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Decompression Theory

Discussion in 'Advanced Scuba Discussions' started by acreichman, Jun 3, 2020.

  1. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    This is a really hard idea for a lot of people to grasp. I am going to say roughly the same thing in a way that may help some people understand the issue of "decompression efficiency" in different terms. I may just confuse people more, but I will give it a shot.

    Let's say I want to do a dive that requires X minutes at depth, and my algorithm tells me I will have to do 30 minutes of decompression at various depths. If I instead choose to do X + 5 minutes at depth, I will have to do additional decompression time. No one disputes that.

    Let's say I instead do X minutes at depth and then add 5 minutes of deep stops to my ascent. Are those decompressions stops, or are they added bottom time? If I add additional shallow decompression time to compensate, then what I am really doing is considering the deep stops to be added bottom time that requires additional decompression.

    But is there a benefit to adding that additional, shallower bottom time/deep stop, whether you don't add extra shallow time (as some people advocated) or do add the extra time, making the ascent less efficient? That is the big question right now. With the Buhlmann algorithm, the most efficient way to the surface is 100% Buhlmann, but no one I know one is advocating that. Most people I know are thinking that there is some benefit to starting a little deeper than 100% Buhlmann and then adding the extra shallow time. They just don't think it was a good idea to stop as deep as people were advocating before, even with the extra time shallow. There is no value to that much added time.

    So everyone using Buhlman with GFs is doing deep stops to some degree, and those stops are a blend of extra bottom time and decompression time. They have to do additional shallow stop time to make up for the increased bottom time, but they hope to achieve a benefit from that added decompression time as well.
    UCFKnightDiver and fsardone like this.
  2. fsardone

    fsardone Solo Diver ScubaBoard Supporter

    # of Dives: I just don't log dives
    Location: Rome, Italy
    ^What he says, he said it better! :D
  3. Dr Simon Mitchell

    Dr Simon Mitchell ScubaBoard Supporter Staff Member ScubaBoard Supporter


    The concept of "decompression efficiency" is tricky to explain. I recently gave an on-line lecture for a Polish group on "optimal decompression". It is an update of the one I gave in South Africa 5 or so years ago, and some of you might find it interesting. It is a bit laboured and long because I was asked to speak slowly because it was simultaneously translated into Polish (I am told the translator did an amazing job), but despite the pace and length, for topic-area enthusiasts it covers off many of the concepts and controversies that get debated on these forums. Early on I explain decompression efficiency. You can find it here. Its in English once I start talking (obviously - my Polish isn't that great!).

    Regarding the debate about Mark Powell's position on deep stops from earlier in the thread, some perspective is required. I should preface this by saying that Mark is a personal friend and a "top bloke" as we say in the Antipodes, and I doubt he would disagree with anything that I am about to say. When Mark wrote the first edition of Deco for Divers he did so as a diving instructor - not as a decompression physiologist. That is actually what made the book so good. He researched the area carefully and pulled the information together from the perspective of a diver who wanted to know more but couldn't find "approachable" explanations in the literature. The way he wrote it really worked for people without formal training in diving physiology. The position he took on deep stops reflected the widespread beliefs of the time which I have spoken about previously. Suffice it to say that like just about everyone, and based almost entirely on theoretical attraction, Mark believed in deep stops. In the second edition he backed away from that position to some extent, but I agree that one could still form the view that he was cautiously supportive (of deep stops). But the second edition arrived at a time when the implications of the NEDU study were still being dissected and debated, and prior to the emergence of other relevant data. I think that naturally enough, Mark had trouble 'letting go' of his first edition position. I also think that the position he has taken in the recent on line article cited earlier is entirely reasonable, and consistent with my own. He acknowledges that bubble models probably over-emphasise deep stops, but he does not know how far to back away from them. He does not advocate endlessly shallower first stop approaches. I think this is the correct position as I have said on many occasions. I also think that Mark's experience with the two editions of his book, and his recent alliance with DAN-Europe in a pragmatic deep stop vs shallower stop study has seen him evolve into a classic citizen-scientist. I am looking forward to the formal report of that study.

    Simon M
  4. clownfishsydney

    clownfishsydney Loggerhead Turtle

    # of Dives: 2,500 - 4,999
    Location: Sydney Australia
    Thanks Simon, as always, enlightening.

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