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Thread: How long to get a homogeneous mix?

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    RBW Member maxguru is an unknown quantity at this point maxguru's Avatar
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    How long to get a homogeneous mix?

    So...

    I had a 100 cf supply cylinder of He with about 1500 psi in it, and didn't want to pay another month's rent on the cylinder, so I boosted as much of it as I could into an HP130 that already had about 1000psi of 14/55 in it. I wound up with about 2700 psi if 5/80, more or less. After the boosting, I used a transfill whip to connect that cylinder to another cylinder containing about 2000 psi of 14/55, and let the two cylinders equalize. I let them sit, whipped together, over the weekend. When I removed the transfill whip, one cylinder contained 8/77 and the other contained 17/50.

    Will they eventually settle out to having the same mix in both cylinders?
    How long will it take?
    Any ideas for how to accelerate the process?

    The point of this exercise was to avoid wasting helium. I figured to be able to doctor whatever the blend turned out to be and get something reasonable for diving later on.

    Thanks,
    max

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    RBW Member TCVA is an unknown quantity at this point TCVA's Avatar
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    Re: How long to get a homogeneous mix?

    Most commercial transfill whips have a one way check valve to prevent backflow to the parent cylinder...so in my opinion...no

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    RBW Member rjack will become famous soon enough rjack will become famous soon enough rjack will become famous soon enough rjack's Avatar
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    Meg

    Re: How long to get a homogeneous mix?

    Quote Originally Posted by TCVA  View Original Post
    Most commercial transfill whips have a one way check valve to prevent backflow to the parent cylinder...so in my opinion...no
    even without the check valve they will never mix.

    this has been demonstrated years ago with a set of doubles that were filled with air on one side and helium on the other. after filling the isolation valve was opened and they rode around in the mixer's truck for weeks - bottom line there's no mixing

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    RBW Member w3dge is an unknown quantity at this point w3dge's Avatar
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    JJ CCR

    Re: How long to get a homogeneous mix?

    If there's no pressure gradient, they will take a VERY long time to mix (practically speaking, they never will). isobaric mixing is driven by random chance of particles moving around - the chance of enough particles randomly moving through the whip and into the other cylinder is very low.

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    RBW Member broncobowsher is on a distinguished road broncobowsher is on a distinguished road broncobowsher's Avatar
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    Re: How long to get a homogeneous mix?

    I would check back in 100 years and see if it is halfway there yet.


    If you look at the size of the connection, the diameter of the hose, the length of the hose, and no driving force. There will be a gradient through the length of the hose. But it will be rather stagnant.


    You could try to force it by heating one tank (leave in the sun) and put the other in a freezer. Switch tanks every few hours. That would put a driving force to flow through the hose. But you would probably still be in it for days if not weeks. Even then there is the question of how close to each other you will accept?

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    Re: How long to get a homogeneous mix?

    Quote Originally Posted by w3dge  View Original Post
    isobaric mixing is driven by random chance of particles moving around - the chance of enough particles randomly moving through the whip and into the other cylinder is very low.

    Eventually they will mix though.. part of the randomness concept as well..
    weird, eh ;)
    question therefore is not will they mix but when... and how many lifetimes are needed :)

  7. #7
    Dave Tomblin wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc's Avatar
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    Re: How long to get a homogeneous mix?

    boost the 8/77 into the 17/50 as much as possible then equalize them again.
    Cheers,

    Dave....

    www.wedivebc.com

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    RBW Member ECho is on a distinguished road ECho is on a distinguished road ECho's Avatar
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    Re: How long to get a homogeneous mix?

    OK, just to be an ass... by most of the arguments below, there should never be a case of isobaric counterdifusion driven DCI... the differential partial pressures necessitate that the two cylinders will eventually reach the same mix, assuming there are no barriers.... however, as broncobowsher rightly pointed out, the cross-sectional area and length of the hose connecting the two gas volumes will be the determining factors to the mix time... if one had a booster, they could push the gas volume back and forth a few times and, likely, get a pretty close to homogeneous mix...

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    Jst anothr Breather Diver Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius is a splendid one to behold Nitrogenius's Avatar
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    Re: How long to get a homogeneous mix?

    Quote Originally Posted by ECho  View Original Post
    OK, just to be an ass... by most of the arguments below, there should never be a case of isobaric counterdifusion driven DCI...

    you might have forgotten smart before the a word, but maybe it was on purpose ;)
    You are aware that the isobaric is only referring to the ambient pressure..
    The partial pressures of the (in case of trimix two) inert gases though are in fact differing (and in the cases documented even quite significant), so I am not sure how you would want to argue the case that the arguments mentioned in this thread would oppose the occurrence of isobaric counterdiffusion induced DCI ?

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    RBW Member w3dge is an unknown quantity at this point w3dge's Avatar
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    Re: How long to get a homogeneous mix?

    Quote Originally Posted by ECho  View Original Post
    OK, just to be an ass... by most of the arguments below, there should never be a case of isobaric counterdifusion driven DCI... the differential partial pressures necessitate that the two cylinders will eventually reach the same mix, assuming there are no barriers.... however, as broncobowsher rightly pointed out, the cross-sectional area and length of the hose connecting the two gas volumes will be the determining factors to the mix time... if one had a booster, they could push the gas volume back and forth a few times and, likely, get a pretty close to homogeneous mix...
    Within the body there is a much larger surface area for the molecular diffusion to happen between tissues (blood supply), although the diffusion is still random chance. The rate of on/off gassing is largely perfusion limited (ie blood flow), and is analogous to the transfill whip in this scenario. Tissues with poor blood flow on-off gas slower.

    Compared to the surface area of the cylinders, the orifice for the gas to enter/leave is tiny, resulting in a very low probability of the molecules entering it.

    ICD happens because higher pressure He enters the inner ear tissue from the adjacent fluid reservoir at the same time as higher pressure N2 enters the inner ear tissue from the adjacent blood vessels, resulting in a higher overall tissue tension. There is a (relatively) large surface area for this diffusion to happen between the tissue types.

    Something like that anyways...

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