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Nitrox for shallow water artifact diving??

Discussion in 'Advanced Scuba' started by calabash digger, Jun 8, 2021.

  1. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    You are thinking of a diver's body as if it were one tissue either on gassing or off gassing. When we dive, different tissues absorb nitrogen at different rates, and they off-gas at different rates. When we ascend, the faster tissues will be off-gassing, but slower tissues will still be on-gassing.

    If we do a proper ascent at a proper rate, then enough off-gassing of the tissues will have occurred so that none of them have too much pressure in relation to ambient pressure. A safety stop helps make sure that all tissues are in a safe range. During that safety stop, the faster tissues will continue to off-gas, but the slower tissues will still be on-gassing. Although they are on-gassing, they are still in a safe range relative to ambient pressure.

    Off-gassing of all tissues does not begin until the diver surfaces. In the case of technical divers who decompress with oxygen, the faster tissues will actually on-gas during the surface interval.
     
  2. BRT

    BRT not a soft touch

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    At 33 feet a diver's body is offgassing, if they came up from deeper than 33 feet. It is ongassing if they just came down from 20 feet.
     
    sabbe and DiveClimbRide like this.
  3. Wibble

    Wibble Contributor

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    it’s the change in pressure that results in on and off gassing. The standard models use different compartments that on/off gas at different rates. This results in some compartments on gassing whilst others offgas if the diver changes depth.

    To use an analogy, imagine 16 glass jars each with a sealed lid. You then laser cut a microscopic hole into the lid of one jar, two holes in the next jar, three in the next and so on. If you then put them all in a decompression chamber which is pressurised to 3 times air pressure (3ATA) that would be the same as diving to 20m/66'. The jar with 16 microscopic holes would let in air faster than the jar with just one hole. This is on gassing.

    After a while the chamber pressure is reduced to 2ATA (the same as 10m/33'), the jars that ongassed faster (lots of holes) would now start letting out gas because the pressure in the jars is nearer to 3ATA compared with the ambient 2ATA pressure. However, the jars with few holes would not have filled during the 3ATA and may be still close to air pressure (1ATA) and so would continue to ongas at 2ATA.

    The mathematical decompression models such as Bulhmann have "compartments" that on and offgas quickly — fast compartments — taking minutes, But the slower compartments taking hours.

    The human body can only stand a certain amount of over pressure in these compartments before causing problems, e.g. DCS. This you have to control the speed of the ascent to stop getting DCS.

    It is the compartments with the greatest pressure differential which controls the decompression schedule the model creates.


    Could the OP's fatigue be showing signs of DCS in their shallow but long dives?



    N.B. The above is just an analogy as an aid to explaining the model.
     
  4. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    People get confused about tissues being "saturated." When a tissue is "saturated," it means that it is at the same partial pressure as the air the diver is breathing. If the diver is at 33 FSW, the diver is breathing gas at total pressure of 2.0. When a tissue is saturated, it means it, too, is at a total pressure of 2.0. The nitrogen at the surface is 79% of the total pressure, so its partial pressure is .79. When a tissue is saturated at 2.0 ATA, the nitrogen partial pressure will be 1.58.

    If the the diver descends to 66 FSW, then the total pressure is 3.0, so that tissue is no longer saturated and the tissue will begin to on-gas. If instead the diver goes to 20 feet, then the tissue is supersaturated and will begin to off-gas.

    When a diver is doing a safety stop, some tissues are supersaturated and are off-gassing, and some tissues are still on-gassing. At safety stop depth, the highest total pressure a tissue can reach is about 1.5. So if the diver were to miraculously stay at the safety stop until all tissues have on-gassed all they can possibly do at that depth, the tissues will still be close enough to surface pressure for the diver to go directly to the surface. That is why there is no limit to how long you can stay at safety stop depth.
     
    kelemvor, Bob DBF and Esprise Me like this.
  5. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    Nor is there evidence it doesn't.
    But *I* feel better, so that's what matters to me.
     
    yle, rjack321, DiveClimbRide and 6 others like this.
  6. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    33 ft is 2 ATM, not one.
    You are offgassing at 20 ft because your body still has gas in it from deeper.
     
  7. 100days-a-year

    100days-a-year Solo Diver

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    Never noticed any difference in shallow dives, midl level dives or deep dives. Have done over 1000 shallow artifact, fossil and lobster dives and always used whatever tank was full in the garage. Artifact diving was a lot less strenuous than the lobstering.
     
    calabash digger likes this.
  8. Bowers

    Bowers Public Safety Diver

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    There’s a lot to be said for slowing your ascent rate even if your not deco diving. Coming up from 20’ at 10’/minute and slowing down even more for the last 5 feet will offgas much more efficiently than just coming up at 30’/min and your tissues will be far less fatigued from nitrogen stress.
     
    Subcooled and MaxBottomtime like this.
  9. AfterDark

    AfterDark Solo Diver

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    Actual 33fsw is 2ATM absolute. For most discussions of this nature 1ATM is used but you can have it your way.

    The fact is nitrox is used to extend NDLs. Above 33FSW or 2ATMA there is no NDL so nitrox is no benefit to the OP at those shallow depths. Below 33FSW is where any benefit would be incurred.
     
    calabash digger likes this.
  10. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    Nitrox is used to lessen nitrogen uptake (compared to air) at ANY depth.
     

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