Dive planning and OTU's

[PfcAJ]

I feel like the CNS clock is a very poor model. You can tox with less than 100% of your 'clock' used up, and not tox with WAY over 100% used.

Perhaps its worth considering on multi day nitrox diving, like a week long live aboard or something or a cave trip where you're diving your face off every day. But for single dives, o2 breaks and low bottom ppo2s seem to be the way to go. Personally, I completely disregard the 'clock' thing for single dives.

[jj1987]

Hi James

Assuming hitting 100% CNS Clock on a single dive is dangerous and that it will kill you if you breach it (over-reacting on this one), I find it much easier to hit the CNS clock limits for my dives versus getting anywhere close to reaching OTU limits. I try to stay below 80% and its often fairly easy to do so w/ the standard gas being pumped in Cave Country and doing my last stop at 10'
Typical cave diving day for me is doing a single dive in the 100-110min range. Rarely doing 2 dives if doing that long w/ deco. I will do 2 90min dives at Peacock on the other hand..

Now, to air breaks..ive learnt the hard way on my last dive where I chose to skip the air break and tough out O2 for 27mins since my buddy was a jerk laughing at me since he cleared 15mins earlier than I did. On the surface, I had an unpleasant dry cough for a few minutes. Perhaps it is time to change my 25/75 GF on petrel

I was more curious why you assume that? If it's just a gut instinct then fair enough, I have just never seen any science to support the 100% CNS limit in diving like what FL cave divers do (with back gas breaks).

[patpicos]

Its more referencing what we are taught in class and following it to the letter.

Remember in rec courses where reaching NDL was drilled into you as the devil...and now you have done tech diving....its laughable..

On a serious note, oxygen limits is not something I want to play guinea pig much! I've already seen a friend being pulled out of a cave from toxing. That was graphic enough for me.

[gschaut]

One of the doctors here can chime in, but my understanding is that there are TWO separate issues with oxygen toxicity.

Neurological toxicity and pulmonary toxicity. The immediate PPO2 being the majority factor in Neuro toxicity (convulsions, ect) and the OTU clock being the majority factor in pulmonary toxicity (damage to lung,cardiac, venous tissues).

Pulmonary toxicity being the cumulative damage that occurs to tissues thru long term exposure (days/weeks) to high PPO2. If exposure is limited to one 24 hr period, there is a much higher OTU that is tolerated. But for repeated exposures, several times a day over multiple days, much lower OTU tolerances are used.

AND... the OTU schedule used on the dive site should be conservative enough to allow Table 6 recompression (if needed) without causing pulmonary damage.

[Kelly Jessop]

Yes, and he was following the suggested PPO2 of the time. the max PPO2 for deco is about half of what it was back then.

Was a time that O2 at 30' was acceptable, as well as doing dives at Devils on 36 to 38% too.

[bent]

One of the doctors here can chime in, but my understanding is that there are TWO separate issues with oxygen toxicity.

Neurological toxicity and pulmonary toxicity. The immediate PPO2 being the majority factor in Neuro toxicity (convulsions, ect) and the OTU clock being the majority factor in pulmonary toxicity (damage to lung,cardiac, venous tissues).

Pulmonary toxicity being the cumulative damage that occurs to tissues thru long term exposure (days/weeks) to high PPO2. If exposure is limited to one 24 hr period, there is a much higher OTU that is tolerated. But for repeated exposures, several times a day over multiple days, much lower OTU tolerances are used.

AND... the OTU schedule used on the dive site should be conservative enough to allow Table 6 recompression (if needed) without causing pulmonary damage.

There you go. This is what I meant. From what you guys are posting you seem to confuse those two. That is what you should know from a basic Nitrox course. From exceeding your clock % over extended periods of time you will screw your lungs. Getting cramps you will get ONLY from high ppo2 at a point in time, not because of a high clock %.

[Bobby]

I am in no way an expert & didn't stay in a Holiday Inn last night. In the past I would have a higher working PO2 and push the PO2 past 1.6 as was standard practice. Then the limit of 1.4 for the working part of the dive and 1.6 for deco was brought in and I adopted it. I have had two hard DCS hits and have been checked for a PFO (I don't have one). I would get skin bends after decompression dives and sometimes even on regular depth decompression dives such as devils. Simon explained to me that even on shorter dives "OTU's" can become an issue and suggested lowering my working PO2 down to 1.0. He also suggested when running on CCR to run it like OC on deeper decompression stops by jacking the PO2 up at a stop then letting it drop as it would on OC which I adopted. Since doing this I have not had a single case of skin bends even with some rather deep (well past 300 feet) and rather long (well over 6 hours) dives.

Everyone is different with their personal limit of CNS. Finding the limit can have dire consequences. I have had dives that are way over the CNS clock limit and have pushed OTU's as well. Repetitive dives over days are a bigger factor then one big dive from my experience. The lungs have to be able to exchange gas to off load on decompression, keep the PO2 low on the working part of the dive to preserve the lungs for decompression. When doing deeper dives with deeper stops jack the PO2 up for off gassing then let it fall to give the lungs a rest. I normally use 275, 130, & 50 feet simply because it matches up well with the profiles I run for OC BO & the gases that are kind of standard.

[nakatomi]

From what you guys are posting you seem to confuse those two. That is what you should know from a basic Nitrox course. From exceeding your clock % over extended periods of time you will screw your lungs. Getting cramps you will get ONLY from high ppo2 at a point in time, not because of a high clock %.

I hear that a lot and do not think it is true.

The risk of a CNS oxygen toxicity event is a factor of an elevated PPO2 and exposure time, thus the CNS percentage as a tracking mechanism.
The OTU's are meant to track the pulmonary and possibly the ocular toxicity risks, but both CNS and OTU incorporate exposure time as a major factor.

Here is an excerpt from:
http://"http://www1.maths.leeds.ac.u...ba/baker3.pdf"

"Two oxygen toxicity parameters are typically “tracked” in technical diving calculations. The first is
pulmonary oxygen toxicity which primarily concerns the effects to the lungs of long-term exposures
to oxygen at elevated partial pressures. The second is central nervous system (CNS) oxygen toxicity
which primarily concerns the effects to the brain of short-to-medium-term exposures to oxygen at
elevated partial pressures. Both oxygen toxicity parameters are a function of the partial pressure of
oxygen ( PO2) in the inspired breathing gas and the time of exposure. "

[whodunit68]

...& didn't stay in a Holiday Inn last night.

IT's a Holiday Inn Express, Bobby. How can you expect to have any credibility?
https://www.youtube.com/watch?v=ZlCLuIwuVgQ

[bent]

I hear that a lot and do not think it is true.

The risk of a CNS oxygen toxicity event is a factor of an elevated PPO2 and exposure time, thus the CNS percentage as a tracking mechanism.
The OTU's are meant to track the pulmonary and possibly the ocular toxicity risks, but both CNS and OTU incorporate exposure time as a major factor. "

I hear you. I am only repeating what I read, I sure don't have the expertise to add anything to that. I just posted that because people seemed to be confusing those two.

Ollie, you probably already have it, but in case you don't. You should get the "Ehm", he has got really good sections on oxtox that were backed up by actual testing. Don't think it is available in English though. You can borrow it if you want. It's the best book on diving physiology I have come across.

There must be a bunch of people on here that have done dives with the cns clock going through the roof. Where are you guys?

Ok, just read Bobbys post. Your lights are awesome btw!