UTD Ratio deco discussion

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Well the thrust of modern research has taken credibility away from deep stops. Both NEDU study and UTD's own Italian study have shown that deep stops may not be as great as they were initially believed. This is being acknowledged by all sides of the discussion so I do not understand how NEDU study will do anything more than that. That point is well taken. Discussion has now shifted to "if not deep stops then how shallow?" UTD has gone from 75% depth to 66% depth and Dr. Mitchell own preference has become Buhlmann 50/80 gradient factors. So 66% of depth and 50/80 gradient factors are the two extremes between which between you better end up in your first stop otherwise no one in the world would like to dive with you! At least we are getting somewhere right?

Problem is that neither of these two parameters (66% depth by UTD and 50/80 by Dr. Mitchell) seem to have enough science behind them to be considered more preferable than the other. Dr. Mitchell has openly acknowledged that his 50/80 gfs are very subjective and he has no science to recommend them. UTD has not shown any solid reason why 66% depth is the ideal spot to aim for other than the fact that it is obviously better than the deeper 75% they were doing.

Conclusion is:

Don't be any deeper than 66% or you are insane! Dont be shallower than 50/80 gf or you are insane too! As long as Ratio Deco (or whatever the heck it is that you are using) is putting you between these two just shut and dive and wait for some more people to get bent so that we have some real data to talk about.
 
Using most rec computers will put you at a GF of much higher than 50/80 when still within NDL.

I had a SNAFU one day and dived a rec dive with a student with the Perdix set on tec mode with GF30/70. I ended up "in deco" while he was still well within NDL. Since then I've been playing with some student dives with an i300 and my Nitek set to 50/80 to see what happens, my Perdix is the driving computer and it's on 45/80 (rec medium).

It's interesting to see how much NDL is left on some computers when the Perdix says TTG.
 
Well the thrust of modern research has taken credibility away from deep stops. Both NEDU study and UTD's own Italian study have shown that deep stops may not be as great as they were initially believed. This is being acknowledged by all sides of the discussion so I do not understand how NEDU study will do anything more than that. That point is well taken. Discussion has now shifted to "if not deep stops then how shallow?" UTD has gone from 75% depth to 66% depth and Dr. Mitchell own preference has become Buhlmann 50/80 gradient factors. So 66% of depth and 50/80 gradient factors are the two extremes between which between you better end up in your first stop otherwise no one in the world would like to dive with you! At least we are getting somewhere right?

Problem is that neither of these two parameters (66% depth by UTD and 50/80 by Dr. Mitchell) seem to have enough science behind them to be considered more preferable than the other. Dr. Mitchell has openly acknowledged that his 50/80 gfs are very subjective and he has no science to recommend them. UTD has not shown any solid reason why 66% depth is the ideal spot to aim for other than the fact that it is obviously better than the deeper 75% they were doing.

Conclusion is:

Don't be any deeper than 66% or you are insane! Dont be shallower than 50/80 gf or you are insane too! As long as Ratio Deco (or whatever the heck it is that you are using) is putting you between these two just shut and dive and wait for some more people to get bent so that we have some real data to talk about.
Well, if you still believe in protecting the Fast Tissues from supersaturation early in the deco profile schedule with deepstops, then be prepared to extend your shallow Oxygen 6m profile to reduce decompression stress and surfacing supersaturation on the Slow Tissues.

Otherwise, if you believe the Fast Tissues are robust enough to handle early supersaturation deco stress, for a more efficient deco profile, by pushing the M-value and ambient pressure gradients, then use pure Buhlmann or Buhlmann GF 50/80 for starters.
 
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Well, if you still believe in protecting the Fast Tissues from supersaturation early in the deco profile schedule with deepstops, then be prepared to extend your shallow Oxygen 6m profile to reduce decompression stress and surfacing supersaturation on the Slow Tissues.

That makes total sense and that is what Ratio Deco 2.0 is doing too now I believe. The earlier post that Mikeny9 posted had relatively shorter initial stops and they were padding their shallower stops to make them longer.
 
Kevin gets it, and believe me, that wasn't an easy transition either.

If you look at the discussion from a mile high then we're talking in part about efficiency. How fast can you get to the surface? Which ascent strategies will allow for shorter total deco obligations and which will not?

I think one of the conclusions of the NEDU study we all talk about is that deep stops DO protect fast tissues. The fact that fast tissues are NOT the ones we need to worry about is the issue that NEDU makes clear.

The issue is this: if you make a deep stop in order to protect your fast tissues and you're happy to spend the extra shallow time in order to compensate for that stop then we really don't have an issue. Many ascent strategies will allow for a "safe" ascent if you don't care how long it takes. However, the going "paradigm" is that "deep stops" allow for shorter shallow stops later. THIS idea needs to go. It is wrong and NEDU proved it.

