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    RBW Member piernegri is an unknown quantity at this point piernegri's Avatar
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    Question of CO2 sensor

    Why do some Reb measuring the percentage of CO2, and not partial pressure, as it does for oxygen? What do you feel is the safe limit of the partial pressure of CO2?

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    Nils Möllerström nilmol will become famous soon enough nilmol will become famous soon enough nilmol will become famous soon enough nilmol will become famous soon enough nilmol's Avatar
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    Re: Question of CO2 sensor

    Quote Originally Posted by piernegri  View Original Post
    Why do some Reb measuring the percentage of CO2, and not partial pressure, as it does for oxygen? What do you feel is the safe limit of the partial pressure of CO2?
    It's expained in this post:
    http://www.rebreatherworld.com/showthread.php?t=27800

    /nils

  3. #3
    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: Question of CO2 sensor

    Quote Originally Posted by piernegri  View Original Post
    Why do some Reb measuring the percentage of CO2, and not partial pressure, as it does for oxygen? What do you feel is the safe limit of the partial pressure of CO2?
    Hello,

    It was predicted in the thread linked by Nils that displaying PCO2 as a percentage surface equivalent would cause confusion and this thread confirms it. I agree that it would have been more sensible to display the PCO2 as a PCO2! This applies to the Apoc of course, and many on this board have written it off because of the interminable delays. However, Alex Deas has popped up elsewhere suggesting that the unit is still coming. We will see. The method by which they are trying to derive an end tidal CO2 has changed since the early debates on this thread and it will be illuminating to evaluate its accuracy. Personally, I think it would be great if it works, but as I say, we will see.

    As to "safe" limits, that is a tricky question. A normal end tidal CO2 is about 5kPa. We know that divers can behave irrationally and become unable to self rescue if it climbs higher than 8 -8.5 kPa, so that is definitely unsafe. But it does not imply that anything up to 8 kPa is "safe". For example, elevated CO2 can predispose to cerebral oxygen toxicity, and will be synergistic with nitrogen narcosis. "As close to normal as possible" is the safe level, but the unfortunate truth is that CO2 often climbs during diving. There is reason to believe that levels around 6.5 - 7 may be fairly common when using breathing apparatus underwater, and while not ideal for the reasons stated above, this does not seem to cause overt harm very often. Anything over 7 - 7.5 I think is becoming dangerous.

    An interesting point that arises from this discussion is that you can see the difference between safe and unsafe (these numbers are not far apart) is small, and that is why it is vital that an end tidal CO2 monitor used in diving is accurate... particularly if the number is going to be used to drive warnings and autobailout as in the Apoc. Even an inaccuacy of 1kPa is significant in this context.

    Simon M

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    Re: Question of CO2 sensor

    The astronauts of Apollo 13 were facing a rise in CO2 levels when the lithium scrubber cartridges from the command module did not fit the scrubber in the lunar module. I recall distinctly from the film that the flight surgeon was worried about mental impairment if the level reached 15. After the creative rigging to allow the use of square cartridges in a round scrubber, the level quickly dropped to 9.

    I don't know what units they were measuring. Maybe not kPa. Possibly ppm. Don't even know if it was pCO2 they were measuring. But you have raised my interest level and I will try to find out.
    Ken

    Quote Originally Posted by Dsix36  View Original Post
    Just remember that listening to an idiot such as myself may very well get your arse dead.

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    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: Question of CO2 sensor

    Quote Originally Posted by kwinter  View Original Post
    The astronauts of Apollo 13 were facing a rise in CO2 levels when the lithium scrubber cartridges from the command module did not fit the scrubber in the lunar module. I recall distinctly from the film that the flight surgeon was worried about mental impairment if the level reached 15. After the creative rigging to allow the use of square cartridges in a round scrubber, the level quickly dropped to 9.

    I don't know what units they were measuring. Maybe not kPa. Possibly ppm. Don't even know if it was pCO2 they were measuring. But you have raised my interest level and I will try to find out.
    Hi Ken,

    Yes, I recall the problem (but not the numbers) from the film.

    They were measuring inspired CO2 rather than end tidal CO2 (expired) which my numbers related to. Either way though, I can promise you that it was not 15 kPa. Inspired or expired you would be unconscious or dead. Possibly millibars I suspect.

    Simon M

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    RBW Member Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q is a name known to all Tony_Q's Avatar
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    Re: Question of CO2 sensor

    Quote Originally Posted by kwinter  View Original Post
    The astronauts of Apollo 13 were facing a rise in CO2 levels when the lithium scrubber cartridges from the command module did not fit the scrubber in the lunar module. I recall distinctly from the film that the flight surgeon was worried about mental impairment if the level reached 15. After the creative rigging to allow the use of square cartridges in a round scrubber, the level quickly dropped to 9.

    I don't know what units they were measuring. Maybe not kPa. Possibly ppm. Don't even know if it was pCO2 they were measuring. But you have raised my interest level and I will try to find out.
    Hello Ken,

    There is an interesting documentary series on the true story behind certain films, Apollo 13 being one of them (which I saw a few weeks ago on television).

    Problem of rising CO2 levels and scrubber media was identified and fixed before hazardous levels of CO2 were reached. Film just took some artistic licence for dramatic effect with the rising gauge (numbers were meaningless).

    A bit like the alleged argument after the tank explodes (What did you do?).

    Regards,
    Tony

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    Down to no good kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter has a reputation beyond repute kwinter's Avatar
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    Re: Question of CO2 sensor

    Thanks Tony. That's good to know because I was not finding any information about what units they were measuring. I will now abandon my fools errand.
    Ken

    Quote Originally Posted by Dsix36  View Original Post
    Just remember that listening to an idiot such as myself may very well get your arse dead.

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    RBW Member SEABEAR is an unknown quantity at this point SEABEAR's Avatar
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    Re: Question of CO2 sensor

    Hello,
    maybe the reason is simple, the OEM CO2 sensor that some rebreather manufacturers are currently using outputs % CO2 or ppm, but not pCO2 .... :)
    Best regards, Arne

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