CNS accumulation rate question

[cmint]

Ok, I'm comparing my buddies profile and my profile for a dive done at Ginnie. The dive was about 90 minutes. We went up to about the half way point on the double lines circuit. I think we ended up with about 15 or 20 minutes of deco.

there was a slight difference in our average depth (4 feet) and I did my last stop at 10 feet and he did his last stop at about 15 feet.

Here's the thing I can't figure out:

My cns clock was at 40 percent at the end of the dive, his was at 60.

That seems like an awful big difference for two divers doing the same dive, and if I'm reading this correctly he could not do that same dive again in a 24 hour period, right? Or would a surface interval reduce that percentage in the petrel? (we usually only do one dive a day so I've not really paid much attention to how multiple dives affect the cns clock on the petrel other than to make sure it's not at 100)

Are there different settings similar to gradient factors in a petrel for the cns clock that we might have set differently?

What factors affect CNS loading like that?

I can post both profiles but it'll be tonight before I can get a screen shot of them. The gradient factors are 35/75, 92 minutes dive, 99ft max depth, 64 ft average depth if I remember correctly.

Thoughts on why such a difference in the CNS clocks?

[tbone1004]

if you look at the NOAA table, he finished his deco with a PO2 of about 1.5. The clock says you have 120 minutes for that. You finished yours at PO2 of 1.3 where you have roughly 180 minutes. By that, he racks up 30% more exposure in the same time that you did.

Shearwater follows the NOAA chart, and drew a curve between the points *note they only follow the single exposure, not the 24 hour exposure table*. Not sure what your PO2 was throughout the dive, but if you did 20 minutes of deco at the depths you posted, he is going to add 16.7% on top of what he already had, and you will add 11.1%. Based on the average depth and dive time you posted, and EAN32, you racked up 25% of your CNS clock. I would expect about 40% for him, but not sure why he had 60% unless he stayed at 20ft for longer than you did. 10 minutes at 20ft eats up about 25% of your cns clock already. All depends on how and where you did your deco. PO2's of 1.6 add up REAL fast for your CNS clock. Going up to 15ft from 20ft triples the allowable exposure time on 100% O2, that's a HUGE difference and you're only going from 1.6 to 1.45 PO2...

https://www.shearwater.com/news/shea...-oxygen-clock/

[cmint]

So doing your 10 ft stop at 10 ft instead of 20 ft would be a lot better on days that you are doing multiple dives?

[tbone1004]

staying at 20ft for your stops has no real benefit per a bunch of studies. You have to stay there until the bubbles are ready for you to head up to 15ft due to the pressure, but deco is no faster at 20ft than it is at 5ft due to being on pure o2. I have my Petrel on 10ft last stop and as soon as it clicks over I get out of the 20ft section. I don't go all the way up to 10ft usually, but go to 15ft pretty quick. Total deco time doesn't change, and less free radicals are bouncing around your head trying to give you a seizure.

[Jax]

tbone, thanks! Excellent explanation!

[tbone1004]

note that the research I mentioned is basically simple math saying that because the PN2 gradient is not changing, staying deeper on the pure O2 stops doesn't expedite your decompression time at all. Not advocating 10ft stops, just going from 20ft to say 17 or 15ft for the OTU's. This is also my belief on the subject. I understand the theoretical efficiency improvement of smaller bubbles coming out at 20ft vs anything shallower, but O2 seizures scare me, and after reading up I've decided to hold the 20ft stop until it clears, and then move up to 17 or 15 ft to finish my decompression to keep the exposure to a minimum. It is a balance, and I am choosing to sacrifice a bit of efficiency in terms of my understanding on how easily the bubbles go out of the tissues due to size, vs the ever counting CNS clock.

[lv2dive]

If you were diving 30% and your buddy 32%, that would have impacted it, in addition to the deco stop depths.

Not that I am advocating this, by any means, but there are several people active on this board who exceed 100% (and even 200% or more) on a single dive on a regular basis. One of the reasons a lot of us do last stops at 10' is to decrease the oxygen exposure on longer dives, therefore (theoretically) decreasing the risk of a CNS oxygen toxicity event. For the record, I believe all the training materials indicate to stay under 100%, but within two hours, your clock will reset to half of whatever it was, and it will continue to do so as time goes on.

I would note two things:
- There are some well-respected individuals in the community who feel strongly that 100% should not be exceeded.
- There are many exploration cave divers who have exceeded 500% without CNS toxicity events

[FW]

If you were diving 30% and your buddy 32%, that would have impacted it, in addition to the deco stop depths.

Not that I am advocating this, by any means, but there are several people active on this board who exceed 100% (and even 200% or more) on a single dive on a regular basis. One of the reasons a lot of us do last stops at 10' is to decrease the oxygen exposure on longer dives, therefore (theoretically) decreasing the risk of a CNS oxygen toxicity event. For the record, I believe all the training materials indicate to stay under 100%, but within two hours, your clock will reset to half of whatever it was, and it will continue to do so as time goes on.

I would note two things:
- There are some well-respected individuals in the community who feel strongly that 100% should not be exceeded.
- There are many exploration cave divers who have exceeded 500% without CNS toxicity events

Not unlike a lot of other physiological things, different people will a lower or a higher tolerance to O2.

[Kelly Jessop]

Not unlike a lot of other physiological things, different people will a lower or a higher tolerance to O2.

Plus the alteration of thresholds from medication like Sudafed and gabapentin

[JohnAd]

staying at 20ft for your stops has no real benefit per a bunch of studies. You have to stay there until the bubbles are ready for you to head up to 15ft due to the pressure, but deco is no faster at 20ft than it is at 5ft due to being on pure o2. I have my Petrel on 10ft last stop and as soon as it clicks over I get out of the 20ft section. I don't go all the way up to 10ft usually, but go to 15ft pretty quick. Total deco time doesn't change, and less free radicals are bouncing around your head trying to give you a seizure.

That is my preferred process precisely, and for the exact same reasons. I will add one more comment about that practice.

When you are doing extended open ocean deco, with waves, surge, and all rather than sitting on a log or a rocky ledge, doing the 20 foot stop with a computer like the Petrel can be annoying. Drop for a second to 21 feet, and you get a high PO2 warning. Rise to 19 feet for a second, and you get a decompression stop warning. You con't actually have to change your depth--it can happen if you drop your arm too low for some reason. If you are wearing two such computers, you may have to push two different buttons to confirm that you just experienced a high PO2. When the Petrel switches you to 10 feet, it is a relief to be able to go up enough to get rid of that annoyance. As tbone1004 says, your actual depth at that point does not matter, as long as it is below 10 feet.

Another solution is to lie to the computer and tel it you are breathing 98%--which may be true, anyway.