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Loop Volume Question

Discussion in 'Rebreather Diving' started by AdamSa, Dec 2, 2019.

  1. JonG1

    JonG1 Nassau Grouper

    # of Dives: 2,500 - 4,999
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    I thought I'd read that the actual volume of inert gas passing from capillaries to lungs was so small that this phenomenon of loop volume change due to offgassing rate was debunked.

    The proposed reality was that it takes time for unmixed diluent in the nooks and crannies of the unit to be displaced, and when it does the po2 drops?
     
  2. rjack321

    rjack321 Captain

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    It depends on the depth of the dive but its on the order of <10liters IIRC. There were some posts with references about inert volumes in the NEDU vs VPM threads over the last couple years but they are too buried for me to find now.

    Its a far smaller volume (even at 1 ATA) than most realize.
     
  3. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    Are you sure it's a few liters? I'd bet it's a lot less even on a tech dive where supersaturation exists.
     
  4. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    I have never been able to notice enough volume increase during deco to to even register a difference, what I usually notice is a PO2 drop in a perfectly oxygenated loop ie. 1.6 at 20 ft that starts to drop as inert gasses from my tissue dilute the oxygen. The rate of drop slows down as the gas gradient decreases.
     
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  5. jgttrey

    jgttrey ScubaBoard Supporter ScubaBoard Supporter

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    Yes, that's how I notice it most easily as well. I'm on a stop adding nothing but O2 periodically yet loop PO2 keeps dropping. Ipso facto, there is inert gas being added to the loop by my offgassing. It isn't coming from anywhere else. It's a significant volume and does impact loop volume noticeably, but like you, I first notice the PO2 decline. Eventually, I need to O2 flush again to get rid of all the inert gas that's been offgassed.
     
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  6. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Would be interested in @Dr Simon Mitchell's input on this interesting physiology question...
     
  7. rjack321

    rjack321 Captain

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    Same

    I notice a ppO2 drop but no substantive increase in volume. I end up dumping part of my loop volume to avoid having to inject tons of O2 to counter the drop though - which probably explains not noticing a volume increase.
     
  8. davehicks

    davehicks Barracuda

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    The reaction of the scrubber is also producing H2O some of which will be water vapor in the loop. So there is additional gas entering the loop with the scrubber reaction. The solid sorb granules themselves contain about 20% water before reaction and this gets released by the reaction.

    Soda lime - Wikipedia

    The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:
    1) CO2(g) → CO2(aq) (CO2 dissolves in water – slow and rate-determining)
    2) CO2(aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
    3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2)
     
  9. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

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    I think it is the solenoid action in an eccr which leads to buoyancy effect and thus noticing the volume increase here.
     
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  10. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    So the question of how much saturated gas is held in the average diver's tissue during a dive has been discussed before on another forum and I watched intently while the dive nerds who had way more schooling than I duked it out. The final conclusion of that debate matched a comment I had previously found in an old PDIC open water manual that stated we "hold about 1 quart of nitrogen in our bodies after a dive" .
     
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