My Journey into UTD Ratio Deco

[Kevrumbo]

If you're referring to Decompression from He-N2-O2 (trimix) bounce dives is not more efficient than from He-O2 (heliox) bounce dives., the study does not claim that Helium and Nitrogen should be treated the same, but that Helium should be treated no worse than Nitrogen, hence all the the noise about eliminating the Helium penalty.

However, it does not prove that Nitrogen is the same as Helium with regard to decompression. Therefore, Helium might actually be better, but there is no research that proves (or disproves) that theory.

@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. . ."

 

[Kevrumbo]

Particularly given the research from David Doolette that we should be treating He and N the same in aqueous tissues in terms of washout. Wonder if there's some lingering "helium is faster" thinking going on. Or maybe the profiles are tame enough that it doesn't matter much.

Still though, I wouldn't use 25/25 or 30/30 Triox without Oxygen deco gas as clean-up even if the Triox was breathed well within "no-stop NDL" parameters -had two cases of the "chokes" about ten years ago using 30/30 Triox only within the GUE Nitrox 32 Min Deco Table at that time (25min at 33msw):

Helium Triox mixes in the recreational range like 30/30, 25/25 or NAUI 26/14 are not the perfect gas or most convenient choice of gas for me however. Even staying within shortened NDLs with proper ascent & safety stops on breathing Triox only, I still suffered "the chokes" (shortness-of-breath) after two repetitive dives with a 90min SIT in between because of residual fine helium bubbles clogging my lungs' alveolar bed.

I now utilize a deco bottle of oxygen to help clean-up during the last repetitive dive of the day, for any set of recreational depth type dives using Triox (haven't had problems with the chokes since). . .

 

[Michael Guerrero]

So...if RD, as taught by UTD, produces ascent profiles that result in more negative indicators, and those negative effects are likely to be amplified with deeper and longer dives, then it loses a lot of appeal to me. I'm not very interested in learning how to come up with what I consider a bad decompression schedule (I consider the one Capt S posted originally to be bad) in my head...particularly not for real deco dives, let alone recreational/light deco ones.

Instead, I prefer to use an incredibly reliable computer that produces results backed up by science I understand (at least from the perspective of a layman), with a backup bottom timer and tables. Sufficient experience in doing decompression dives makes me feel strongly that I could adequately pad conservatism on the fly using these tools and still perform an ascent I'm comfortable with, should things go sideways. And that's as a solo diver. In a team, I'm not even a little worried.

RD seems like an anachronism.

 

[Kevrumbo]

So...if RD, as taught by UTD, produces ascent profiles that result in more negative indicators, and those negative effects are likely to be amplified with deeper and longer dives, then it loses a lot of appeal to me. I'm not very interested in learning how to come up with what I consider a bad decompression schedule (I consider the one Capt S posted originally to be bad) in my head...particularly not for real deco dives, let alone recreational/light deco ones. . . RD seems like an anachronism.

Ratio Deco attempts to protect and limit supersaturation and potential bubble formation in the fast tissues early in the deco profile with deeper/longer deepstops; in contrast Bühlmann dissolved gas model aggressively approaches these fast tissue M value tensions or optionally moderates them with Gradient Factors, but still not as radically deep or as long as a RD Deepstop(s).

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 (and significantly much longer than implied by that RD Oxygen profile in the Spisni Study) in order to effectively clear those tissues that were still on-gassing as a result of deepstops -which therefore makes RD more conservative and a much longer total deco runtime compared to a Bühlmann GF profile... (i.e. If you believe in protecting the fast tissues as well as effectively washing out the slow tissues later in the decompression profile, then you are taking a qualitative "conservative" approach with "more deco time" in the water per RD).

It is now thought that the more perfused Fast Tissues are robust enough and can take the peak tensions and lesser time integral of critical supersaturation earlier in the deco profile without bubbling, thus not requiring the need of deepstops, and therefore making a Bühlmann profile more "efficient". In contrast, RD claims a better strategy of not driving Fast Tissue anywhere near to its M-Value critical supersaturation by using deepstops to prevent potential bubbling, thus making its profiles more "conservative" (but as the NEDU Deepstops Study showed, not necessarily optimal later in the profile at the expense and deco stress of the Slow Tissues' surfacing supersaturation tensions).

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???

