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Thread: SCR on a CCR and Computer Settings

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    RBW Member Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545 is a splendid one to behold Lancer4545's Avatar
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    SCR on a CCR and Computer Settings

    I have been thinking about contingency planning in the event you have to use SCR on your CCR unit and what settings you would set your computer to when you went to SCR.

    My thinking is this. You're at a depth of around 65m and you have been there for 45mins and BOOM.....you have lost your O2 and cannot access it any more. I know you could bailout to OC and probably should, but lets say for the point of this exercise you have decided to use SCR as you have plenty of dil and you can access your offboard which is the same gas as your dil. Lets say for this exercise it's somewhere around TX16/45.

    You have a fairly lengthy deco commitment and you would want to have your computer set correctly otherwise you're in "No Man's Land" and haven't got a clue what you should be doing.

    What would you set your computer to........Would you set the current gas to TX16/45 and set the computer to OC or is there a better way to set it and why.

    Thanks,

    Lance

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    Re: SCR on a CCR and Computer Settings

    Quote Originally Posted by Lancer4545  View Original Post
    I have been thinking about contingency planning in the event you have to use SCR on your CCR unit and what settings you would set your computer to when you went to SCR.

    My thinking is this. You're at a depth of around 65m and you have been there for 45mins and BOOM.....you have lost your O2 and cannot access it any more. I know you could bailout to OC and probably should, but lets say for the point of this exercise you have decided to use SCR as you have plenty of dil and you can access your offboard which is the same gas as your dil. Lets say for this exercise it's somewhere around TX16/45.

    You have a fairly lengthy deco commitment and you would want to have your computer set correctly otherwise you're in "No Man's Land" and haven't got a clue what you should be doing.

    What would you set your computer to........Would you set the current gas to TX16/45 and set the computer to OC or is there a better way to set it and why.
    HI Lance,

    There's two main reasons for SCR, one of wich is trivial, wich is the case above where you have lost (or run out) of O2, but still have viable PPO-monitoring capabilities. Assuming you dont have inline PPO-monitoring, you would set the PPO-setpoint to match the PPO of the diluent when it is breathed down somewhat. So on a blend of Tx16/45 you could beathe it down comfortable to say PPO=0.4 and do a SCR-flush. Keep doing that and get out of the water. Switch to rich bailout-gas for the SCR-part when ever possible, and switch diluent on the computer, again keeping a comfortably low setpoint to have effective use of the diluent.

    Actual PPO-setpoint depends on what you bring for bailout (again this is the trivial part)

    The hard part is not when the O2-side goes boom, but when the PPO-monitoring goes sigh.... And no longer display meaningfull PPO-information.
    You're then forced to run blind SCR wich is much more interesting.

    There was some threads on this previously wich got closed, but basically thers at least wellknown 3 ways of doing the blind SCR:

    1) Constant breaths-cycle ratio exchange (effectively running the CCR as a paSCR) typically with ratios as 1:3.

    2) Breath-ratio proportional to absolute pressure i theory giving you constant FO2 (SCR-mode working as normal SCR).

    3) Joe Radomskis mix: ratio=(5+depth/10) ofr high PO2-gasses only.

    Bisides the 3 wellknown methods theres also the master-technique, using counter-lung volume at constant depth to maintain known PPO.

    Lastly you could just plug in your EAN50-gas, and maintain a normal setpoint while ascending at least until you reach 12-14msw. In thiscase you will most likely be conservative inreagrds to deco by not changing the dil-selection asd the He is slowly removed through the exhale-cycles. But this is just a brainfart I wroteabout in another thread.

    I am actually planning on wrinting a comprehensive SCR-article at some point, dont hold your breath-ratio (pun intended).

    So for loss of 02, no special setting just a lower setpoint (and dill changes).
    Loss of PPO-monitoring I would use method 2 with lower FO2-setting than pure OC.

    See earlier ramblings: http://www.rebreatherworld.com/showthread.php?t=16375

    Usual discalimer: I known jack shit about Tx-dives so its all ramblings. Dont trust what I write - verify it yourself yada yada yada.

    Nicolai
    Last edited by Hanssing; 15th August 2008 at 14:50. Reason: Disclaimer and brainfart.

  3. #3
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    Re: SCR on a CCR and Computer Settings

    Quote Originally Posted by Hanssing  View Original Post
    HI Lance,

    There's two main reasons for SCR, one of wich is trivial, wich is the case above where you have lost (or run out) of O2, but still have viable PPO-monitoring capabilities. Assuming you dont have inline PPO-monitoring, you would set the PPO-setpoint to match the PPO of the diluent when it is breathed down somewhat. So on a blend of Tx16/45 you could beathe it down comfortable to say PPO=0.4 and do a SCR-flush. Keep doing that and get out of the water. Switch to rich bailout-gas for the SCR-part when ever possible, and switch diluent on the computer, again keeping a comfortably low setpoint to have effective use of the diluent.