To bring it back to this thread..... In terms of RD, the tactic allows for redistributing time deeper rather than shallower. This is all done based on a "best guess" or the diver's personal psychological "comfort zone" but it is not done base upon science. The science says to do the stops shallower rather than deeper. The deeply ingrained "instinct" of technical divers (and I will admit to having to fight this instinct myself even to this day) is to NOT "bend and mend" or go too shallow to fast, regardless of what my computer says is OK.

R..
 
Kevin gets it, and believe me, that wasn't an easy transition either.

The issue is this: if you make a deep stop in order to protect your fast tissues and you're happy to spend the extra shallow time in order to compensate for that stop then we really don't have an issue. Many ascent strategies will allow for a "safe" ascent if you don't care how long it takes. However, the going "paradigm" is that "deep stops" allow for shorter shallow stops later. THIS idea needs to go. It is wrong and NEDU proved it.

To bring it back to this thread..... In terms of RD, the tactic allows for redistributing time deeper rather than shallower. This is all done based on a "best guess" or the diver's personal psychological "comfort zone" but it is not done base upon science. The science says to do the stops shallower rather than deeper. The deeply ingrained "instinct" of technical divers (and I will admit to having to fight this instinct myself even to this day) is to NOT "bend and mend" or go too shallow to fast, regardless of what my computer says is OK.

R..

Agreed on both points. Deeper stops must be padded. This is exactly what UTD tried to do in RD 2.0 the way I am looking at it. If you compare the two profiles, old RD vs New RD given below you will notice that they have firstly shortened the stops at 70 and 60 ft depths and increased the one at 30 ft depth.

One of the other changes in RD 2.0 is not only the 66% deep stop from 75%, but the way the S-curve is shaped.

Example with old RD using a 50% bottle and 30 min of total deco:
70': 5min
60': 5min
50': 2min
40': 2min
30': 3 min
20': 12 min
10': 3 min

Example with RD 2.0 using a 50% bottle and 30 min of total deco:
70': 3 min
60': 3 min
50': 2 min
40': 2 min
30': 5 min
20': 12 min
10': 3 min
 
More great @Dr Simon Mitchell snippets (sifting through the Deep Stops Increases DCS thread again):

...I am saying (based largely on the results of the NEDU study) that transient high / peak supersaturation in fast tissues does not seem to matter as much as we thought it might, and therefore that protecting fast tissues from supersaturation early in the ascent by using deep stops does not seem as effective as assumed by bubble models. This is especially so when it comes at the cost of increased supersaturation (both in terms of peak levels and duration) in slower tissues later in the ascent. The NEDU study is telling us that this is where the problems seem to come from.

@Kevrumbo asks:
. . .Explain from a physiological basis why allowing supersaturation of Fast Tissues in this instance is less risky to those tissues and results in a lower overall incidence of DCS in the shallow stops trials of the NEDU Study. . . ?

Hello Kev,

The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum I attended last week. Hopefully this will find its way into the literature at some point soon. In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).

Simon M

Deep Stops Increases DCS
 
That makes total sense and that is what Ratio Deco 2.0 is doing too now I believe. The earlier post that Mikeny9 posted had relatively shorter initial stops and they were padding their shallower stops to make them longer.

Really? You mean compared to what they did in the past, I assume, but not in comparison to other algorithms.
Look at the comparison of a RD 2.0 profile to the Buhlmann profile I provided earlier.

Comparison of RD to Buhlmann at sea level:
  1. RD first stop at 120 feet; Buhlmann at 90.
  2. RD time on ascent between 180-80 = 9 minutes; Bulmann = 5
  3. RD 70-60 time = 10 minutes (oxygen window theory); Buhlmann = 4
  4. RD total ascent time to 50 feet = 19 minutes; Buhlmann = 9
  5. RD total time at last two stops = 20 minutes; Buhlmann = 25
  6. RD total Run time = 83 minutes; Buhlmann = 78
Comparison of RD to Buhlmann at 6000 feet:

  1. RD first stop at 120 feet; Buhlmann at 90.
  2. RD time on ascent between 180-80 = 9 minutes; Bulmann = 5
  3. RD 70-60 time = 10 minutes (oxygen window theory); Buhlmann = 3
  4. RD total ascent time to 50 feet = 19 minutes; Buhlmann = 8
  5. RD total time at last two stops = 20 minutes; Buhlmann = 36
  6. RD total Run time = 83 minutes; Buhlmann = 87
As you can see:
  • In both cases, RD made the first stops much deeper than the Buhlmann profile.
  • In both cases, the RD profile spent 10-11 minutes more time in the deepest portion of the dive than the Buhlmann profile.
  • In both cases, the RD profile spent less time in the shallow stops than the Buhlmann profile. At sea level, it spent 5 fewer minutes. At altitude, it spent 16 fewer minutes.
So, how is this evidence that the RD profile protects the faster tissues by slower ascents and then makes up for it by extending the shallower stops to compensate for that extra time at depth?
 
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