 

[Scuba J7]

Conservative seems to be subjective in this matter. I personally don’t buy into it.

Add more N2 to tissue A in order to protect tissue B. Now you MUST do more deco to clear tissue A. Just because there is more deco time doesn’t make it more conservative. Adding more deco after the tissue has been cleared may be considered conservative.

 

[Kevrumbo]

Conservative seems to be subjective in this matter. I personally don’t buy into it.

Add more N2 to tissue A in order to protect tissue B. Now you MUST do more deco to clear tissue A. Just because there is more deco time doesn’t make it more conservative. Adding more deco after the tissue has been cleared may be considered conservative.

Again, what is the unknown latent and long-term cost in not protecting tissue B?

That is the conservatism supposedly inherent in RD -deepstops to protect the Fast Tissues from Critical Supersaturation and potential bubbling early in the deco profile. . . (But as shown by the NEDU Study, it comes with the cost of greater surfacing supersaturation and deco stress of the Intermediate/Slow Tissues. RD users must objectively compensate by conservatively extending out their Oxygen profile times. . .)

 

[victorzamora]

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 think that you're misusing the term "efficiency" Kev. 75/75 is, generally, more conservative than 100/100. However, it's not clear which is most efficient. The questions I've highlighted here are fairly non sequitur.

The terms "efficient" and "conservative" are too specific, I think, for our current understanding of deco to be used as vaguely as is common to use them.

 

[Kevrumbo]

I think that you're misusing the term "efficiency" Kev. 75/75 is, generally, more conservative than 100/100. However, it's not clear which is most efficient. The questions I've highlighted here are fairly non sequitur.

The terms "efficient" and "conservative" are too specific, I think, for our current understanding of deco to be used as vaguely as is common to use them.

The point is again if you believe in "conservatively" protecting your Fast Tissues with deepstops as per RD (or ostensibly by choice with a low value GFLo like 20 or 30 in Bühlmann Gradient Factors), then compensating "conservatively" enough in the deco schedule with an extended O2 profile to account for the resultant Slow Tissue on-gassing and later surfacing supersaturation has to be a mandatory add-on procedure for better efficacy in overall inert gas washout and less deco stress & risk of DCI.

Extrapolating it out even further, given a 11L/AL80 O2 cylinder supply, you can eventually do as much shallow Oxygen decompression as needed to clear slow tissues of surfacing inert gas supersaturating tensions such that it doesn't matter what amount of RD Deepstops or GFLo value you did earlier to load those tissues -this instance can be thought of as the maximum in decompression "conservatism" and "efficacy", but obviously not in "efficiency".

Again Vic in contrast, the main takeaway and motivation with regards to the NEDU Study and decompression algorithm efficiency -->What matters most is a deco schedule which gets a Navy Diver or SEAL Team quickly recovered from the surface -especially in a "denied operations area" mission- with the least amount of in-water decompression time at depth, for an "isorisk" allowable level of peak Fast Tissue supersaturation early in the deco profile and minimal Slow Tissue integral time of supersaturation upon surfacing.

 

[mikeny9]

I'm not very interested in learning how to come up with what I consider a bad decompression schedule (I consider the one Capt S posted originally to be bad) in my head...particularly not for real deco dives, let alone recreational/light deco ones.

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

 

[rjack321]

Sorry I am late to the party...
I read through most of the better posts here (I skipped the word salad, and went for dessert) and offer a few thoughts...
I started off with min and ratio deco using a bottom timer in about 2002 and GUE-F and later tech1. I think taking UTD tech1 is a good idea to expand your universe, even if I think some of the theory that is taught now is not state of the science.

1) Depth averaging: I do this like Hu Porter, I don't try and remember 6 or 7 different depths and compute an average at the end. I keep a rolling average in my head, "if I had to ascent right now what depth would I use?" If the next five mins ends up deeper than that depth the average slides down, if the next 5 mins is shallower the average slides up. Near the end of the dive, initial ascent-ish, I look back over the profile in my head and decide whether I have been shallowing or deepening and if there is an element of reverse profiling in my sawtooths (if any). I make a final call on what my average for deco purposes is then (it could end up being my max depth ala OW tables even)

2) Min deco and 32% and repetitive dives - seems to have a reliable track record for my purposes. At least here in cold water we don't do 3+ dives a day with square profiles and minimum surface intervals. Places were this is common (Bonaire) have profiles where you can easily and pragmatically add a ton of shallow "deco" time based on what you've been doing. So the tools provided for repetitive 32% diving with just a bottom timer just seem to work out in practice. The handwringing over residual N2 time has been ongoing since the early 2000s and it you are really doing 5 square profile dives a day on air the min deco rules are going to bite you in the butt. But with the widespread availability of EANx, the recognition of shallow stops, and the really that few divers race out of the water on repetitive multileveling dives anyway it just doesn't seem to be a problem in practice.

