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Thread: Switching from CCR to SCR at depth

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    RBW Member Paul T will become famous soon enough Paul T will become famous soon enough Paul T will become famous soon enough Paul T will become famous soon enough Paul T's Avatar
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    Switching from CCR to SCR at depth

    Hi all

    I was recently at Eurotek listening to Rick Stanton talking about his home built Rebreather

    He telling us that he runs ccr on decent, accent, and deco, but on the bottom phase of the dive (180+ mtr) he switches the unit to semi-closed

    I understand that he does this to reduce the work the scrubber had to do at these depths, and to reduce the chance of any CO2 issues

    All this makes perfect sense to me, and i was wondering what everyone else thinks

    Are there any Commercial / military units designed to do this, or has anyone done this themselves

    Cheers

    Paul

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    Re: Switching from CCR to SCR at depth

    Quote Originally Posted by Paul T  View Original Post
    Hi all

    I was recently at Eurotek listening to Rick Stanton talking about his home built Rebreather

    He telling us that he runs ccr on decent, accent, and deco, but on the bottom phase of the dive (180+ mtr) he switches the unit to semi-closed

    I understand that he does this to reduce the work the scrubber had to do at these depths, and to reduce the chance of any CO2 issues

    All this makes perfect sense to me, and i was wondering what everyone else thinks

    Are there any Commercial / military units designed to do this, or has anyone done this themselves

    Cheers

    Paul

    I was not at the presentation but why is the risk of CO2/ scrubber failure less during the long deco phase a dive to 185m would generate?

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    Re: Switching from CCR to SCR at depth

    Quote Originally Posted by tecdivertraining  View Original Post
    I was not at the presentation but why is the risk of CO2/ scrubber failure less during the long deco phase a dive to 185m would generate?
    I don't think that's it myself. It's more that at depth the scrubber needs to work more efficiently in order to nab the X molecules of CO2 out of the massive number of O2/inerts, whereas at a shallow stop, there are much less other molecules around so the scrubber capacity can be lower.

    If you bleed off a fixed number of inerts/O2/CO2 via SCR then it lessens the scrubbers' work.

    David

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    Re: Switching from CCR to SCR at depth

    Quote Originally Posted by David Pye  View Original Post
    I don't think that's it myself. It's more that at depth the scrubber needs to work more efficiently in order to nab the X molecules of CO2 out of the massive number of O2/inerts, whereas at a shallow stop, there are much less other molecules around so the scrubber capacity can be lower.

    If you bleed off a fixed number of inerts/O2/CO2 via SCR then it lessens the scrubbers' work.

    David
    I get the point at depth but the CO2 issues on extreme depth/dives is quite possibly a Physiological CO2 retention issue. That said scrubber failure is just as possible during the long deco phase especially on this type of dive, it would be interesting to know what provision is in place to consider a failure at this point of the dive.

    either OC / SCR
    additional / duel CCR

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    Re: Switching from CCR to SCR at depth

    I admit this perplexes me.

    We all know that at 180m a scrubber is running a fearful mass flow rate so the efficiency is going to be way down but so little of the gas (mass wize) that you exhale is CO2 I'm not sure I see a significant saving.

    Modeling it in my head I assume to get any advantage you'd need to dump part of each breath through your nose (or a pucka dump) as if you wait until it has passed the scrubber you gain nothing. However suddenly you are using some large fraction of 19 times SAC of bottom gas. If you dump a third the scrubber is working at a mass flow rate more applicable to 120m but that's a lot of gas bubbling off.

    So if you're going tooled up for huge amounts of bottom mix why not just use the CCR as a deco-machine and do the bottom segment on OC? Actually staying on the loop seems counterproductive to the argument. As I see it I can reduce the mass-flow/unit cross-section area far more simply by getting a bigger piece of pipe and building a bigger scrubber.

    I'm probably missing something obvious but the physics of this doesn't seem to work for me.

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    Re: Switching from CCR to SCR at depth

    Quote Originally Posted by nigelh  View Original Post
    I'm probably missing something obvious but the physics of this doesn't seem to work for me.
    The idea is to put less stress on the scrubber. (and the Reccy scrubber is bigger than a Kiss or inspo) That particular Reccy (SP?) is 10:1 ratio, so one reccy and a 20L cylinder would equate to 9 or 10 20L cylinders, even with a lot of D rings, that's an impossible volume of gas to carry for O/C

    I would think that at 180+m with a standard inspo or Kiss scrubber you are at best "gathering data for the manufacturer". Done it and I'm not doing it again.

    I've gone down a different route: Fully closed with larger scrubber and dump the loop every so often, however look at numbers of deep deaths on rebreathers and fully closed appears more dangerous, although I'm not convinced there are enough data points yet to draw any definite conclusions.

    It is clear that a modest increase in scrubber mass provides a much greater duration or safety margin. For this reason alone switching from a Kiss to a Reccy is worth the trouble.

    Has anyone yet thought about how the deco is worked out?

    John

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    Re: Switching from CCR to SCR at depth

    MAYBE, we can get Rick on here to answer your questions?

    PM me his email if someone has it?

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    Re: Switching from CCR to SCR at depth

    OK, I did a search and came up empty. So I'm totally ignorant. What's a "pucka?"

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    Re: Switching from CCR to SCR at depth

    Quote Originally Posted by kwinter  View Original Post
    OK, I did a search and came up empty. So I'm totally ignorant. What's a "pucka?"
    A typo

    pukka

    mea culpa

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    Re: Switching from CCR to SCR at depth

    Quote Originally Posted by johnv  View Original Post
    The idea is to put less stress on the scrubber. (and the Reccy scrubber is bigger than a Kiss or inspo) That particular Reccy (SP?) is 10:1 ratio, so one reccy and a 20L cylinder would equate to 9 or 10 20L cylinders, even with a lot of D rings, that's an impossible volume of gas to carry for O/C

    I would think that at 180+m with a standard inspo or Kiss scrubber you are at best "gathering data for the manufacturer". Done it and I'm not doing it again.

    I've gone down a different route: Fully closed with larger scrubber and dump the loop every so often, however look at numbers of deep deaths on rebreathers and fully closed appears more dangerous, although I'm not convinced there are enough data points yet to draw any definite conclusions.

    It is clear that a modest increase in scrubber mass provides a much greater duration or safety margin. For this reason alone switching from a Kiss to a Reccy is worth the trouble.

    Has anyone yet thought about how the deco is worked out?

    John
    hI John

    When you say you've gone a different route,larger scrubber and dump the loop every so often. I'd be interested to know what amount of scrubber your putting in and how often your dumping the loop.

    Cheers Andyg

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