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Thread: Large CNS Spike

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    Established TDF Member OutOfTest's Avatar
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    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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    Tamaso Maa Jyotir-Gamaya MadUKDiver's Avatar
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    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?

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    Established TDF Member OutOfTest's Avatar
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    Sorry should have said, the black line is depth (upwards shallower?)

    And the green line on bottom is ppO2


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    Where'd The Bubbles Go ....? Capt Morgan's Avatar
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    What depth did you switch to 50℅ ?

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    Established TDF Member OutOfTest's Avatar
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    Quote Originally Posted by Capt Morgan View Post
    What depth did you switch to 50℅ ?
    21-ish metres. May have been half a metre deeper or so.

    Cave profiles and all that...

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    Coastal Member dwhitlow's Avatar
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    Quote Originally Posted by OutOfTest View Post
    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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    Established TDF Member nigel hewitt's Avatar
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    Quote Originally Posted by OutOfTest View Post
    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.
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    Established TDF Member OutOfTest's Avatar
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    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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    TDF Member IainC's Avatar
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    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)

  10. #10
    Established TDF Member nigel hewitt's Avatar
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    Quote Originally Posted by OutOfTest View Post
    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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