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Thread: So what is this "SCR Mode" talk all about?

  1. #1
    RBW Member jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk has a reputation beyond repute jkaterenchuk's Avatar
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    HammerMeg

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    So what is this "SCR Mode" talk all about?

    On another thread the debate rages regarding the value and need for SCR Mode in rebreather diving and training. Over the years I have heard several different versions of what divers have been told about how to properly conduct SCR on CCR.

    So I thought it might be worthwhile to start a thread to have the many different methods that people were taught/teach or utilize to run SCR on their CCR and then hash out the pro's/cons's and inconsistencies.

    So go ahead and post the procedure you were taught or have adopted for SCR operation on your CCR. Of particular interest would be the sharing of the CCR instructors here so that we might see the differences agency to agency.

    John

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    SF2 | rEvo

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    To add to this I'd like to hear what others have done for deco planning with SCR bailout.

    I was taught 3:1 as a rule of thumb for SCR.
    -Capt Tom McCarthy
    Stone Rust | East Coast Wreck Diving
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    RBW Member JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF is a jewel in the rough JasonF's Avatar
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    rEvo III Hyb Min, rEvo II STD

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    Re: So what is this "SCR Mode" talk all about?

    SCR Mode on a CCR in a MOD 1 Level Course is nothing more than a Gas extension of onboard Diluent when the diver experiences 1 of 2 situations.

    Situation 1) No Oxygen

    Situation 2) No Displays

    It's safe to say that you can handle each case differently however for the purpose of teaching a MOD 1 course I prefer to teach one method which clearly illustrates to the students how to manage the situation.

    (disclaimer-I feel that this is a minimalist skill and doesn't really belong in a MOD 1 course. I also believe that everyone should carry enough bailout to get them out of a dive safely from the worst possible point of the dive /disclaimer)

    I present the worst case scenario for a MOD 1 course "Being below 20'/6m" and loosing all of your displays. Your only "know" safe breathable gas below 20'/6m is your onboard diluent.

    What's the solution?

    Evacuate the loop with all "unknown" contents by performing an adequate diluent flush. Now you have a safe breathable loop, not "ideal" for the depth but a safe gas to breathe.

    Take 3 breaths off of the loop and 3 out the nose (some teach just 2 out the nose, once again, just trying to simplify it for a MOD 1 course)






    Every time you exhale out of your nose that depletes counter lung volume which will activate the ADV and brings in more safe breathable Diluent gas.

    Every time your breathing off of the loop without exhaling out you nose, your just rebreathing the safe breathable diluent gas. This particular part of the SCR Mode is where you are extending your distance of travel underwater without depleting any of the diluent.

    So it looks like this:
    Dil flush first , three on the loop, three out the nose etc..

    So, if you have no displays, how do you know what your breathing/PO2 in the loop? The answer is simple, your breathing your diluent gas (MOD 1 course 21% O2). You can calculate your PO2 in your head or by using a CCR dive computer. Calculation: Depth in ATA * FO2. So if you were in 99'/30m of water you would be breathing a .84 PO2
    99'/30m=4ata/4bar, 4ata/4bar*.21= .84PO2

    The SCR Mode is most efficient at a constant depth while moving horizontally to your exit location.

    Calculate deco/NDL based on diluent gas.

    You can ascend while doing SCR, I recommend bailing out to O.C. at 30ft

    P.S. Some of the Smilies are not showing up, is there a moderator that can fix it for me?
    Cheers,

    Jason

  4. #4
    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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    Hollis Prism 2, Megalodon

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    Re: So what is this "SCR Mode" talk all about?

    At a air diluent, beginner level I teach open loop mode which is 1:1 ratio using the ADV more as a open circuit regulator.
    In more advanced diving I teach first and foremost that SCR is a stopgap technique and not a substitute for adequate bailout.
    We experiment with different ratios while closely monitoring PO2. When using air diluent most find a 3:1 ratio maintains a reasonable PO2. I also teach the use of deco bailout gas fed through the bypass valve in SCR mode as the diver gets shallower. I also agree with the above comment that SCR mode should not be used above 30ft.
    Cheers,

    Dave....

    www.wedivebc.com

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    Doing What Works jcook is an unknown quantity at this point jcook's Avatar
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    Re: So what is this "SCR Mode" talk all about?

    We never used our DIL for SCR mode, we used our optimized PO2 bailout. I was taught 3:1 ratio and if on deco you can go up to 4-5:1 if the PO2 is high enough on the gas. This is only if the O2 is gone.

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    Mk15.5, Prism, Prism2

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    Re: So what is this "SCR Mode" talk all about?

