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Thread: Discussion on Co2 monitoring (split from another Apoc thread)

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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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    Discussion on Co2 monitoring (split from another Apoc thread)

    All posts following have been split from this thread http://www.rebreatherworld.com/open-revolution-rebreather/27743-make-your-point-state-your-whinge-2.html please try and keep this one to a discussion on co2 and the two (so far!) co2 monitors that are close to market.
    Cheers, CD
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    Quote Originally Posted by jasondrake  View Original Post
    Measures actual end of breath c02, and records.
    Can someone in the know please tell me if this is one of the parameters the automatic bailout is triggered by?

    Thanks,

    Simon M
    Last edited by sadave; 13th July 2009 at 01:20.

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Simon Mitchell  View Original Post
    Can someone in the know please tell me if this is one of the parameters the automatic bailout is triggered by?

    Thanks,

    Simon M
    Simon, Your best off asking Alex or OSEL direct (neither seem to be very active on here currently) but as far as I am aware yes, one of the auto bailout triggers is excessive exhaled CO2.

    When not breathed off displays 0% RebreatherWorld Gallery - Message - Powered by PhotoPost
    When breathed off usually it should display 4% CO2
    IIRC Bailout trips somewhere around 6-6.5% CO2

    Not sure if this answers your question: does end of breath CO2 mean the same thing as exhaled CO2?

    Regards
    Brad

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Brad_Horn  View Original Post
    Simon, Your best off asking Alex or OSEL direct (neither seem to be very active on here currently) but as far as I am aware yes, one of the auto bailout triggers is excessive exhaled CO2.

    When not breathed off displays 0% RebreatherWorld Gallery - Message - Powered by PhotoPost
    When breathed off usually it should display 4% CO2
    IIRC Bailout trips somewhere around 6-6.5% CO2


    Not sure if this answers your question: does end of breath CO2 mean the same thing as exhaled CO2?

    Regards
    Brad
    Thank you Brad,

    But now I must admit to being somewhat confused!

    We use CO2 monitoring every day in anaesthesia. The analyser samples the 'end tidal' gas, meaning the gas at the end of each exhalation. The CO2 is measured in the end-tidal gas because it is assumed that this gas has come from the deepest part of the lung, ie the alveoli. Alveolar gas tensions are essentially in equilibrium with those in the arterial blood, and so we assume that the PCO2 in the end tidal gas is a reasonable estimate of the arterial PCO2, which in turn is the important physiological parameter. I don't know how the device shown in the photograph handles the expired gas to derive an end-tidal (end of breath) reading; maybe it just reports the peak CO2 and assumes that must be the end tidal value. I would have some concerns about the sampling protocol and the possibility for gas resident in the hoses to dilute the end tidal CO2 concentration. I would be interested in knowing how that has been addressed.

    However, of more importance (and concern) is the curious nature of the display. It is labelled 'PCO2' (a convention for partial pressure of CO2) but the read-out is expressed as a percentage. These are not the same and mean completely different things, especially in the context of diving where ambient and respired gas pressures change. The normal arterial PCO2 and therefore end tidal PCO2 is about 40mmHg, and will be about 5-6% of the exhaled gas at 1 atm abs. At 90msw (10 atm abs) the arterial and therefore end tidal PCO2 should still be 40mmHg, but the percentage of CO2 in the expired gas will be more like 0.5%. Thus, I cannot see how the bailout could be triggered on the basis of a threshold CO2 percentage. If it waited for '6-6.5% CO2' at 90m the diver would be well dead before it even reached 1.5%. Indeed, irrespective of how the bailout is triggered, I cannot see the point in expressing the reading as a percentage because it is so hard to interpret the number at different depths.

    Warm regards,

    Simon M

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Simon Mitchell  View Original Post
    Indeed, irrespective of how the bailout is triggered, I cannot see the point in expressing the reading as a percentage because it is so hard to interpret the number at different depths.

    There you have a Pox paradox. Diver complacency is supposed to be an issue, one reason to shy away from an eCCR, but the display is useless since the readings are in odd units and the diver can't see it anyway. The diver just needs to sit back breathe the koolaid fumes and trust the bailout mechanism will save them. Ok that's an exageration. If they speak english there's a audio warning; more electronics.

