Modern Ratio Deco usage?

[Michael Guerrero]

But there's a lot of talk in this thread now about tissues. Phrases like "sacrificing slow tissues" like they're real things that the models are tracking.

Fast and slow tissues are real things. And I don't think there is any confusion that the decompression models are attempting to approximate these real tissues using a range of halftimes that produce theoretical compartments to track on-/off-gassing.

Am I overstating something?

 

[Michael Guerrero]

BTW, no one could pay me enough to use VPM. I think it's been sufficiently discredited.

Does Ratio Deco emulate a VPM schedule?

 

[Ayisha]

Anyone have a link to the post/article where Simon is talking about using 50/80 now? I've read his posts in the past where he talks about 40/70 but haven't seen anything recently with him discussing now using 50/80. There is quite a jump/change in schedule from 40/70 to 50/80.

He may have said 50/80 in the decompression controversies video. The numbers definitely evolved over the last few years between the original posts in the deep stops thread(s), the link that Dr Mitchell posted leading to UWSojourner's post with slightly higher GF LO's and HI's, and then to 50/80, which I also mentally noted.

I recently re-watched a David Doolette presentation on the NEDU study, and if memory serves, the difference in DCS incidence between their deep stop profile and their shallow stop profile was significant (double). However, the actual rate of DCS for both profiles was minuscule.

The traditional schedule produced 0.02% DCS and the deep stops one produced 0.04% DCS--if I'm remembering correctly.

Just remember though that Dr Mitchell said something to the effect that they had to stop when the rate of DCS in the deep stops participants reached statistical significance. It wouldn't be ethical to continue.

 

[Kevrumbo]

Does Ratio Deco emulate a VPM schedule?

Per the UTD Student Diver and Procedure Manual 2.0, RD is similar to Buhlmann 30/85 GF's and VPM set at +2 conservatism, but the profile graph of depth vs time is very different due to RD's prescribed deepstops and S-curve shaping of the intermediate Eanx50 deco stop profile. In my experience, I find RD profiles to have slightly longer run times and deeper deepstops than 30/85 GF's and VPM+2.

Here's a simple Ratio Deco example and strategy, easy to plan for a novice open circuit dive team with double AL80's (twinset 11L cylinders) with practical & economical use of only the standard mixes of 21/35 and Eanx50:

At a setpoint 150'/45m for Ratio Deco 1:1 Schedule, one 40cf/5.5L Deco Bottle of Nitrox 50% can safely cover a total deco profile time of 20 minutes for a deco Surface Consumption Rate of around 0.6 cfm/17 lpm, as well as cover a Buddy in a lost deco bottle contingency. So a pre-dive "insta-profile" depth & bottom time choices, using RD rules (i.e. adding 5min deco for every 10'/3m you go over the setpoint of 150'/45m, and subtracting 5min for every 10'/3m interval less than the setpoint), calculates out like this:

140'/42m for 25min;
150'/45m for 20min [RD 1:1 setpoint]
160'/48m for 15min

(Choose one of the above for a square profile at the particular depth of interest, or stay around the average depths of the RD 1:1 setpoint).

The Deco Ascent Profile and total time on Nitrox 50% is the same (20min) whichever depth and corresponding bottom time you choose above. DeepStops start at 120'/36m with 1min (30 seconds stop/30 seconds to ascend) every 10'/3m to 90'/27m, and then 2min at 80'/24m. The deco stop times in minutes starting at 70'/21m with 10'/3m interval stop depths ascending to the surface has a S-curve progression like this: 3,3,1,1,2,4 and finally a slow 3fpm/1mpm ascent from 20'/6m to surface. Total run time 48 to 50 minutes.

Alternatively, this is one simple and easy back-up deco profile for the novice tech diver's wet notes, if the dive computer malfunctions out and all that's left is a back-up bottom timer.

 

[Kevrumbo]

He may have said 50/80 in the decompression controversies video. The numbers definitely evolved over the last few years between the original posts in the deep stops thread(s), the link that Dr Mitchell posted leading to UWSojourner's post with slightly higher GF LO's and HI's, and then to 50/80, which I also mentally noted.. . .

