• Welcome to ScubaBoard

  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Deep Stops Increases DCS

Discussion in 'Technical Diving' started by Divetech99, Dec 8, 2014.

Thread Status:
Not open for further replies.
  1. Akimbo

    Akimbo Just a diver Staff Member ScubaBoard Supporter

    10,635
    8,875
    The way it was described to me by US Navy hyperbaric physiologists, stops aren’t part of decompression algorithms that calculate bubble formation — which includes the original Haldane work at the beginning of the last century. There are no stops in the “ideal world” of a hyperbaric physiologist, only a continually declining ascent rate possibly interrupted by sleeping periods.

    Stops are a nuisance imposed by the pesky practicalities of operating in the ocean. A second and somewhat independent set of algorithms looks at bubble formation for a slice in time to create stops based on an arbitrary fixed ascent rate like 30'/minute.

    Stops are a game of continually under and over-shooting the ideal. You arrive at a stop faster than the “model” indicates you should and stay there a little longer. The squirrely part is when slower tissues are still outgassing while others start re-absorbing. Deep stops are just another way of biasing those compromises.
     
    Last edited: Dec 12, 2014
  2. Kevrumbo

    Kevrumbo Banned

    # of Dives: 1,000 - 2,499
    Location: South Santa Monica Bay/Los Angeles California, USA
    5,659
    1,349
    It's been hypothesized & conjectured before (Deep Stops) many years ago by Erik Baker (see link below). . .

    Baker also considers this in his original article on Deep Stops:

    http://www.scubaboard.com/forums/attachment.php?attachmentid=118570&d=1331862212
     
  3. PfcAJ

    PfcAJ Contributor

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
    7,920
    7,284
    your "ratio deco" is made up. It could tell you anything and you'd call it the best.

    Longer deco? It's better because it's more conservative.

    Shorter deco? It's better because it gets you out of the water faster.

    B. A. L. O. N. E. Y.
     
  4. Kevrumbo

    Kevrumbo Banned

    # of Dives: 1,000 - 2,499
    Location: South Santa Monica Bay/Los Angeles California, USA
    5,659
    1,349
    Yeah sure AJ, whatever you say : Made it all up & pulled it out from between my Ischial Tuberosities -just for you.:flush:
     
  5. Slamfire

    Slamfire Solo Diver

    # of Dives: I'm a Fish!
    Location: Langley, British Columbia, Canada
    1,322
    258
    Through simple algebra anybody can verify that the Combined Gas Law is truly an amalgamation of Gay-Lussac, Boyle, and Charles. Having AG or Kevrumbo proclaim RD an amalgamation is simply not proof enough.

    In this post I asked why it would seem that VPM (not just Buhlmann GF) is also unfairly penalizing for using helium: While on VPM, if I start stops as deep as RD starts calling for, I will have to extend shallow stops longer than what RD says. And yes, the stock answer is that helium has lower solubility and higher diffusivity than nitrogen, but nobody ever expands this qualitative statement into the realm of the quantitative.

    Why is the quantitative important? Because both buhlmann gf and vpm will yield lower deco times for any given dive when helium is present. This points to the fact that both models do take into account some deco advantages of helium. Qualitatively they are no different than RD in this aspect. The difference is quantitative. Explain it, please, because I've never seen anybody capable of explaining it. Is it even possible to explain it quantitatively?

    RD proponents cite bubble mechanics and dual phase models as the reason for starting stops so deep without adding extra time to shallow stops. They cite Yount and Wienke as their basis. Then why is it that RD won't comply with Yount's VPM?

    Now we see NEDU studies that seem to go counter to starting stops as deep as VPM calls for without adding extra time to shallow stops. Some people seem to be backing away from vpm level deep stops. RD is even more aggressive than vpm in this regard, unexplainably so. And yet there is still no shortage of proclamations that it is the superior deco strategy. All this while providing no shred of quantitative or statistical evidence.
     
    GrimSleeper and LiteHedded like this.
  6. Kevrumbo

    Kevrumbo Banned

    # of Dives: 1,000 - 2,499
    Location: South Santa Monica Bay/Los Angeles California, USA
    5,659
    1,349
    From Bruce Wienke (abridged from article "RGBM: A Simplified Overview and Update"):

     
    Last edited: Dec 13, 2014
  7. PfcAJ

    PfcAJ Contributor

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
    7,920
    7,284
    "Seasoned tech divers all had their own recipes for this process. And sure, what works works in the diving world. What doesn't is usually trashed."

    We know. Hence the willingness to make changes.
     
    rjack321 likes this.
  8. Kevrumbo

    Kevrumbo Banned

    # of Dives: 1,000 - 2,499
    Location: South Santa Monica Bay/Los Angeles California, USA
    5,659
    1,349
    With regards to the "deeper Deep Stops" of Ratio Deco on standard high Helium mixes, my qualitative take on this is due to Helium's quantitative high diffusivity -and potential affinity for diffusing into any micro-nuclei/bubble seeds- which is why you want to keep these micro bubbles small with high bubble skin surface tension, and kept there by higher ambient pressure of the deeper Deep Stop.


    From Bruce Wienke, Technical Diving in Depth, Reduced Gradient Bubble Model (RGBM) In Depth:

    Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so. . . The size of bubbles formed with various inert gases depends upon the amount of gas dissolved, and hence the solubilities. Higher gas solubilities promote bigger bubbles.

    Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.

     
    Last edited: Dec 13, 2014
  9. Divetech99

    Divetech99 Solo Diver

    232
    19
    Kevrumbo, thank you for sharing the article in post #36. Quite interesting and compelling. The link does not mention the date it was written. Do you know if this article was written before or after the NEDU study?
     
  10. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    11,272
    9,135
    According to the document properties, it is from March 2005.
    The NEDU publication is 2011, with funding shown from 2005-2011.
    But note the first pages of the NEDU report:
    "The investigators are grateful to the following individuals who reviewed the design of this study by participating in the Gas Content Versus Bubble Volume Models Workshop, held 12–13 May 2004 at the Hyatt Regency Baltimore, Baltimore, MD: (any names and then) Dr. Bruce R. Wienke Los Alamos National Laboratory"

    Interesting...
     
Thread Status:
Not open for further replies.

Share This Page