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Thread: Evidence based guidelines on gas density planning

  1. #41
    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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    Re: Evidence based guidelines on gas density planning

    ah the old is O2 narcotic or not debate again, feels like 2002


    Don't think this was Simon's intent here; O2 is certainly much closer in density to N2 than He.

  2. #42
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    Re: Evidence based guidelines on gas density planning

    Quote Originally Posted by rjack  View Original Post
    ah the old is O2 narcotic or not debate again, feels like 2002


    Don't think this was Simon's intent here; O2 is certainly much closer in density to N2 than He.

    Hello rjack, not meaning to side track, just legitimately confused about what it is or supposed to be. Must have missed those discussions, but I guess it was bound to come up again in a context like this... -Andy

  3. #43
    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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    Re: Evidence based guidelines on gas density planning

    Quote Originally Posted by silent running  View Original Post
    Hello rjack, not meaning to side track, just legitimately confused about what it is or supposed to be. Must have missed those discussions, but I guess it was bound to come up again in a context like this... -Andy
    Its been a long running debate based on myers-overton hypothesis, based on lipid solubility of O2 and N2 being similar. In like 2 sentences, "narcosis" is a generalized anesthetic effect created by gas molecules dissolving into the lipid layer of nerve cells disrupting their signaling potential. Because O2 and N2 have similar lipid solubilities (similar being within a factor of 2) they can be considered to behave similarly - especially because you only metabolize a small fraction of the inspired O2. Is the difference measurable on some sort of objective "narcosis scale"? Of course not. Its hard enough to measure objective narcosis in the first place despite all of us recognizing it at the extremes - like on a 75m air dive.


    Longer version here: http://www.tech-ccr.com/pdf/study-lo...er-Overton.pdf


    The wiki has a good explanation of why the myer-overton anesthetic principle is incomplete (although not specific to O2).
    https://en.wikipedia.org/wiki/Theori...sthetic_action

  4. #44
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    Re: Evidence based guidelines on gas density planning

    Quote Originally Posted by rjack  View Original Post
    Its been a long running debate based on myers-overton hypothesis, based on lipid solubility of O2 and N2 being similar. In like 2 sentences, "narcosis" is a generalized anesthetic effect created by gas molecules dissolving into the lipid layer of nerve cells disrupting their signaling potential. Because O2 and N2 have similar lipid solubilities (similar being within a factor of 2) they can be considered to behave similarly - especially because you only metabolize a small fraction of the inspired O2. Is the difference measurable on some sort of objective "narcosis scale"? Of course not. Its hard enough to measure objective narcosis in the first place despite all of us recognizing it at the extremes - like on a 75m air dive.



    Longer version here: http://www.tech-ccr.com/pdf/study-lo...er-Overton.pdf


    The wiki has a good explanation of why the myer-overton anesthetic principle is incomplete (although not specific to O2).


    https://en.wikipedia.org/wiki/Theori...sthetic_action
    Thanks for the links, will dig in, -Andy

  5. #45
    RBW Member manni-yunk is an unknown quantity at this point manni-yunk's Avatar
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    Re: Evidence based guidelines on gas density planning

    Quote Originally Posted by Mike Ferguson  View Original Post
    Mark

    I looked at a few common gases.

    air hits the lower limit at 30m
    18/45 hits the lower limit at 55m with an END of 20m
    15/55 hits the lower limit at 65m with an END of 18m
    10/52 hits the lower limit at 63m with an END of 25m
    10/70 hits the lower limit at 90m with an END of 15m

    Thank you for this.

  6. #46
    RBW Member Dive Africa is an unknown quantity at this point Dive Africa's Avatar
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    Re: Evidence based guidelines on gas density planning

    Formula as follows:



    Gas Density (g L-1 ) = [(fO2 x 1.428) + (fN x 1.251) + (fHe x 0.179)] x (y ATA)

    (y being the atmospheric pressure (i.e 70m = 8 ATA)


    So a Tx 18/45 dive to 60m as follows

    GD @ 1 ATA = (0.18 x 1.428) + (0.37 x 1.251) + (0.45 x 0.179)
    = 0.257 + 0.463 + 0.081
    = 0.801

    Dive to 60m = 7 ATA x 0.801 = 5.6 g L-1

    Lower limit is 5.2 g L-1 and higher limit is 6.2 g L-1 – so if a hard working dive reduce gas density.

  7. #47
    RBW Member matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram is just really nice matthewoutram's Avatar
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    Re: Evidence based guidelines on gas density planning

    Quote Originally Posted by Mike Ferguson  View Original Post
    Mark

    I looked at a few common gases.

    air hits the lower limit at 30m
    18/45 hits the lower limit at 55m with an END of 20m
    15/55 hits the lower limit at 65m with an END of 18m
    10/52 hits the lower limit at 63m with an END of 25m
    10/70 hits the lower limit at 90m with an END of 15m
    I wondered if a simple chart would be useful. Of course plotting all combinations isn't useful so I made the below.

    For your gasses, to nearest match I got:
    18/45 - 55m
    15/55 - 65m
    10/50 - 60m
    10/70 - 90m

    I've made a few assumption (for those interested):

    - PO2 in gas at depth is 1.3
    - He is He in gas
    - N2 is 100-He-O2@1.3
    - Values rounded to 1DP
    - Yellow >= 5.2
    - Red >= 6.2

    Not tested other values.

    Matt.


  8. #48
    RBW Member jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner is a glorious beacon of light jturner's Avatar
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    Re: Evidence based guidelines on gas density planning

    Quote Originally Posted by matthewoutram  View Original Post
    I wondered if a simple chart would be useful. Of course plotting all combinations isn't useful so I made the below.

    For your gasses, to nearest match I got:
    18/45 - 55m
    15/55 - 65m
    10/50 - 60m
    10/70 - 90m

    I've made a few assumption (for those interested):

    - PO2 in gas at depth is 1.3
    - He is He in gas
    - N2 is 100-He-O2@1.3
    - Values rounded to 1DP
    - Yellow >= 5.2
    - Red >= 6.2

    Not tested other values.

    Matt.

    Looks good to me, though I might give the 100% He a miss - gas density would be the least of my problems!!!!

  9. #49
    RBW Member SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK is a glorious beacon of light SimonK's Avatar
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    Re: Evidence based guidelines on gas density planning

    Weekends diving blown out and having had an unpleasant experience using 10/50 at 90m I had a stab at working out RB gas densities.

    The starting densities used above (g/l):

    rN 1.251
    rO2 1.428
    rHe 0.179

    are at standard temp and pressure, that is 1 ata, 0 deg C. At normal temperature and pressure, 1 ata, 20 deg C, a more realistic temperature, the densities are:

    rN 1.165
    rO2 1.331
    rHe 0.166

    The density of gas in a rebreather is

    SPO2*rO2+ (Pd-SPO2)*(FN*rN/(FN+FHe)+ FHe*rHe/(FN+FHe))

    SPO2 oxygen set point
    Pd Pressure at depth
    FN fraction nitrogen in the dil
    FHe fraction helium in the dil

    Running this for the mixes listed I got:
    18/45 hits the lower limit at 60m
    15/55 hits the lower limit at 70m
    10/52 hits the lower limit at 62m
    10/70 hits the lower limit at 93m
    which is close enough to Mikes numbers to make the whole effort a bit of a waste of time!

    Off to open another bottle!
    S

  10. #50
    RBW Member turtle dude is an unknown quantity at this point turtle dude's Avatar
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    Re: Evidence based guidelines on gas density planning

    Could this be a function\value that is displayed in CCR mixer? Like the end\ead.
    Regards
    Baz

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