    Actual PPO-setpoint depends on what you bring for bailout (again this is the trivial part)

    The hard part is not when the O2-side goes boom, but when the PPO-monitoring goes sigh.... And no longer display meaningfull PPO-information.
    You're then forced to run blind SCR wich is much more interesting.

    There was some threads on this previously wich got closed, but basically thers at least wellknown 3 ways of doing the blind SCR:

    1) Constant breaths-cycle ratio exchange (effectively running the CCR as a paSCR) typically with ratios as 1:3.

    2) Breath-ratio proportional to absolute pressure i theory giving you constant FO2 (SCR-mode working as normal SCR).

    3) Joe Radomskis mix: ratio=(5+depth/10) ofr high PO2-gasses only.

    Bisides the 3 wellknown methods theres also the master-technique, using counter-lung volume at constant depth to maintain known PPO.

    Lastly you could just plug in your EAN50-gas, and maintain a normal setpoint while ascending at least until you reach 12-14msw. In thiscase you will most likely be conservative inreagrds to deco by not changing the dil-selection asd the He is slowly removed through the exhale-cycles. But this is just a brainfart I wroteabout in another thread.

    I am actually planning on wrinting a comprehensive SCR-article at some point, dont hold your breath-ratio (pun intended).

    So for loss of 02, no special setting just a lower setpoint (and dill changes).
    Loss of PPO-monitoring I would use method 2 with lower FO2-setting than pure OC.

    See earlier ramblings: http://www.rebreatherworld.com/showthread.php?t=16375

    Usual discalimer: I known jack shit about Tx-dives so its all ramblings. Dont trust what I write - verify it yourself yada yada yada.

    Nicolai
    Hi Nicolai,

    Thanks for the input.

    I have done SCR during training dives and am quite comfortable with it and understand the mechanics of it but when you do the sums on monitoring you breathing loop gas it gets a bit pear shaped hence the post.

    With regard to gas selection I would think the best choice would be what you had already chosen to carry for bailout and deco gas and change the gasses when you would have done so if you had opted for OC bailout.

    The bit I was having trouble with was how to set the computer to give you an idea of your remaining deco commitment. I can see the benefit of setting a lower PO2 and monitoring the gas you are breathing whilst leaving it set to CC mode as you have pointed out and it does make sense.

    I would guess the ideal situation would have your computer monitoring the loop and would do it for you......But not all of us have that.

    It's an interesting topic and one I have not seen a discussion on before.

    Regards,

    Lance

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    Re: SCR on a CCR and Computer Settings

    Personally for dives where SCR bailout is a required option I plan for it and have a schedule on my slate. That way I don't have to worry about for what reason I've gone SC, I just follow the table and worry about getting the hell out of Dodge. I see SCR as a gas saver on the bottom...unless I have a lost gas scenario I would be OC on ascent and deco.

    Now for the scenario you propose (assuming for whatever reason we have to be SCR in deco) I would set the computer to OC for two reasons: First setting a lower ppO2 and staying CCR will penalize you unreasonably or have you chasing an unobtainable ppO2 in the ascent. And secondly since with the repeated flushing required in an SCR ascent you are pretty much OC anyway...and this lets you do an old trick I used to play with my azimuth... just set the computer to 3-5% less O2 than actual to account for FiO2 drop...you'll have plenty of time to get this pretty accurate while on your extra long hang!

    Cheers
    Andrew

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    Re: SCR on a CCR and Computer Settings

    I was thnking about this last night and you saved me the bother of writing a thread.

    Thanks

    :D

  6. #6
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    Re: SCR on a CCR and Computer Settings

    Quote Originally Posted by Lancer4545  View Original Post

    I would guess the ideal situation would have your computer monitoring the loop and would do it for you......But not all of us have that.

    It's an interesting topic and one I have not seen a discussion on before.

    Regards,

    Lance
    you gota love the new shearwater meg on this one:D

  7. #7
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    Re: SCR on a CCR and Computer Settings

    Quote Originally Posted by tecdivertraining  View Original Post
    you gota love the new shearwater meg on this one:D
    Yeah,

    I forgot you had a "LOAN OUT" unit. I should pick it up for next week on the way through.......

    Now don't forget!!

    Regards,

    Lance

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