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Hypothetical question

Discussion in 'Basic Scuba Discussions' started by boulderjohn, Mar 12, 2019.

See the first post for an explanation of the question related to the time to wait before flying.

  1. No wait at all

    55 vote(s)
    66.3%
  2. 6 hours

    6 vote(s)
    7.2%
  3. 12 hours

    7 vote(s)
    8.4%
  4. 18 hours

    5 vote(s)
    6.0%
  5. 24 hours

    10 vote(s)
    12.0%
  6. 48 hours

    0 vote(s)
    0.0%
  1. Diver0001

    Diver0001 Instructor, Scuba

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    In metric.

    EAD = (Depth + 10) × Fraction of N2 / 0.79 − 10

    (9+10)*(.6/.79)-10

    that's 4.4(etc.) which I rounded up to 4.5

    R..
     
    RainPilot likes this.
  2. Dan

    Dan Orca

    # of Dives: 500 - 999
    Location: Lake Jackson, Texas
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    Isn’t Fraction of N2 0.4, not 0.6? If that’s correct then the math would give EAD = -0.4 m
     
    scubadada likes this.
  3. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
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    Yep. EAN60 is 0.4 fN2, not 0.6.

    Easy mistake to make. After all EAN60 is rather uncommon, EAN40 not so much.
     
  4. Diver0001

    Diver0001 Instructor, Scuba

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    ... incidentally, I needed to look up this equation because although we all learned it during our EAN class the only thing that is really relevant to diving outside of a technical context is that you know how to enter the EAN % in your computer. Even for technical dives we use software that does all the math for us.

    That said, it's pretty much inexcusable to forget concepts like EAD even if one can't remember the math.

    R..
     
  5. swimlikethefish

    swimlikethefish Divemaster

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    306
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    My first question would be, have you completed any dives before your planned dive. Or is this a single dive profile for the day. If the planned hypothetical dive only, fly away. If other dives subsequent to that then my answer will vary from 18 to 48 depending on saturation.
     
    Diver0001 likes this.
  6. Diver0001

    Diver0001 Instructor, Scuba

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    Just goes to show you that you can never stop "sharpening the saw".
     
  7. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: New York City
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    I participated in that thread. I think that the OP's goal was to demonstrate that despite breathing underwater, the diver was breathing a gas with a lower PPN2 than surface air, and to "catch" people who just followed the rules without thinking about it. No ongassing, so no DCS risk, right? Seems a simple conclusion.

    However, the thread did generate some good discussion, and one thing that came up was this - I'm still not sure about how much of a clinical issue this is, maybe one of the deco gods can chime in...

    Ascending from 30 FSW to the surface on EAN60 means a drop in ambient pressure, which means a risk for bubble formation. Unless you are breathing 100% O2, you have some N2 in your tissues, and it's the drop in ambient pressure that start bubble growth, which correlates with DCS risk. As was mentioned in another thread here, if you switch from your back gas to 100% O2 at 20 FSW, you will suddenly put all of your compartments into strong supersaturation, but since there is no change in ambient pressure if you stay at one depth, decoing on O2 doesn't increase DCS risk.

    On the other hand, any ascent means a reduction in ambient pressure, and the risk for bubble formation. You don't need to load your tissues more than they are at the surface to be at risk for DCS. The example of that is astronauts - they breathe 100% O2 before a spacewalk, because any sudden drop in ambient pressure can cause bubble formation. So whether that happens from being exposed to a partial vacuum or by ascending from 1.9 ATA to 1.0 ATA, it's still the same process.

    The question then is, once bubbles are formed, do they put you at risk if you fly immediately (and further drop your ambient pressure to the equivalent of 8000 feet above sea level)? Maybe not for the diver described by the OP, maybe that ascent is clinically insignificant. I don't know.

    But I thought that it was an interesting topic, even though I can't stand following busy threads on that FB page because everyone just heads down side branches and makes the same points over and over.

    Unlike here! :D
     
    txgoose, Snoweman and swimlikethefish like this.
  8. Diver0001

    Diver0001 Instructor, Scuba

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    31
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  9. Storker

    Storker Divemaster

    # of Dives: 100 - 199
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    Is it? According to everything I've read, and, incidentally, also according to Henry's law, it's the drop in partial pressure that matters.

    Theoretically, one might be concerned about O2 bubbles forming, though. At 9m on EAN60, the pPO2 is 1.14 bar, compared to 0.21 at the surface on air.
     
    RyanT, markmud and Dan like this.
  10. Dan

    Dan Orca

    # of Dives: 500 - 999
    Location: Lake Jackson, Texas
    5,664
    3,200
    113
    fN2 means volumetric (or mole) fraction of nitrogen. EAN60 contains 60% of oxygen or fO2 = 0.6. If we assume other gases are negligible and the gas mostly contains of oxygen & nitrogen, i.e., fN2 + fO2 = 1, then fN2 = 1 - fO2 = 0.4.
     

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