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"Nitrox" Incident - Full analysis

Discussion in 'Near Misses & Lessons Learned' started by jagfish, Mar 2, 2021.

  1. jagfish

    jagfish The man behind the fish ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
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    Nitrox Incident Scuba Diving - Full Analysis
    As the final culmination of my scuba gasses series, this is my full analysis of this "nitrox" event. As many have noted, though this was posted originally as a nitrox incident, the nitrox itself was not the culprit, but there were many issues to discuss. Also, this incident led to a very spirited discussion of how nitrox gas is made/mixed and if this could have contributed to the original poster's issue. In particular, the question, "is tank rolling a thing" became the hot button. Have a listen and post your thoughts.

     
    chillyinCanada likes this.
  2. John C. Ratliff

    John C. Ratliff Instructor, Scuba

    # of Dives: I'm a Fish!
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    I haven’t seen this yet, but why not place it in the same thread? By the way, I did answer before seeing what you put out here.

    Concerning the issue of tank stratification, take a look here:
    Stratification of Nitrox Mixes

    SeaRat
     
    jagfish likes this.
  3. John C. Ratliff

    John C. Ratliff Instructor, Scuba

    # of Dives: I'm a Fish!
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    ‘Glad to see that your thoughts align with mine on this one.

    SeaRat
     
  4. jagfish

    jagfish The man behind the fish ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Kanagawa and Florida
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    Saw that analysis, John, very comprehensive and we shared a lot of ground. Fair point about posting on the other thread...it got a bit long, so I have this it's won space...
     
    John C. Ratliff likes this.
  5. MaxTorque

    MaxTorque Registered

    # of Dives: 0 - 24
    Location: United Kingdown
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    From a pure physics perspective "tank rolling" to avoid gas stratification is clearly total bunkum!

    Gases are driven by Brownian motion, the vibration and and random movement of their molecules in that gas. This movement is what gives us "pressure" because that pressure is actually those molecules hitting and bouncing off the walls of the tank. The more you compress something, the higher the pressure, because in any given volume there are more molecules hitting the confining walls at any given moment.

    So what would be the mechanism for stratification of gases in a tank? Clearly the main one, assuming there is no direct chemical reaction, is gravity seperation due to the differences in gas density.

    Here are the densities for these gases at 1bar 0degC

    Oxygen: 1.43 g/l
    Nitrogen: 1.25 g/l

    Therefore oxygen is just 14% more dense, ie it experiences a 14% greater force in the direction of gravity.

    This is clearly a small difference and driven only by the force of gravity and means that brownian motion is many time more dominant even at 1bar, and certainly way too small an effect to overcome the forces of brownian motion at hundreds of bars of pressure.

    Industrial gas centrifuges that are expressely designed to seperate different densities of gas require high speed rotation that acts with tens of thousands of times the force of gravity in order to practically seperate gases with such small density differences.

    And of course, rolling a tank itself doesn't mean the gas inside actually moves, as that gas has an inertia and only the small amount of boundary layer friction between the gas and the moving tank walls would drive the gas around with the tank.

    Finally, the hole that routes the gas into a cylinder is pretty small, a cross section in the order of tens of square millimeters at most, which means the incomming gasstream velocity is pretty high, (which is also why you fill tanks slowly to avoid dynamic high pressure, and hence high temperature, zones) and that high velocity stream causes massive swirling and mixing of the gases inside the cylinder, once again rendering "rolling the tank" as a total waste of time.

    So, we are left with the conclusion:

    1) tank rolling doesn't cause the contents to mix
    2) The contents mix themselves via brownian motion
    3) the force of gravity is insufficient over any time scale to cause stratification due to different constituent densities




    Finally, with regard to breathing pure nitrogen, ie at a PPo2 of zero bar. The old airforce tables give an expected time to loss of conciousness of as little as 5 seconds at pressures below 0.05 bar. At a true zero PPo2, your lungs will actually REMOVE the existing o2 from your bloodstream. Chances are, i'm going to suggest that a single full breath of pure N2, is likely to result in pretty much instaneous LoC, and assuming the next breath wasn't suitably hyperoxic death within a minute.
     
    lukeb, Kriet, DiveClimbRide and 3 others like this.
  6. johndiver999

    johndiver999 Solo Diver

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    LOL,, you are wrong... I completely agree with you about why you should be right, but you are wrong. I would never have believed tank rolling makes a difference.. but it does. I have seen it way too many times. Don't believe me? I can certainly understand that and don't blame you.
     
    OTF likes this.
  7. OTF

    OTF Contributor

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    This conversation has popped up in a few threads recently and perhaps deserves a new one of its own. Because now I don't know what to believe. On one hand tank rolling seems convincingly debunked by several scientific-sounding posts. Yet other people swear it makes a measurable difference. The latter group includes experienced blenders and tech instructors at my local dive shop who, while not infallible, I respect and trust immensely.
     
    Lorenzoid likes this.
  8. HKGuns

    HKGuns Contributor

    # of Dives: 25 - 49
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    The portion of that video that rang most true with me is three-some dive buddies.

    I dove with a three-some one time and will never do it again. I was with my normal buddy and a Rescue Diver we were assigned on the boat. Needless to say it didn't work out. Rescue diver refused to even remotely resemble anything that would represent a buddy and kept swimming off on his own. As I was the least experienced, my normal buddy stuck with me and let Rescue Diver worry about himself.
     
  9. MaxTorque

    MaxTorque Registered

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    Ok, simple question:

    Demonstrate that tank rolling makes a difference.

    Should be easy right?

    But, how are you going to do this? How are you going to demonstrate that the gas inside your high pressure cylinder is stratified to a statistically valid level of certainty?

    Someone just said "i have seen it with my own eyes"? er, what have you seen? Both Oxygen and Nitrogen are transparent gases, of similar density and kinematic viscousity, and are contained within a solid metalic high pressure container? So how have you "seen it"?

    and don't say "my gas analyser showed me" because the variables in that sampling are too numerous to determine. In fact, to determine the true distribution of a gas sample in such situations you would need an expensively modified cylinder that allowed you to draw off, without contamination from the ambient air, samples from different parts of the cylinder, and a precision analyser that had an inert damper medium, or operated in a near vacuum to avoid cross contamination. Simply sticking a typical commercial gas analyser on the tank valve and saying look the composition is different or whatever is NOT proof of any kind.


    And finally, if in fact O2 and N2 did stratify, it would happen with our atmosphere, which is made up of mostly Nitrogen and oxygen. But our atmosphere, although reducing in pressure with altitude, does not in fact change it's broad composition as you gain height (until you get high enough that solar formation of ozone etc occur) And it would of course happen in normal scuba tanks containing air under pressure! Imagine that massive bank of cylinders at your dive shop, that sit fixed to the wall, probably over 1,000 litres (at 1atm) of gas volume and with a single pipe comming of the neck of each (usually at the top) they too would stratify and the first person on monday morning to get a fill would get all the Nitrogen and no oxygen.........
     
    davehicks likes this.
  10. MaxTorque

    MaxTorque Registered

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    Location: United Kingdown
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    Oh, and BTW, if tank rolling did do anything, it would actually cause the gases to seperate more, because the centripetal acceleration would drive the gas with the highest density to the outside and the low density gas to the inside of the tank!
     
    Kriet and davehicks like this.

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