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Simon Mitchell  View Original Post
    We use CO2 monitoring every day in anaesthesia. The analyser samples the 'end tidal' gas, meaning the gas at the end of each exhalation. The CO2 is measured in the end-tidal gas because it is assumed that this gas has come from the deepest part of the lung, ie the alveoli. Alveolar gas tensions are essentially in equilibrium with those in the arterial blood, and so we assume that the PCO2 in the end tidal gas is a reasonable estimate of the arterial PCO2, which in turn is the important physiological parameter. I don't know how the device shown in the photograph handles the expired gas to derive an end-tidal (end of breath) reading; maybe it just reports the peak CO2 and assumes that must be the end tidal value. I would have some concerns about the sampling protocol and the possibility for gas resident in the hoses to dilute the end tidal CO2 concentration. I would be interested in knowing how that has been addressed.

    However, of more importance (and concern) is the curious nature of the display. It is labelled 'PCO2' (a convention for partial pressure of CO2) but the read-out is expressed as a percentage. These are not the same and mean completely different things, especially in the context of diving where ambient and respired gas pressures change. The normal arterial PCO2 and therefore end tidal PCO2 is about 40mmHg, and will be about 5-6% of the exhaled gas at 1 atm abs. At 90msw (10 atm abs) the arterial and therefore end tidal PCO2 should still be 40mmHg, but the percentage of CO2 in the expired gas will be more like 0.5%. Thus, I cannot see how the bailout could be triggered on the basis of a threshold CO2 percentage. If it waited for '6-6.5% CO2' at 90m the diver would be well dead before it even reached 1.5%. Indeed, irrespective of how the bailout is triggered, I cannot see the point in expressing the reading as a percentage because it is so hard to interpret the number at different depths.
    By AD_Ward9 in Open Revolution Apocalypse

    A scrubber breakthrough is 2% under NORSOK U-101 (and one is allowed peaks in EN14143 also). 2% is a good figure because it produces symptoms after a few hours: obviously breakthrough would expose the diver to this for just minutes.

    The 0.37% means a PPCO2 of 0.0037 atm. Breathing onto the sensor in the unit produces about 4% - out in the air like in the picture the figure varies from about 0.3 to 1.8% depending on when the shutter is pressed and how directly you blow on the sensor. Blockages from WOB, scrubber breakthrough etc, takes it over 6%, where it warns, and alarms at 6.5%.

    The monitor is on the exhale side, because we found the inhale figure doesn't say much. Exhale end of breath CO2 gives you a figure proportional to the retained CO2 and is a far better measure: it picks up things like the flapper valve failure, WOB failures, skip breathers, as well as scrubber issues.
    Simon, I don't know the answer to your question other then IIRC, the end of tidal is added to the peak tidal if that makes sense. Hopefully answers reside in the rigs manual, but the above from Alex posted circa Dec 08 may assist in the matter, equally it may not as well.

    Be interesting to compare what the Apoc displays to what the Sentinal displays with regards CO2.

    Didn't NEDU publish some work on end tidal CO2 and CO2 tolerance?

    Regards
    brad
    Last edited by Brad_Horn; 11th July 2009 at 11:27.

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Brad_Horn  View Original Post
    Simon, I don't know the answer to your question other then IIRC, the end of tidal is added to the peak tidal if that makes sense. Hopefully answers reside in the rigs manual, but the above from Alex posted circa Dec 08 may assist in the matter, equally it may not as well.

    Be interesting to compare what the Apoc displays to what the Sentinal displays with regards CO2.

    Didn't NEDU publish some work on end tidal CO2 and CO2 tolerance?

    Regards
    brad
    Quote Originally Posted by AD_Ward9
    Quote:
    By AD_Ward9 in Open Revolution Apocalypse

    A scrubber breakthrough is 2% under NORSOK U-101 (and one is allowed peaks in EN14143 also). 2% is a good figure because it produces symptoms after a few hours: obviously breakthrough would expose the diver to this for just minutes.

