O2 Toxicity in deco question

[Nick_Radov]

We don't have much experimental data on CNS oxygen toxicity above 1.6 atm; the NOAA tables only go up to 1.6 atm. It's easy to temporarily exceed that value if you drop down to 71 ft while breathing 50%, or 21 ft while breathing 100%. So, without hard data, modern dive computers have resorted to various arbitrary hacks which don't necessarily align to physiological reality. Don't take them too seriously. Finish your deco. Maintain good buoyancy control and err on the side of slightly shallower instead of slightly deeper. But do take occasional back gas breaks if the deco plan calls for more than about 20 minutes on 100% oxygen; this is not so much for CNS issues but rather to maintain lung vital capacity.

Shearwater and the CNS Oxygen Clock

The question of what CNS oxygen exposure is safe can spark heated debate among technical divers. This reflects the seriousness of CNS oxygen toxicity when diving. This article sidesteps that debate, and focuses solely on how Shearwater dive computers calculate the CNS value.

www.shearwater.com

 

[Nick_Radov]

100% doesn't mean that is as far as you can go. I still like the last helicoptor ride I took. The tachometer reads in percent, not RPM. Take off speed is something like 104%.

NASA also ran the Space Shuttle main engines at 104% of rated thrust.

 

[Akimbo]

We don't have much experimental data on CNS oxygen toxicity above 1.6 atm

The US and Royal Navies have a great deal of data. The Royal Navy's limit was 3.0 ATA at the start of World War II and the in-water limit was 2.0 ATA for around 50 years in both navies. The NEDU had a large body of work to draw on developing Treatment Tables 5 & 6 which run at 2.82 ATA. I can't remember from reading old manuals but I believe these are the tables where air breaks were introduced.

Info - Oxygen Toxicity Limits & Symptoms

Oxygen Toxicity Limits & Symptoms Oxygen toxicity limits can be very confusing, especially for PPO2 (Partial Pressure of Oxygen) levels above 1.6 ATA used in chamber-based hyperbaric treatment (recompression) and decompression tables. For example, here is a chart of one of the most common DCS...

scubaboard.com

 

[Jack Hammer]

We don't have much experimental data on CNS oxygen toxicity above 1.6 atm; the NOAA tables only go up to 1.6 atm. It's easy to temporarily exceed that value if you drop down to 71 ft while breathing 50%, or 21 ft while breathing 100%.

Shearwater and the CNS Oxygen Clock

The question of what CNS oxygen exposure is safe can spark heated debate among technical divers. This reflects the seriousness of CNS oxygen toxicity when diving. This article sidesteps that debate, and focuses solely on how Shearwater dive computers calculate the CNS value.

www.shearwater.com

You may want to recheck your numbers and sources for most of this statement.

 

[Merida Cave Diver]

D. Spend 96 minutes thinking about what CCR model you'll be getting to avoid such long deco times

lol, yeah. I had thought about it and was not going to go the CCR route but this dive is making me revisit that decision.

 

[Merida Cave Diver]

Great answers thank you, appreciate the replies. I do try to dive safely and responsibly so I'm happy to hear that the number isn't a life or death thing. First time I've ever had that long a deco run and seen my CNS go up.

 

[PfcAJ]

lol, yeah. I had thought about it and was not going to go the CCR route but this dive is making me revisit that decision.

Ccr isn’t going to impact your decompression that much.

 

[Manatee Diver]

gradual reduction of po2 as you ascend

How do you go about that? Just a slow ascent? Or switching to lower oxygen content gas like 50% (if you are carrying it)?

 

[PfcAJ]

How do you go about that? Just a slow ascent? Or switching to lower oxygen content gas like 50% (if you are carrying it)?

On OC or pscr just ascending reduces the po2. Ccr dudes can spike and let it decay on the way up. They can speak to the techniques they use.