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

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

  1. Dennis Guichard

    Dennis Guichard Angel Fish

    # of Dives: 500 - 999
    Location: Durban, South Africa
    15
    4
    3
    I've got my mind into a muddle thinking through all this decompression theory and hoping someone can kindly switch my light bulb back on...

    Where I dive it has become common practice to do the last dive deepest simply because people controlling the dives think that a short deep dive (31m for 40 minutes) is safer than a long shallow one (say 18m max for 60 minutes)... so we would have done two 18-24m-ish dives on Friday, same again on Saturday, and then a 'quick' deep dive on the Sunday before getting away to drive back home after the long weekend away.

    On the one hand I understand that tissue saturation is a time vs depth equation... it's driven by half-life's of the various tissues... so on a short dive you'd in-gas the fast tissues but not as much of the slower tissues as you would on a longer dive. But Henry’s Law of diving physics states that the amount of gas absorbed by a tissue is directly proportional to the partial pressure of the gas in contact with the tissue...

    I know that decompression theory is not a 'simplistic' thing but is it rather at a simple level just a time driven thing, based on tissue half life's...?

    In recreational no-deco-stop diving, IS a deep short dive safer than a longer shallower dive, assuming all else is equal in terms of ascent rate control, safety stops, hydration, etc...?

    Research debated at the "Reverse Dive Profiles Workshop", conducted at the Smithsonian Institution in Washington D.C., in October 1999, resulted in the conclusion that in fact reverse profile diving (on the same dive or over a sequence of multi-day dives) was perfectly acceptable as long as the generally accepted 40m depth limit was not exceeded and that any repetitive dive was not more than 12m deeper than the preceding dive. This has subsequently however been disputed more recently (2013) in some dive medical books I've read and all seems to be endlessly in debate (as much around decompression theory seems to be).

    If on both dives a diver has breathed their cylinder 'empty' (from say 230bar down to 50bar), haven't they both in fact breathed and thus in-gassed the same volume of inert gas? As an aside I've always questioned in my mind whether gas loading is influenced or not then by our breathing rate too rather than just something time-based...? So, to my original question, if you breathe the same volume of gas on both the deep and shallow dives isn't the in-gassing near enough the same and thus one isn't necessarily safer than the other at all...?

    I appreciate and value any guidance and clarity...
     
    Bent Benny likes this.
  2. Steve_C

    Steve_C Divemaster

    # of Dives: 500 - 999
    Location: Raleigh, NC USA
    4,080
    2,647
    113
    Ongassing depends on pressure. I can breath air all day at the surface and I do not ongas. I maintain the same pressures internally. I gan breath my tank dry at 20ft in the local quarry or on a shallow reef and I will not have ongassed much. That is what the NDL tells us. Deeper I go the faster I on gas.
     
    Blueringocto_73 likes this.
  3. Dennis Guichard

    Dennis Guichard Angel Fish

    # of Dives: 500 - 999
    Location: Durban, South Africa
    15
    4
    3
    Thanks Steve, I would agree, but I'm confused about how that then relates to tissue half-life's in decompression theory that are all just time based... what am I missing...?
     
  4. Bowers

    Bowers Public Safety Diver

    # of Dives: 200 - 499
    Location: Ohio
    253
    130
    43
    they are time AND depth based. the deeper you go the greater potential you have for ongasing, but you still require time for it to happen. Thats why the tables allow less time on deeper dives.
     
  5. Dennis Guichard

    Dennis Guichard Angel Fish

    # of Dives: 500 - 999
    Location: Durban, South Africa
    15
    4
    3
    Thanks Bowers, but a 5-minute tissue (say) would always take 5-minutes to 50% saturate with inert gas... unless tissue half-life's are stated at surface (atmospheric pressure) levels and those in-gassing times change in relationship to absolute pressure...? But I haven't seen that said anywhere that I've read... I'm seemingly missing the connection in my mind about how depth/pressure relates to in-gassing times...
     
  6. Dennis Guichard

    Dennis Guichard Angel Fish

    # of Dives: 500 - 999
    Location: Durban, South Africa
    15
    4
    3
    ah.. hang on... tissue saturation IS time based... but it's related to ambient pressure too... so at 10m depth it would take a 5-minute tissue 5 minutes to 50% saturate to 2 bar ambient pressure (30 minutes or 6 half life times to be considered near enough 100% saturated). At 100m depth it would still take 5-minutes to 50% saturate but to 11bar absolute... same half-life time period but substantially different levels of absorbed inert gas. It's the partial pressure of this gas that then dictates decompression times/ceilings etc... (thinking it out 'aloud' to myself...) :-D
     
    Bowers and Steve_C like this.
  7. Dennis Guichard

    Dennis Guichard Angel Fish

    # of Dives: 500 - 999
    Location: Durban, South Africa
    15
    4
    3
    So question... because I'm having a questiony deep-thinking kind of Sunday... if a diver breathes all their gas out with a higher breathing rate in say 20-minutes vs another with a slower breathing rate doing the same but over say 40-minutes... have they both in-gassed the same amount of inert gas at depth...? So the first diver has spent less time at depth but he's breathed the same volume of gas...? Do tissue half-life's dictate that the first diver is 'safer' relatively because he's in-gassed less, regardless of his breathing rate...? There has to be some level of correlation...?
     
  8. Steve_C

    Steve_C Divemaster

    # of Dives: 500 - 999
    Location: Raleigh, NC USA
    4,080
    2,647
    113
    Since the body does not metabolize nitrogen at a given depth it is mostly the time of exposure. At a given pressure, the tissue absorbs at a certain rate. Most of the air is wasted in OC. Forget how much air the diver asks . Just ask what happens if in a large room at given dept for 20 minutes vs 2 hours. Now it is totally different with CO2 since we produce CO2 and the breathing flushes what we produce out.
     
  9. Dennis Guichard

    Dennis Guichard Angel Fish

    # of Dives: 500 - 999
    Location: Durban, South Africa
    15
    4
    3
    I think I've thought it through... so Henry's Law is correct (naturally) because at increased depth / ambient pressure substantially more volume of gas is being driven into the tissues in the same half-life time period... 50% or 100% tissue saturation at 100m is substantially more gas absorption than in the same time period (half life period) at shallower depths... I think I'll go put the kettle on and make some tea...
     
  10. dberry

    dberry Hydrophilic ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Philadelphia
    984
    507
    93
    Minor quibble: you refer to "Henry's Law of diving physics." Henry was a chemist, and the law of partial pressures is invariably taught in general chemistry classes.

    In fairness, the subject is probably best classified as "Physical Chemistry".

    How's the cuppa?
     
    Storker likes this.

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