this study:
https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00944.2014

". . .On the basis of these results, it is inappropriate to assign substantially different time constants for nitrogen and helium in all compartments in decompression algorithms. . ."

And yet, in a conversation I had with David about this 2 years ago, the "ALL" tissues he wrote above, became less certain. His private recommendation at that time, was to make them equal at the fast end, but he was not confident to do this at the slow end. He seemed to want to still observe the heliox style testing and results, meaning we continue to observe the faster helium in the slower tissues.

Bottom line of the above is you get a gradual change, from N2-He being equal at the fast tissue end, through to status quo 2.65 at the slow tissue end.

.

 

[Michael Guerrero]

In my post I said aqueous tissues, and I'm pretty sure I got that directly from @David Doolette'
s study.

 

[Michael Guerrero]

Can you please refer to me where is this profile posted by Sinbad?

Post #15.

 

[Michael Guerrero]

Ratio Deco attempts to protect...

However with regards to later slow/intermediate tissue surfacing supersaturation (your "negative indicators" @Michael Guerrero), RD users have to practically extend their shallower O2 stops by some greater & longer amount of time

So given your comment @Michael Guerrero that "RD seems like an anachronism", the question now becomes how hard are you willing the drive your Fast Tissues into Supersaturation for the sake of decompression efficiency?

IOW, in terms of Bühlmann GF's, would you now dive deco profiles of 75/75? 80/80?? Full Bühlmann 100/100 implementation???

I get all that Kev. I think the thing we have to recognize is that if I do the deep stops as prescribed by RD and then totally disregard the short shallow stops, instead extending them significantly to execute a good washout for slow tissues, then I'm not really using RD anymore. It seems like you're advocating the use of RD as an analog to figuring out a GF lo. Likewise, when we use GFs we're not really using Buhlmann, we're using a modification that allows us to focus on a dissolved gas model with modifications about proximity to the m value.

And to answer your question about what I'm comfortable with, I dive a 70/70 GF and would try 100/70 if Shearwater would ever implement my change request.

 

[mikeny9]

@Michael Guerrero is this the profile you are referring to in post #15?

After finishing the dive, you ascend to 50% of your depth @ 33 ft / minute and then stop for 1 minute every 10 feet until you reach the surface. For a hundred ft average depth you are doing 5 stops of 1 minute each all the way up to the top so that would make it a 5 minute ascent schedule. For 80 feet dive your first stop will be at 40 feet and then 30, 20, 10 so 4 minute schedule.

This is hardly a technical profile and is considered "min-deco" by GUE and UTD. You'll clear any Buhlmann GF-based computer set with common GFs using this ascent strategy. How is this decompression schedule "bad" as you say?

UTD RD is not an algorithm, it's an ascent strategy. You can change the ascent based on a variety of factors. A computer doesn't know all of these factors. For example, are you stressed, cold, didn't have much sleep the night before, dehydrated, etc.? You should probably adjust your profile based on these factors, and some of them can change while on the dive.

The practical (i.e. not theoretical) differences between a Buhlmann GF profile and one that UTD RD 2.0 would suggest are minor. Compare a technical profile created with Buhlmann GFs and one that UTD RD 2.0 would suggest. A Buhlmann GF profile might theoretically be more "efficient" - it gets you out of the water quicker, but "quicker" usually means a few minutes in most cases. I'd rather hang a bit longer; I'm not a Navy SEAL trying to get to my next mission and addressing your fast tissues with deep stops is far more important and less risky.

Furthermore, RD (both UTD and GUE versions) gives you the advantage of understanding how you might be able to change your profile in the middle of your dive.

For example, you've averaged 60' for 30 min and there is another wreck nearby at 80' you want to visit because you are board of the wreck you are currently on. How long can you stay down at 80'? What if you spend 10 minutes at 80' and want to go to another wreck at 60'? How long can you stay at the third wreck at 60'?

To find these answers without RD you'd have to do the dive and see what your computer tells you after the fact because the computer is a lagging indicator. RD provides a practical estimation of these numbers before you ever start descending to that second wreck. This knowledge allows you to make good judgement calls before they become a problem. RD is another tool in the toolbox.

