Acceptable CNS/OTU's - How far do you push it

[kensuf]

CNS O2 toxicity is an issue of dose and time, people have toxed very quickly at very high PO2s. A medical doctor friend of mine, that's quite well read up on diving physiology (and has done some incredible diving), has told me he couldn't find a documented case of CNS OxTox at a PO2 below 1.3.

On OC, I shoot for a 1.2-1.3 MAX PO2 at depth and the traditional 1.6->1.0 gas switch protocol for deco gases (50% @ 30' is 0.95). During the oxygen phase (20' and shallower) I'll take gas breaks (I do 15+5 for 3 breaks per hour), but for me it's less about the CNS clock and more about trying to avoid frying my lungs.

On CCR I've been playing with a 1.1-1.2 PO2 on the bottom and just staying at 1.3 on deco. I've done this on up to 400 minute runtimes. Unfortunately my teaching schedule has been so busy the last year that I haven't had many chances to do this, so my sample size is quite small (1/2 dozen dives). The difference in maintaining a 1.3 @ 20' versus pure oxygen in the loop is maybe 5 minutes on a long dive but the difference in the battering my lungs take is huge. I also try to get off helium based DIL by 100', that will shave an hour off right there courtesy of the helium penalty.

OTU's - I don't worry about it. Single day maximum OTU exposure is 850, that's a huge dive. Anyone racking up that many OTU's in a day isn't likely to be diving the next day. Even multiple day diving is 300 OTU's per day, which is quite significant. If I was diving 4-5 hours a day for a week to ten days straight I'd pay attention to OTU's and I'd also be concerned with hyperoxic lenticular myopia, it's really a thing. You'd manage that by lowering your PO2's and taking the deco hit.

 

[Heat Miser]

On OC, I shoot for a 1.2-1.3 MAX PO2 at depth and the traditional 1.6->1.0 gas switch protocol for deco gases (50% @ 30' is 0.95). During the oxygen phase (20' and shallower) I'll take gas breaks (I do 15+5 for 3 breaks per hour), but for me it's less about the CNS clock and more about trying to avoid frying my lungs.

When you say "frying" are you actually conscious of a different feeling/discomfort after being on higher O2. Or is it precautionary because of your proclivity to a lot of deco diving?

 

[kensuf]

Yes. Spend an hour or two on pure O2 and your lungs feel it.

 

[tbone1004]

When you say "frying" are you actually conscious of a different feeling/discomfort after being on higher O2. Or is it precautionary because of your proclivity to a lot of deco diving?

you can feel it in your lungs, it's unpleasant. On top of that, as I mentioned above because the O2 acts as a vasoconstrictor you are decreasing your offgasing efficiency so it helps to open everything back up.

@kensuf I've been running 1.1 bottom on CCR since I started, largely because of how the 2-led HUD works on the Meg. I had a single blink and as soon as I saw the red light indicating 1.0 I knew to add and if I saw a double blink I knew I had to pay attention.
On deco what I've been doing for the long ones is flushing up to 1.5-1.6 then letting it fall down to about 1.1 then push it back up which has worked pretty well for me on the 3-4hr deco's. I will take OC gas breaks on 32% which definitely helps my lungs and sinuses *usually combined with blowing my nose* about once an hour or so as dictated by my sinuses.

 

[PfcAJ]

Ive gotta say, Ive never really felt it in my lungs. IDK why. A lot of folks seem to experience that, though.

 

[kensuf]

On deco what I've been doing for the long ones is flushing up to 1.5-1.6 then letting it fall down to about 1.1 then push it back up which has worked pretty well for me on the 3-4hr deco's.

Yeah I don't do that. What I do is based on a lengthy discussion I had with a guy that knows a lot more about diving physiology and long decos than I do.

 

[tbone1004]

Yeah I don't do that. What I do is based on a lengthy discussion I had with a guy that knows a lot more about diving physiology and long decos than I do.

Will have to hear that over a beer some night. It doesn't stay up above 1.3 for more than a few minutes normally as it's getting flushed down pretty quick, but curious to hear what this guy said.

 

[rjack321]

OTU's - I don't worry about it. Single day maximum OTU exposure is 850, that's a huge dive. Anyone racking up that many OTU's in a day isn't likely to be diving the next day. Even multiple day diving is 300 OTU's per day, which is quite significant. If I was diving 4-5 hours a day for a week to ten days straight I'd pay attention to OTU's and I'd also be concerned with hyperoxic lenticular myopia, it's really a thing. You'd manage that by lowering your PO2's and taking the deco hit.

I've done 250-300 OTUs/day for a week in Mexico. Vision definitely starts to get a little squirly at the end of the week but thankfully that isn't a long term effect. If I was there for 2 weeks I'd look at taking a rest day to hike around the jungle or something.

 

[100days-a-year]

Oxygen Toxicity and Special Operations Forces Diving: Hidden and Dangerous - PMC

In Special Operations Forces (SOF) closed-circuit rebreathers with 100% oxygen are commonly utilized for covert diving operations. Exposure to high partial pressures of oxygen (PO2) could cause damage to the central nervous system (CNS) and ...

www.ncbi.nlm.nih.gov

Saw this good read.

 

[kensuf]

I've done 250-300 OTUs/day for a week in Mexico. Vision definitely starts to get a little squirly at the end of the week but thankfully that isn't a long term effect. If I was there for 2 weeks I'd look at taking a rest day to hike around the jungle or something.

Yeah, I'm supposed to be spending 9 days on a liveaboard somewhere diving some incredible wrecks on that once in a lifetime trip. My plan for that was 4 hours a day, 0.9po2 and maybe a rest day in the middle. But country is still closed, so who knows.