    The 0.37% means a PPCO2 of 0.0037 atm. Breathing onto the sensor in the unit produces about 4% - out in the air like in the picture the figure varies from about 0.3 to 1.8% depending on when the shutter is pressed and how directly you blow on the sensor. Blockages from WOB, scrubber breakthrough etc, takes it over 6%, where it warns, and alarms at 6.5%.

    The monitor is on the exhale side, because we found the inhale figure doesn't say much. Exhale end of breath CO2 gives you a figure proportional to the retained CO2 and is a far better measure: it picks up things like the flapper valve failure, WOB failures, skip breathers, as well as scrubber issues.
    Brad,

    Dan Warkander who now works at NEDU has published extensively on this issue. So have others. In all of this work end tidal CO2 is measured and expressed as a pressure of CO2 because that is the physiological parameter that matters; not a percentage.

    I'm not sure what the numbers Alex quotes apply to, but in principle, "0.37% CO2 means a PPCO2 of 0.0037 atm" only at an ambient pressure of 1 atm abs. For example, 0.37% CO2 means a PPCO2 of 0.0074 atm if the ambient pressure is 2 atm abs, or a PPCO2 of 0.037atm if the ambient pressure is 10 atm abs. But of most concern, he seriously seems to be suggesting that the CO2 alarms are based on percentages of CO2 in the exhaled gas that remain constant across the depth range. I can only assume I'm interpreting this incorrectly and that the % is being indexed back to a surface equivalent in some way, but if so it surprises me he has not said that, and he should just display the proper PCO2 rather than a confusing percentage. If not, the whole concept is seriously flawed.

    Warm regards,

    Simon M

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Simon Mitchell  View Original Post
    But of most concern, he seriously seems to be suggesting that the CO2 alarms are based on percentages of CO2 in the exhaled gas that remain constant across the depth range. I can only assume I'm interpreting this incorrectly and that the % is being indexed back to a surface equivalent in some way, but if so it surprises me he has not said that, and he should just display the proper PCO2 rather than a confusing percentage. If not, the whole concept is seriously flawed.
    Simon, Thanks, do you mind if I cross post your query to the newsletter thread, it 'might' shake loose a definitive answer from OSEL on the subject and is somewhat an important topic. Be interesting to know what the Sentinal Expedition displays, if that ships in Sept could start generating some odd CO2 related questions on here.

    It would make sense to a layman (this one anyway) that the % is indexed back to a surface equivalent in some way as that would make training a buddy and for that matter the diver significantly easier > otherwise if I read you right then one would need a table to compare the CO2 readout against the depth to find out if it is a safe level or not. Not exactly practical for a recreational dive.

    Fascinating topic for a rebreather thread.

    Regards
    Brad

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Brad_Horn  View Original Post
    Simon, Thanks, do you mind if I cross post your query to the newsletter thread, it 'might' shake loose a definitive answer from OSEL on the subject and is somewhat an important topic. Regards
    Brad
    Be my guest.

    Simon

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    Re: Make your point, state your whinge, change my mind

    Quote Originally Posted by Brad_Horn  View Original Post
    Simon, Thanks, do you mind if I cross post your query to the newsletter thread, it 'might' shake loose a definitive answer from OSEL on the subject and is somewhat an important topic. Be interesting to know what the Sentinal Expedition displays, if that ships in Sept could start generating some odd CO2 related questions on here.

    It would make sense to a layman (this one anyway) that the % is indexed back to a surface equivalent in some way as that would make training a buddy and for that matter the diver significantly easier > otherwise if I read you right then one would need a table to compare the CO2 readout against the depth to find out if it is a safe level or not. Not exactly practical for a recreational dive.

    Fascinating topic for a rebreather thread.

    Regards
    Brad

    You may want to ask if their CO2 monitoring gadget has ever made it underwater, and if so, to what depths.

    It sounds a lot like it's still a work in progress in the lab, and that it still needs a lot of developement.

    It wouldn't take much to dummy up something that changed a digital readout when you blow on it for demonstration purposes, even if it couldn't tell CO2 from C3PO.

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    Re: Make your point, state your whinge, change my mind

    When viewing the picture, one can see it IS PCO2 what is being monitored..


    Cheers

    Ivan

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