    3:1 and I don't teach it in MOD 1 beyond a brief mention. I drone on about planning adequate bailout to fill in its time slot
    "However beautiful the strategy, you should occasionally look at the results." Winston Churchill
    A is A

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    JJ, Flex

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    Re: So what is this "SCR Mode" talk all about?

    Love the explanation Jason.
    I think the smilie version of the dil flush as I learned it is


    Then just as he said



    If your loop is completely flooded or otherwise out of commission, you can do the same thing using a second stage and your wing. Been there, done that.
    Ken

    Quote Originally Posted by Dsix36  View Original Post
    Just remember that listening to an idiot such as myself may very well get your arse dead.

  8. #8
    So many CCR So little etc Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase's Avatar
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    JJ Hybrid

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    Re: So what is this "SCR Mode" talk all about?

    Jasons explination is prety much exactly what i was taught


    I was taught this on Mod 1 and asked to demonstrate my understanding of it on Mod3 but in both instances it was a 5-10min exercise. In Mod3 we were also taught pure 02 CCR blind.

    We were taught that SCR could be used if we felt the scrubber had failed on the premis it would reduce the amount of C02 being managed by the scrubber. Something that i feel was part of the reasion very long durations have been possable on the RB80 PSCR

    Having finished Mod 3 six years ago I have since discovered a few problems with SCR


    PERSONAL experiance with SCR bailout



    NOSE VENTING:

    repeted nose venting in salt water became painfull after about an hour and a half. Also the nose piece of the mask acted in much the same way as a diaghpragm on a regulator in that it added to the physical effort involved in exhaling as you had to blow past it. obviously masks will differ in their effect. I was using a Cressi Big Eye back then with a clear silicon body


    WOB:

    I cant say for sure if it was the nose venting or the act of operating the ADV that caused the issues of chest pain and breathing fategue but the issue became so accute i had to abandon the SCR attempt after about two hours.

    The ADV on the Inspo Classic is one of the best I have used in so far as minimum loop was easy to maintain but its still light in operation. I here claim that the Meg ADV is so light the WOB is nominal. I had a similarly light cracking resistance on my rEvo but i found it very hard to dive minimum loop and maintain PP02. My 02 consumption went through the roof untill i significantly increased the cracking presure on the unit. On a 180min dive ill usualy use 50-60bar 02. With the rEvo that shot up to 80-100bar and came back down to 50-60 once id sorted the ADV.


    WOB RELATED C02:

    I have no proof of this but after my attempt at a full on SCR ascent I suffered a monster headach which couldent be fixed with water and pills. I beleive it was as a result of retained Co2 from the WOB issues of SCR but I cant be sure.


    BOYANCY:

    Not a big issue but maintaining free hang whilst loop venting and flushing was an effort and that effort got worse as I got shalower and the effort lead to fategue (include headach and sore nose to this factor)

    Doing it for 10mins it was unnoticable but after an hour or so it becomes a noticable effort.


    DECO:


    Not a biggy as once at 6m you can go pure 02 CCR and extend deco but I realy didnt have much idea how to recalculate deco once on SCR. TBH I still dont.



    By Comparison:


    ISSUES WITH OC BAILOUT


    Being a BOV user OC bailout was a simple issue of turning a knob. Being out of practice with OC and having at first owned a Golum and Tec4 BOV I got a rude shock when on my JJ I breathed water on the first breath.

    Note to self: button purge your bov dont spit purge and use your last breath.

    Gas switching from CCR to OC was simple on all my units excepth the standard JJ bov as on this it ment letting go of the shot line to close the loop. Not good if the tide is running and of course the tide was running at the time. So one arm arround the shot to allow two handed operation of the BOV.

    With a Golum,Tecme, Tec4 BOV its easy to do single handed but if you want the next reg to hand you need to wrap one arm around the shot and do the BOV and loop removal with it, whilst the other holds the reg.

    Once on OC i switched off 02 to the CCR and ignored it other than to go vertical ocasionaly to vent the CLs.


    Thats about the full extent of the OC bailout issues i can think of.

    ATB

    Mark
    Last edited by Mark Chase; 13th April 2012 at 09:44.

  9. #9
    RBW Member steve6690 has a spectacular aura about steve6690 has a spectacular aura about steve6690 has a spectacular aura about steve6690 has a spectacular aura about steve6690 has a spectacular aura about steve6690's Avatar
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    Re: So what is this "SCR Mode" talk all about?

    Quick question re the ADV. Could you not switch it off and use the MAV instead ?

  10. #10
    So many CCR So little etc Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase's Avatar
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    Re: So what is this "SCR Mode" talk all about?

    Quote Originally Posted by steve6690  View Original Post
    Quick question re the ADV. Could you not switch it off and use the MAV instead ?

    On some units yes but on others no.

    JJ KISS for example dont have MAVs for dill but you can of course add one.

    ATB

    Mark

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