". . .Equally, I do believe the data are strong enough (and bear in mind they are the only data) to consider a change in practice if you are a strong "emphasizer" of deep stops. In practical terms, "de-emphasizing" deep stops (or lessening any potential disadvantage) would mean using bubble models on very high conservatism settings, and with gradient factors, avoiding very low GF-lo values. I have been evolving my own use of GFs and am currently around 50/80 ...sometimes as low as 70 for the high value when we are at places like Bikini and I am the only diving physician. Pre-NEDU study I was GF lo of 20. This is my personal perception of a sensible graduated response to the way the evidence is currently evolving. I may well go further in future (guided by the evidence).

Sorry about the long post. Hope it makes sense."

Simon M

Deep stops debate (split from ascent rate thread) - Page 13

 

[PfcAJ]

Fast and slow tissues are real things. And I don't think there is any confusion that the decompression models are attempting to approximate these real tissues using a range of halftimes that produce theoretical compartments to track on-/off-gassing.

Am I overstating something?

A 16 compartment model does not correlate to 16 "tissues" that actually exist. Same thing with a 3 compartment model, or an 18 compartment model.

It's a nice way to conceptualize things but as far as one of those compartments actually tracking a real biological structure? No, they don't work like that.

 

[KenGordon]

I recently re-watched a David Doolette presentation on the NEDU study, and if memory serves, the difference in DCS incidence between their deep stop profile and their shallow stop profile was significant (double). However, the actual rate of DCS for both profiles was minuscule.

The traditional schedule produced 0.02% DCS and the deep stops one produced 0.04% DCS--if I'm remembering correctly.

My point is that moving some minutes around here and there is very unlikely to have any significant impact on your outcomes, as all decompression strategies that fall within these two approaches appear to be very, very safe.

You are writing the probability numeric value (a fraction where 1 = certain) as a percentage (where 100 = certainty), i.e. 100 times smaller than it should be. Really the expectation is one bend in 50 dives to one bend in 25 dives. I do not think either is something I want to be experiencing or should be termed very, very safe.

Factors other than the profile that differ bettween these test dives (which had to be aggressive to give useful bend rates in a practical number of dives) and the dives people generally do dive mean that we don't get bent as often as expected by these results.

 

[KenGordon]

I think the important metric is how much supersaturation the fast tissues can sustain/tolerate before it actually matters. Or are you saying you're not happy with the critical supersaturation M-values previously derived by decompression scientists?

Everyone diving GF is saying that they are not happy with Buhlmann's M-values.

 

[Michael Guerrero]

Just remember though that Dr Mitchell said something to the effect that they had to stop when the rate of DCS in the deep stops participants reached statistical significance. It wouldn't be ethical to continue.

As compared to the dissolved gas model, and at a point where they could make that determination by way of a valid comparison, not some arbitrary stopping point.

I think I was wrong about interpreting his chart. Three of 192 dissolved gas dives resulted in DCS, ten of 198 dual-phase model dives resulted in DCS. His y axis reads about a 0.018 for the "Shallow stops" and about 0.052 for "Deep stops." The label on the y axis says "DCS Incidence," but 3/192 is almost 2% and 10/198 is more than 5%, so the y axis is the fractional representation of % (i.e., multiply the number on the y axis by 100 to get a percentage). So nearing 2% for dissolved gas and 5% for dual-phase in the NEDU experiment.

As far as I know, these were the pure models without any added conservatism like GFs give you. They were also designed to be "provocative" dives (air at 170ft for 30 min while doing moderate work on the bottom), intended to increase the likelihood of bending divers to achieve DCS as an outcome and provide real data.

 

[Michael Guerrero]

A 16 compartment model does not correlate to 16 "tissues" that actually exist. Same thing with a 3 compartment model, or an 18 compartment model.

It's a nice way to conceptualize things but as far as one of those compartments actually tracking a real biological structure? No, they don't work like that.

Am I contradicting this in any of my posts? I even try to explicitly clarify above that the models use theoretical compartments to approximate the various tissues in the body.

However, just because they are theoretical doesn't make them useless, quite the contrary, and doesn't change anything I've said. As we have clearly seen, decompression is a game of horseshoes.

When you make deep stops you are doing so in order to prevent bubble formation in the fast tissues. This necessarily leads to additional on-gassing in the slower tissues, as would be expected. That on-gassing is approximated by the model, not actually tracked in your personal physical tissues. If you don't extend your shallow time to account for the additional on-gassing of the slow tissues then you are sacrificing your slow tissues (I.e., exposing them to greater supersaturation) for the benefit of your fast tissues.

Is that better?