3) 32% and 25/25
When AG started UTD (2008?) the original EADs for 32% was -20% and for 25/25 it was -10%. This is simple and if you drop numbers into decoplanner etc you'll see that -10ft of depth for the 80-130ft range that you'd actually use 25/25 seems to roughly fit. At some point this got changed, with really no evidence to support it, to -20% just like 32%. This is just bogus and frankly I think its a stupid way to save 5 mins. When I was doing ~120ft wreck type (not long cave dives) deco dives on OC with 25/25 and O2 for deco (aka UTD tech1 type of dive) we had 3 different deco times. We did ~8-12mins between 60ft and 20ft and 5, 10, or 15mins on O2 depending on the profile. This was cut with an axe type deco, and to my knowledge in probably 1,000-1,500 dives amongst my buddy group we had no DCS events. When in doubt (because we were scootering big sawtooths around a wreck) we did 5mins more on O2. Pragmatically -10% EAD and O2 'clean-up' deco worked well for us. If I was doing a "no deco" 25/25 dive, which I think we did 2-3x(?) as part of a recreational class I DMed with, the -20% EAD was in theory aggressive but the dives were cut short by at least one student's single tank volumes anyway. So I have never bumped into that -20% EAD more aggressive limit. Personally I think its a stupid way to squeak out a couple of extra minutes of bottom time. But I am also the type of diver who is willing to hang an extra 5 mins because we're cold or the average depths weren't really progressively shallower, or the biggest one is that a chamber ride is far away and no fun at all, padding the deco over UTD's thin baseline is not a big deal on modest wreck dives. Save -20% EAD for 32% and use -10% EAD for 25/25. Whatever rationale UTD has for this wont have a single peer reviewed paper to support it...

4) 21/35 + 50%, 18/45+ 50% + O2. 1:1 and 1:2 setpoint dives.
I have roughly 500 trimix dives, using adapted RD for these kinds of depths. 130 to 210ft, 15 to 30mins BT. I found RD aggressive but do-able. I got skin bends once (in conjunction with a flooded suit) and felt like crap a bunch of times until I worked out something that worked for me. This personal experimentation was long before reliable computers like a shearwater existed, long before the NEDU and Spisi studies. I figured out by trial and error that the 75% deep stops were crazy (by getting sore and really tired). I was able to nudge my buddies up to a "2 ATA" off the bottom deep stop (usually only a minute thankfully) by 2008 or 9 which helped, but in retrospect was still too deep. In combination with trying really hard to get off the bottom and up at least to the point of some offgassing faster, I started adding O2 to even tech1 dives that wouldn't otherwise use it. So all my 150ft, 20 min 21/35 dives were 2 deco gas dives. That helped too. What really ameliorated my aches and pains was doing all of what UTD would call shallow time (combined 20+10ft stop times) at 20ft, then doing 5 more mins at 10ft. So basically my pragmatic rule added 5mins shallow to every deco dive I did with my UTD OC buddies. They knew I needed more time and were ok with that. Personally I think I was reducing their DCS stress in the process too even if they assumed they were "clean" after the 20ft stop.

I don't think the class is outright dangerous, BUT I do think (depending on your age and fitness) that many RD practices (especially around deep stops and total deco time and the distribution of that time) are riskier than other available ideas. Thankfully you have the benefit of a bunch of illuminating studies that I didn't have back in the late 2000s when bubble models and the "randomness" of DCS risks were perhaps all we had to go on. Today you can be more informed about the profiles generated and modify them based on actual science instead of personal experimentation (and consequent aches and pains of likely subclinical DCS). At least in my experience (and perhaps this is age related, I am 49yo at the moment) I have always been adding time, almost always shallow in the 10-30ft range compared to what the RD rules would dictate.

Have fun with your class,
R