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

[GF99/99]

I know there are a few previous discussions about oxygen toxicity but I thought I would start one about what others consider an acceptable level or risk for CNS/OTU levels

At an advanced (key word here is advanced so please lets not flame away about going over 80% lets try and have a serious discussion) technical level of diving I think we all know that the days when you took your first PADI nitrox course and were scared into the magical 80% max CNS level is a conservative number intend for the typical holiday family diver.

So my question is those that are diving over 80% how far do you push it? What are you comfortable with and why? Same for OTU's

Over the years I have tied to read a decent amount of info on oxygen toxicity and its is one of those things that I always have in the back of my head but also it is something that has sooooo many variables it is virtually impossible to put an acceptable value on that is not super conservative. But that being said I have not found alot of info about what levels actual divers underwater are reaching and what the outcomes were.

Additionally what are your thoughts on air brakes and how effective do you think they are?

 

[PfcAJ]

There isn’t a ton of published data. But there are a LOT of dives being done that are far in excess of “100%”. I think the CNS clock is baloney. That’s not to say oxygen toxicity isn’t real, but the clock is just a very poor attempt to measure it.

I know what works:

Low po2 on the bottom (1.2 or less, avg), 1.6 max on deco (decreasing on the way to the next gas switch), and gas breaks (12mins on, 6mins off) on the oxygen stop to a very low po2.

 

[tbone1004]

@PfcAJ what was that profile to get it up that high and do you change the computer to air breaks? Just curious.

I agree with you though, I get warnings about 100% CNS fairly early in deco and know it's being done every day like that. That is not something that I worry about.

 

[PfcAJ]

@PfcAJ what was that profile to get it up that high and do you change the computer to air breaks? Just curious.

I agree with you though, I get warnings about 100% CNS fairly early in deco and know it's being done every day like that. That is not something that I worry about.

200ft avg for 3hrs. I do not switch off for gas breaks, I don’t want the added deco time. It’s my belief that breaks don’t really impact deco that much, and I’m willing to take that risk if it does. Since my pfo closure I’ve had no issues even on the wackiest of profiles so ima run it.

There’s also some advantage baked in with the po2 drop on the rb80, but I keep that to a minimum on deco with frequent gas flushes. And of course on oxygen there is no drop.

 

[tbone1004]

@PfcAJ I'm with you on not changing the computer over, was just curious since it would impact the CNS% being shown

 

[PfcAJ]

@PfcAJ I'm with you on not changing the computer over, was just curious since it would impact the CNS% being shown

Yeah idk for sure.

Probably a less absurd number. But still absurd.

 

[GF99/99]

There isn’t a ton of published data. But there are a LOT of dives being done that are far in excess of “100%”. I think the CNS clock is baloney. That’s not to say oxygen toxicity isn’t real, but the clock is just a very poor attempt to measure it.

I know what works:

Low po2 on the bottom (1.2 or less, avg), 1.6 max on deco (decreasing on the way to the next gas switch), and gas breaks (12mins on, 6mins off) on the oxygen stop to a very low po2. View attachment 702921

@PfcAJ what was that profile to get it up that high and do you change the computer to air breaks? Just curious.

I agree with you though, I get warnings about 100% CNS fairly early in deco and know it's being done every day like that. That is not something that I worry about.

Yes I find pretty much every dive I am going way over 100% and know most of us are also that's why I wanted to start a discussion about it and what we find to be out personal safe limits if we even have them. Personally even on 300-400min dives where your hitting the 200% mark I still don't even do air brakes as I have not been to concerned about it. But I guess based on @pcfaj 3569% that just blows any doubts I had about what is actual high CNS out of the water. Yes I know there are so many factors to take into account for oxygen toxicity but 3569% goes to show you that this magical 80% is a super conservative number

 

[tbone1004]

@marsh9077 you may want to reconsider air breaks. Irrespective of the CNS/OTU's, your lungs/sinus's feel a LOT better after doing them as the high ppO2 is acting as a vasoconstrictor and it helps with the offgasing efficiency.

 

[PfcAJ]

Yes I find pretty much every dive I am going way over 100% and know most of us are also that's why I wanted to start a discussion about it and what we find to be out personal safe limits if we even have them. Personally even on 300-400min dives where your hitting the 200% mark I still don't even do air brakes as I have not been to concerned about it. But I guess based on @pcfaj 3569% that just blows any doubts I had about what is actual high CNS out of the water. Yes I know there are so many factors to take into account for oxygen toxicity but 3569% goes to show you that this magical 80% is a super conservative number

I think it would be extremely hazardous to expose yourself to that kind of oxygen dose without mitigating it by the strategy I outlined above.

People have toxed below 100%.

 

[rjack321]

Yes I find pretty much every dive I am going way over 100% and know most of us are also that's why I wanted to start a discussion about it and what we find to be out personal safe limits if we even have them. Personally even on 300-400min dives where your hitting the 200% mark I still don't even do air brakes as I have not been to concerned about it. But I guess based on @pcfaj 3569% that just blows any doubts I had about what is actual high CNS out of the water. Yes I know there are so many factors to take into account for oxygen toxicity but 3569% goes to show you that this magical 80% is a super conservative number

The percentage number isn't useful for much of anything, 60%, 100%, 600%, 2000% it doesnt mean anything useful. People have toxed at <100% and they have avoided toxicity at 1500%

Air breaks ARE the way this gets done with minimal risk

You are playing with fire focused on the wrong end of the burning stick.