Large CNS Spike

[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.

http://uploads.tapatalk-cdn.com/20170109/678dda425205137455282bc2e000ae4f.jpg


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[MadUKDiver]

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?

[OutOfTest]

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]

What depth did you switch to 50℅ ?

[OutOfTest]

What depth did you switch to 50℅ ?

21-ish metres. May have been half a metre deeper or so.

Cave profiles and all that...

[dwhitlow]

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.

http://uploads.tapatalk-cdn.com/20170109/678dda425205137455282bc2e000ae4f.jpg

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.

[nigel hewitt]

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?

[OutOfTest]

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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[IainC]

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)

[nigel hewitt]

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.