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Thread: Measuring the ppO2 indirectly using He sensor

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    RBW Member mazmazak is an unknown quantity at this point mazmazak's Avatar
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    Measuring the ppO2 indirectly using He sensor

    I have a question about the decompression algorithm that we rebreather divers use. My understanding is that these algorithms were designed for OC where the exact gas contents are know.
    When we are using our computers the only thing we know is the ppO2 in the loop because we have the O2 sensor to measure this directly. But how does the computer know the ppN2 or the ppHe in the loop? these are used for the decompression algorithm. The O2 gets metabolized and the other gasses dissolve in our tissue at different rates. For example when we decent to say 100ft the ADV kicks in to maintain our lung vol. lets say the dill tank is filled with 18/50. and for simplicity lets imagine the initial loop contents is close to 18/50 (freshly flushed with dill). So then our solenoid kick in (eCCR) to bring the ppO2 to a setpoint (say 1.3). Of course while the solenoid is kicking in we may need to vent some gas from the loop to maintain our neutral buoyancy at the same time we are breathing and both He and the N2 are getting diffuse into our tissue but at different rates. You can see that keeping track of the exact fraction of each gas other gas in the loop would be challenging. so unless the computer knows the loop/lung vol and the rate of diffusion of each gas at each depth, how does it know the exact N2 or He fraction in the loop? it doesn't, does it? but how does the estimate the FN2 and FHe. does it just base on the dil fraction 0.32 for N2 and 0.5 for He? I know it might not be very important in the grand scheme of things specially when we are dealing with models that are imperial /theoretical and not exact science. But one manufacturer of rebreathers claims to be able to estimate the ppO2 in the loop using a He sensor. Is this possible? It seems interesting

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    Re: Measuring the ppO2 indirectly using He sensor

    I'm not around long enough in CCR to give a decent answer, I can just tell you what I think.
    I don't care so much about the He. As long as I know the PPo2 and I know the max PPn2 I can reach I'm good.
    In OC it's fixed like you said, in CCR you know how much n2 is in your dil but it wil always be less so you have some kind of a safety margin built in.
    The helium content doesn't matter so much if it's a little more. With the n2 less is more (better)

    But I can be totally wrong !!!!!
    (Sorry for the Englisch)



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    Re: Measuring the ppO2 indirectly using He sensor

    Yes our meters calculate He and N2 fractions from ppo2 status of the loop based on diluent fractions. It is easy and when understood you can see it is quite near the truth.
    Same thing using He sensor....based on ppHe and dill fractions the loop fractions are calculated. PpO2 too.

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    RBW Member Capt Morgan is an unknown quantity at this point Capt Morgan's Avatar
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    Re: Measuring the ppO2 indirectly using He sensor

    Dil 18/50
    Setpoint 1.3
    Depth 40m
    Ambient pressure 5bar


    Dil is 18/50 balance being 32% nitrogen
    Daltons law states that the oxygen content is 1.3 / 5 = 26%

    Now 26% O2 + 32% Nitrogen + 50% Helium = 108% ?
    So what changes, as we know the O2 is 26% we have to normalise the other two gases.

    The remaining gas is still at a ratio of 50 parts helium to 32 parts nitrogen.
    So with 74% inert gas (100-26) we can work out the inert gasses.

    Helium
    (74 x 50) / (50+32)
    3700 / 82 = 45%

    Nitrogen
    (74 x 32) / (50+32)
    2368 / 82 = 29%

    So now in the bag we have
    26% O2
    45% Helium
    29% Nitrogen



    Hope this helps.

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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: Measuring the ppO2 indirectly using He sensor

    Someone can probably correct me, but the calculation needs to ignore the O2. It is consumed, leaving only the N2 and He as constants. The on and off gassing should play with the ratio a little, how much I don't know. I'm guessing less than you expect.

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    Re: Measuring the ppO2 indirectly using He sensor

    Consumption of O2 is replaced with injection of O2. And status of O2 is measured. More O2 in loop proportionaly less inerts in the loop.

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    RBW Member Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P is just really nice Igor P's Avatar
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    Re: Measuring the ppO2 indirectly using He sensor

    Quote Originally Posted by Capt Morgan  View Original Post
    Dil 18/50
    Setpoint 1.3
    Depth 40m
    Ambient pressure 5bar


    Dil is 18/50 balance being 32% nitrogen
    Daltons law states that the oxygen content is 1.3 / 5 = 26%

    Now 26% O2 + 32% Nitrogen + 50% Helium = 108% ?
    So what changes, as we know the O2 is 26% we have to normalise the other two gases.

    The remaining gas is still at a ratio of 50 parts helium to 32 parts nitrogen.
    So with 74% inert gas (100-26) we can work out the inert gasses.

    Helium
    (74 x 50) / (50+32)
    3700 / 82 = 45%

    Nitrogen
    (74 x 32) / (50+32)
    2368 / 82 = 29%

    So now in the bag we have
    26% O2
    45% Helium
    29% Nitrogen



    Hope this helps.
    Excelent explanation! The only thing I would add is instead of using "setpoint" use word "Ppoo2". As Ppo2 dictates fraction of oxygen. Setpoint is only desired point (target value of Ppo2 which we try to acchieve and mantain in the loop) of Ppo2. Well in eCCR usualy very near to actual Ppo2.

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    Last edited by Igor P; 22nd February 2016 at 21:14.

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    RBW Member Capt Morgan is an unknown quantity at this point Capt Morgan's Avatar
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    Re: Measuring the ppO2 indirectly using He sensor

    Quote Originally Posted by Igor P  View Original Post
    Excelent explanation! The only thing I would add is instead of using "setpoint" use word "Ppoo2". As Ppo2 dictates fraction of oxygen. Setpoint is only desired point (target value of Ppo2 which we try to acchieve and mantain in the loop) of Ppo2. Well in eCCR usualy very near to actual Ppo2.

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    I have no idea what Ppoo2 means, sorry.

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    Re: Measuring the ppO2 indirectly using He sensor

    Quote Originally Posted by Capt Morgan  View Original Post
    I have no idea what Ppoo2 means, sorry.
    Sory I mistyped. It should read "ppo2" - partial pressure of oxygen

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    RBW Member Capt Morgan is an unknown quantity at this point Capt Morgan's Avatar
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    Re: Measuring the ppO2 indirectly using He sensor

    Quote Originally Posted by Igor P  View Original Post
    Sory I mistyped. It should read "ppo2" - partial pressure of oxygen

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    Thanks, thought it was something new.

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