Megladon Rebreather - Toxicity Query
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Thread: Megladon Rebreather - Toxicity Query

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    Losing the Will... Major Clanger's Avatar
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    Megladon Rebreather - Toxicity Query

    As all will be aware that own a meg, the O2 feed is to the inhale lung. There is talk about a possibility that a faulty valve could allow the injection of low levels of oxygen in to the inhale lung, the build up of which was undetected by the cells as breathed straight to lungs, casuing 02 toxicity. I emphasise that this may just be speculation. If so, what can be done to avoid this. I was thinking of feeding the O2 hose to the offboard valve on the exhaust lung or having a freeflow type switch and leaving it off unless needing manual inject. Thoughts?
    Last edited by Major Clanger; 01-05-12 at 10:56 AM.
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    Jesus don't want me for a sunbeam NotDeadYet's Avatar
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    Would it have been undetectable if it had been a leak? You wouldn't be burning enough O2 for the loop to appear normal by the time it reaches the cells surely? Admittedly you might not be getting particularly well mixed gas on each inhale so I can see why that would be bad. Genuine question, I've never been near a Meg.
    Last edited by NotDeadYet; 30-04-12 at 09:24 PM.
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    The worlds slowest sailor. mala's Avatar
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    run the set point low and add manual to get what you want.
    if you dont need to add you will know that there is extra o2 leaking in.

    i cant see the 02 level being any different if you plug in to the exhale lung-just makes the hose run more complex.
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    Third hand information. I couldn't say for certain. The guidance for manual injection advises short bursts to avoid this type of thing happening. As for the possibility of it occuring due to a faulty valve, I dunno. The person telling me seemed to think that the inhale lung basically was full of O2 that the diver was breathing straight off.
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    Quote Originally Posted by mala View Post
    run the set point low and add manual to get what you want.
    if you dont need to add you will know that there is extra o2 leaking in.

    i cant see the 02 level being any different if you plug in to the exhale lung-just makes the hose run more complex.
    I think the point they were making was that the cells couldn't detect the high O2 as it was in the inhale lung or diver. Plugged in to the exhale lung, the gas would pass the cells before breathed in so changes more detectable in this scenario. Dunno, just mulling.
    Last edited by Major Clanger; 30-04-12 at 09:42 PM.
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    Nigel Hewitt nigelH's Avatar
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    I would say no.
    A slow bleed would cause the loop ppO2 to rise slowly. Remember that you only take in the O2 you need to bring your body up to the loop level.
    You exhale the rest so it goes round again and gets to the cells. If you were inhaling a high level you will exhale what your blood is at so the cells will be reading that within a breath or two.
    Also the lag, even at quite high ppO2s, before symptoms hit is reportedly quite large so the situation must be sustained for quite a while.
    Again big leak would be felt inflating things long before it hurt you - rebreather divers are very bitchy about buoyancy.
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    Quote Originally Posted by nigelH View Post
    I would say no.
    A slow bleed would cause the loop ppO2 to rise slowly. Remember that you only take in the O2 you need to bring your body up to the loop level.
    You exhale the rest so it goes round again and gets to the cells. If you were inhaling a high level you will exhale what your blood is at so the cells will be reading that within a breath or two.
    Also the lag, even at quite high ppO2s, before symptoms hit is reportedly quite large so the situation must be sustained for quite a while.
    Again big leak would be felt inflating things long before it hurt you - rebreather divers are very bitchy about buoyancy.
    Something else is to blame so fixing that won't help.
    That helps put my mind at rest. I thought the leak would be detectable but couldn't quite see how.
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    Quote Originally Posted by Major Clanger View Post
    I think the point they were making was that the cells couldn't detect the high O2 as it was in the inhale lung or diver. Plugged in to the exhale lung, the gas would pass the cells before breathed in so changes more detectable in this scenario. Dunno, just mulling.
    NDY's point was that the high PO2 gas from the inhale lung would still be passed over the cells on the next breath, as you only metabolise some of the O2 you breathe in. Therefore, the higher than expected PO2 should have been obvious.

    I do find myself wondering whether current limited cells or a very high set point that meant the diver was close to a tox anyway could have played a part. In the latter case, injecting into the inhale lung could have raised the PO2 to a suitable level, I guess.

    I prefer the idea of injection on the exhale side, or into the head, as both the Inspo and JJ do, but I had understood that the difference was negligible if everything else was right (good cells and sensible set point).
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    Quote Originally Posted by philsiswick View Post
    NDY's point was that the high PO2 gas from the inhale lung would still be passed over the cells on the next breath, as you only metabolise some of the O2 you breathe in. Therefore, the higher than expected PO2 should have been obvious.
    I would also expect this to be the case and also think that there may be other factors at play. The manual O2 inject is below the loop and the person telling me appeared to be saying that a low level weep had caused a pocket of O2 to collect in the bottom of the inhale lung but, as mentioned, even breathing this for a short time, I would expect O2 levels to still be sensed quickly. Have to keep a listen out to see if more details become available.
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    Senior Member matthewoutram's Avatar
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    This is one of the reasons I don't like the Meg - the O2 coming in on the inhale side is a hazard. However I had thought it was only a hazard for using the MAV (although I confess I didn't study the set-up in minute detail).

    If you are not diving at MLV (and a leak would make it very difficult to do so anyway) then perhaps the oxygen can build-up. But why would it not pass the cells - surely after one breath the expired gas is en-route to the cells (NDY's point)? I have no clue how many breaths of high ppO2 gas are actually needed to cause a problem, I do know it is less in the presence of CO2, but I have no absolute numbers.

    Matt.

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