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Large CNS Spike

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  • OutOfTest
    Established TDF Member
    • Oct 2013
    • 1111

    Large CNS Spike

    Hi guys,

    Recently did a dive and had to bailout the rebreather.

    Looking at the computer log when I switched to 50% it ascended very quickly before slowing down, whilst at a constant ppO2 etc which doesn't work with my knowledge of CNS.

    I hasten to add that I don't follow CNS too much and am very happy to exceed 100%. Just a bit interested by the physics or whatever behind this.




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  • MadUKDiver
    TDF Member
    • Aug 2014
    • 396

    #2
    Your ppO2 looks constant before the switch (straight line increase in CNS) but it appears to me that it changed (went up) when you switched to o/c.

    Can you add a depth scale and ppO2 line to the above graph?
    “We cannot banish dangers, but we can banish fears. We must not demean life by standing in awe of death” -- David Sarnoff

    Comment

    • OutOfTest
      Established TDF Member
      • Oct 2013
      • 1111

      #3
      Sorry should have said, the black line is depth (upwards shallower?)

      And the green line on bottom is ppO2


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      • Capt Morgan
        Where'd The Bubbles Go ....?
        • Dec 2012
        • 1831

        #4
        What depth did you switch to 50℅ ?

        Comment

        • OutOfTest
          Established TDF Member
          • Oct 2013
          • 1111

          #5
          Originally posted by Capt Morgan
          What depth did you switch to 50℅ ?
          21-ish metres. May have been half a metre deeper or so.

          Cave profiles and all that...

          Comment

          • dwhitlow
            Coastal Member
            • Dec 2012
            • 6410

            #6
            Originally posted by OutOfTest
            Hi guys,

            Recently did a dive and had to bailout the rebreather.

            Looking at the computer log when I switched to 50% it ascended very quickly before slowing down, whilst at a constant ppO2 etc which doesn't work with my knowledge of CNS.

            I hasten to add that I don't follow CNS too much and am very happy to exceed 100%. Just a bit interested by the physics or whatever behind this.

            I am not sure what you are concerned as I see only depth and PPO2 and what I see seems to match expectations. If I am reading that correctly, you bailed at 66mins to 15/55 which gave a lower PPO2 than the loop and the PPO2 dropped. Soon after, at 70 mins, you switched to 35/25 and this shows as increase in PPO2. As you ascended the PPO2 dropped and then, at 80 minutes, you switched to 50% and the PP02 again jumped.

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            • nigel hewitt
              Established TDF Member
              • Sep 2013
              • 3199

              #7
              Originally posted by OutOfTest
              21-ish metres. May have been half a metre deeper or so.
              So you went from 1.08bar of O2 to 1.75bar?
              Yup. The CNS rate is going to go up like a rocket. From 0.4 to about 20 %/min

              Am I missing something that says it shouldn't?
              Helium, because I'm worth it.
              Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.

              Comment

              • OutOfTest
                Established TDF Member
                • Oct 2013
                • 1111

                #8
                It's not the fact it rockets that's confusing me.

                It's the fact it rockets and then slows down before the depth/ppO2 changes.

                Additionally...1.75bar? 50% is 1.6bar at 22m


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                Comment

                • IainC
                  TDF Member
                  • Feb 2014
                  • 323

                  #9
                  It's the fact it rockets and then slows down before the depth/ppO2 changes.
                  You're ascending, it rockets up as you start to breathe a higher FO2 gas, then the rate of increase tails off as you ascend (which on open circuit reduces the PO2 proportionally)

                  Comment

                  • nigel hewitt
                    Established TDF Member
                    • Sep 2013
                    • 3199

                    #10
                    Originally posted by OutOfTest
                    Additionally...1.75bar? 50% is 1.6bar at 22m
                    Duh. Sorry. A bit brain dead when I worked that out obviously.
                    I blame all those beef burgers I ate back in the 80s-90s.
                    Helium, because I'm worth it.
                    Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.

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