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How do dive computers calculate nitrogen absorbtion and clearance when using nitrox?

Discussion in 'Computers, Gauges, Watches and Analyzers' started by GJC, Jun 18, 2020.

  1. couv

    couv Instructor, Scuba

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    I like the above question and the interesting and informative answers.

    I think you'd need higher O2 (less N2) or a shallower depth-10' would do it.

    Check my math.
    .79 ata N2 at the surface breathing air.

    .87 ata N2 at 15 feet breathing 60% N2
    (15+33) /33 X .60 = .87
     
    GJC and tmassey like this.
  2. tmassey

    tmassey Manta Ray

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    Math checks. I can’t show my work like you did, but my math says that at 40%, you equal the partial pressure of surface air at 10 feet.

    Ending up with tissues cleaner than you went in is practically only going to happen with pure oxygen.

    And again, only in the fastest tissues: those that take on and push out nitrogen the fastest. So, within minutes, you’ll be very close to normal levels even if you were able to flush all the nitrogen out! :)

    (ETA: I believe the fastest compartment is five minutes. in 6 halftimes you reach 98+ percent saturation. So that means in 30 minutes you would go from 0% to >98% of saturation.)
     
    GJC and couv like this.
  3. couv

    couv Instructor, Scuba

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    :) I was just editing my post as you wrote that.
     
    tmassey likes this.
  4. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    This sounds like you'd use your dive computer's "repetitive group" to help you use the Navy tables. I hope you are aware that not all tables use repetitive groups that are the same as the Navy tables, and dive computers would be even worse. One of the fundamental rules is to NOT transfer information between tables or dive computers
     
  5. GJC

    GJC Solo Diver

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    Yeah, I get that. I would have to use the USN Tables retroactively to figure out what my rep group is in order to use the Ascent to Altitude Table.

    It sure would be nice if the computer would tell me what my nitrogen load is at the end of the day in a way that I could use to figure out my own safe ascent to altitude plan.

    I'll have to save some dive plans from the computer right after I finish some dives and see how they compare to repetitive groups from the tables. I can see how things may not match up well with the computer tracking 16 compartments and the tables not doing that.
     
  6. ATJ

    ATJ Barracuda

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    My Garmin tells me my residual N2. I dived 8 hours ago and my N2 is at 17% (which includes driving home to 260m). I'm not sure how to use that information in planning my next dive but it's there.

    My Perdix shows my tissues in a graph.
     
  7. dmaziuk

    dmaziuk Regular of the Pub

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    It's not that simple: gas exchange is driven by delta-pressure. Start with less N2 in the tissue and it'll be on-gassing faster b/c of greater dP. So it depends on your bottom time and tissue half-time, and on "multi-level" profiles: on other tissue compartments in your model too.

    Simple case: a square profile no-stop dive would be controlled by the fastest tissue, if your computer has it at 5 minutes: 5*6=30 minutes will saturate it regradless of its starting PPN2. But for RGBM's 2.5-minute TC that's only 15 minutes.
     
  8. Esprise Me

    Esprise Me Kelp forest dweller ScubaBoard Supporter

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    I've wondered if people ever do that--breathe O2 or Nitrox on the surface after a dive. Even if there's no easy way to calculate how much it shortens your required surface interval, it could add a margin of safety with repetitive dives that are pushing the limits.
     
  9. Stephen McCallister

    Stephen McCallister Angel Fish

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    what about switching between air and nitrox? Like diving nitrox in the morning and then having a 3 hour lunch break and diving air in the afternoon?
     
  10. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    Dude, you are beginning to worry me. You are looking at DM? Your quesion is in the basic Nitrox class. You set the computer for the gas you are going to use on a dive. So your morning dive you set the computer to (say) 32%, then you set it to 21% for the afternoon dive. The computer knows how much nitrogen is in you after the first dive, and calculates how much is off gassed during your SI.
     

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