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Thread: A couple of questions on rebreather design

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    RBW Member supercritical is an unknown quantity at this point supercritical's Avatar
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    A couple of questions on rebreather design

    - How do depth-compensating counterlungs work?

    - If I used nitrox in an oxygen rebreather and set the point where it injects fresh gas appropriately, would it be safe? (Say you had EAN40 in the loop, and when the loop volume is breathed down to 90% so that it becomes EAN30, more gas is injected.
    I know there has already been a thread dealing with what would happen should gases other than oxygen be present in the loop, but that was not assuming that the injector had been set appropriately for the oxygen fraction.)

    If these questions have already been asked somewhere and have answers, please point me there.

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    RBW Member Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse is a glorious beacon of light Packhorse's Avatar
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    Re: A couple of questions on rebreather design

    1. The internal pressure is the same as the external pressure since they are flexible. I cant think of a non depth compensating counter lung design or why you would use one.

    2. Then its a SCR not a CCR. I think the new VR unit injects a single gas mix to maintain set point.

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    Re: A couple of questions on rebreather design

    Quote Originally Posted by Packhorse  View Original Post
    1. The internal pressure is the same as the external pressure since they are flexible. I cant think of a non depth compensating counter lung design or why you would use one.
    .
    hard to understand clearly, but maybe he's referring to the Halcion fridge, bellow type, to change the ratio of refreshed gas in PSCR
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    RBW Member supercritical is an unknown quantity at this point supercritical's Avatar
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    Re: A couple of questions on rebreather design

    Before I continue, I should say that I'm a non-rebreather diver, so I might not know very much about them.

    @Packhorse: I think you may have misunderstood me. The gas injection system works like a PSCR, but it does not intentionally purge part of the gas. Basically, it injects gas when the bellows is at (in my example) 90% volume on the inhale rather than when it is fully collapsed (like in an oxygen rebreather.)

    @paulraymaekers: How does that vary the ratio?

  5. #5
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    A couple of questions on rebreather design

    The Hollis Explorer works just that way. It uses a nitrox mix and measures the oxygen content in the loop. It then injects nitrox to maintain a somewhat constant PPO2
    The excess volume of gas is vented out
    Cheers,

    Dave....

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    RBW Member supercritical is an unknown quantity at this point supercritical's Avatar
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    Re: A couple of questions on rebreather design

    I meant without electronics. It's a 'dumb' (as in it does not actively measure oxygen content) system. Think of it as an oxygen rebreather which doesn't allow the counterlung to collapse fully.

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    A couple of questions on rebreather design

    Quote Originally Posted by supercritical  View Original Post
    I meant without electronics. It's a 'dumb' (as in it does not actively measure oxygen content) system. Think of it as an oxygen rebreather which doesn't allow the counterlung to collapse fully.
    Then you either would require a constant flow of gas as in active SCR (Dreager dolphin) or passive RMV keyed SCR (halcyon rb80)
    Cheers,

    Dave....

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    RBW Member supercritical is an unknown quantity at this point supercritical's Avatar
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    Re: A couple of questions on rebreather design

    Okay...
    Then my other question: How does a PSCR vary the ratio of the bellows counterlungs?

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    Dave Tomblin wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc's Avatar
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    A couple of questions on rebreather design

    Quote Originally Posted by supercritical  View Original Post
    Okay...
    Then my other question: How does a PSCR vary the ratio of the bellows counterlungs?
    It doesn't the ratio is constant although the KISS gem does have the ability to vary the ratio
    Cheers,

    Dave....

    www.wedivebc.com

  10. #10
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    A couple of questions on rebreather design

    Dan Reynolds who is working on the Lungfish rebreather has developed a very clever device similar to this situation which injects oxygen and maintains a setpoint mechanically. I looked through the schematics and actually tried it in a pool. I think the Russians or Germans have also achieved something similar in the past.

    I'm sure someone will be along shortly to explain that we are too stupid to understand it!

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