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Deep vs shallow SPORTS diving... which is safer?

Discussion in 'Decompression Theory' started by Dennis Guichard, Oct 7, 2018.

  1. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    Since you say you refuse to use words the way they are used by others and prefer to use your own definitions, you may be interested in someone else who has the same belief--Humpty Dumpty in Alice through the Looking Glass. Here is the relevant section so you can see your kinship.

    When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean—neither more nor less.’

    ‘The question is,’ said Alice, ‘whether you can make words mean so many different things.’

    ‘The question is,’ said Humpty Dumpty, ‘which is to be master—that’s all.’

    Alice was too much puzzled to say anything, so after a minute Humpty Dumpty began again. ‘They’ve a temper, some of them—particularly verbs, they’re the proudest—adjectives you can do anything with, but not verbs—however, I can manage the whole lot of them! Impenetrability! That’s what I say!’

    ‘Would you tell me, please,’ said Alice ‘what that means?’

    ‘Now you talk like a reasonable child,’ said Humpty Dumpty, looking very much pleased. ‘I meant by “impenetrability” that we’ve had enough of that subject, and it would be just as well if you’d mention what you mean to do next, as I suppose you don’t mean to stop here all the rest of your life.’

    ‘That’s a great deal to make one word mean,’ Alice said in a thoughtful tone.

    ‘When I make a word do a lot of work like that,’ said Humpty Dumpty, ‘I always pay it extra.’

    ‘Oh!’ said Alice. She was too much puzzled to make any other remark.

    ‘Ah, you should see ‘em come round me of a Saturday night,’ Humpty Dumpty went on, wagging his head gravely from side to side: ‘for to get their wages, you know.’
    TrimixToo likes this.
  2. dmaziuk

    dmaziuk Regular of the Pub

    Alice is the required reading in my profession, but I use the word the way Powell et al used it in the DSAT '94 report.
  3. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    Try using the words the way PADI uses them. It is their table. Powell et al made input to it, a quarter-century ago. Things have evolved.
  4. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

    This addresses your question indirectly, but Hobbs et al have done probabilistic modeling of dive profiles. Below is one we did a few years ago for trimix dives. In general, with all of the decompression models we looked at, the probability of DCS increased with both depth and bottom time.

    Hobbs GW, Murphy FG, Gault KA, Hexdall E, Howle LE, Walker JR. Decompression risk evaluation for trimix divers derived from commercially available desktop decompression algorithms. Abstract/poster, Undersea and Hyperbaric Medical Society annual scientific meeting, June 2014

    Best regards,
    Dennis Guichard and Steve_C like this.
  5. dmaziuk

    dmaziuk Regular of the Pub

    ITYM "they made the table".
  6. dmaziuk

    dmaziuk Regular of the Pub

    Cool. You can plot time vs. depth where the "most conservative" estimate crosses the 2% risk line, but with only 5 points it's hard to tell if the dependency is linear or a curve... but either way longer shallower dives actually look "better" to me on that one.
  7. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

    The exact percentages aren't so significant as the slopes of the lines. The model that was used was based on HeO2 and Nitrox dives, so it isn't perfectly suited for trimix. Can you clarify "only 5 points"?
  8. dmaziuk

    dmaziuk Regular of the Pub

    There's 5 graphs for 5 depths. You can take time vs depth point where estimated risk becomes e.g. 2% on each graph. You get 5 points for 5 depths. I.e. you're taking a derivative to look at the dependency of risk on depth and time.

    Then you plot it, look at the slope, and tell the OP that risk increases with depth faster than it does with time. Or not.

    With the actual numbers you could do it per-model and/or per risk level and see how they stack up. Not sure it's of much practical use for dive medicine, it's just studying software behaviours, but that's what I frequently do at work, so...
    Duke Dive Medicine likes this.
  9. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

    That would be awesome except like you said, part of it is software behavior. I don't think you can draw hard conclusions about specific depths and times from any of this, aside from the idea that none of them are, as Gene Hobbs put it, iso-risk.
  10. dmaziuk

    dmaziuk Regular of the Pub

    Right. Part of the OP's question was about the belief that "deep and short" is "safer" than "long and shallow", I'd say it's about trends rather than specific depth and times. Those plots could show the trends as per the current models and we could see if they match the belief. We do trust the models on real dives to real depth, so why not trust them on trends as well?
    Dennis Guichard likes this.

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