 

[Patoux01]

UTD RD is not an algorithm, it's an ascent strategy. You can change the ascent based on a variety of factors. A computer doesn't know all of these factors. For example, are you stressed, cold, didn't have much sleep the night before, dehydrated, etc.? You should probably adjust your profile based on these factors, and some of them can change while on the dive.

Why do you think (or at least, suggest) you cannot do more deco than what a computer says?

Furthermore, RD (both UTD and GUE versions) gives you the advantage of understanding how you might be able to change your profile in the middle of your dive.

Why would using a science based algorithm mean you don't understand how it will work, from experience one can know that too. Guess what you're doing when using the "ascent strategy"? You're actually relying on... someone else's experience/calculations, and sometimes unfounded guesses. Actually, GUE RD does exactly that, it emulates an actual model-based algorithm.


As for RD 2.0 being closer to buhlmann, I don't know, I still haven't seen the actual rules for computing a schedule using RD 2.0. At least none where people seemed to agree on what they were, we only knew they worked better than previously because it's 2.0.

 

[mikeny9]

Why do you think (or at least, suggest) you cannot do more deco than what a computer says?

I do not think nor have I ever suggested that. I'm illustrating if you perform exactly what the computer is telling you to do, it might not be the best ascent strategy based on a variety of factors that the computer does not know about. If you're a smart enough diver you can probably figure out if you should stay in the water longer and do more deco than your computer is suggesting. UTD RD facilitates this smartness and forces you to think about your profile. In contrast, there's nothing wrong with setting a computer on your wrist to a Buhlmann GF profile and seeing how the two compare for sanity's sake.

Why would using a science based algorithm mean you don't understand how it will work, from experience one can know that too. Guess what you're doing when using the "ascent strategy"? You're actually relying on... someone else's experience/calculations, and sometimes unfounded guesses. Actually, GUE RD does exactly that, it emulates an actual model-based algorithm.

I never said using a "science based" algorithm means you don't understand how it works. Simply following a computer does not allow you to predict future results on your profile. That is exactly the power that RD (GUE or UTD versions) gives you, and as you illustrated yourself how GUE RD does exactly that.

 

[rjack321]

I get all that Kev. I think the thing we have to recognize is that if I do the deep stops as prescribed by RD and then totally disregard the short shallow stops, instead extending them significantly to execute a good washout for slow tissues, then I'm not really using RD anymore. It seems like you're advocating the use of RD as an analog to figuring out a GF lo. Likewise, when we use GFs we're not really using Buhlmann, we're using a modification that allows us to focus on a dissolved gas model with modifications about proximity to the m value.

And to answer your question about what I'm comfortable with, I dive a 70/70 GF and would try 100/70 if Shearwater would ever implement my change request.

First off, there is no one holy grail of RD. To my knowledge there are at least 4 different published/taught versions of "deriving an ascent" from BT and (usually average) depth. So saying someone is or isn't following "ratio deco" is silly. UTD has a version they teach. Parts of what they teach and how they teach it are rigorously followed post-class, parts not. In class you are expected to follow it or explain why not in debrief. But every Rec3/Tech1/Tech2/Trimix1 class I ever participated in as a DM for them articulated quite clearly that the UTD RD mnemonic was a starting point - not something to be slavishly followed.

Second you don't understand Bulhmann if you are requesting a 100lo / 70hi GF setting.

 

[Scuba J7]

Actually a Shearwater computer can tell you exactly how profile changes will turn out (during a dive) if you know how to use it.

 

[boulderjohn]

ctually a Shearwater computer can tell you exactly how profile changes will turn out (during a dive) if you know how to use it.

There are all sorts of things you can do with a Shearwater during a dive. I was in Palau doing nothing but recreational diving for nearly 3 weeks this past month, and I began to amuse myself by going to the tissue loading graph at various times in the dive, especially during an ascent, and watching how things changed with time and depth changes.