Burning the O2 clock

[Blackwood]

Those of you who have experienced lung or throat discomfort potentially from prolonged exposure for excessive time on oxygen clock, how long of a deco were you doing when it happened? I haven't ever experienced such discomfort and wondering how close my own deco times might be to somebody else's personal experience. I know each person is different in personal physiology.

The dives these guys are doing in caves are typically much longer than OC ocean dives. Bottom times in the hours at depths we usually leave after 25-30 minutes.

I don't know how much you've accrued, but I've never done more than 20 minutes on O2. Meister481 mentioned >1 hour O2 exposures. Start talking WKPP and the comparison to what I'm doing or what you're doing gets absurd.

 

[rjack321]

There is still some confusion in this thread about the CNS clock (which has a "max" single exposure of 45min at ppO2 1.6) and the pulmonary toxicity which has an allowable dose per 1 day of 850 units. With 1 unit roughly equaling 1min/bar of O2.

Pulmonary toxicity is predominantly accumulated on the bottom with some racking up on deco. CNS toxicity is mostly accumulated on O2 although if you run a ppO2 of 1.2 for 2 hours on the bottom you're starting to get to the 210min @ ppO2 1.2 limit too.

NOAA CNS limits
PO2 Max single exposure duration Min
0.6 720
0.7 570
0.8 450
0.9 360
1.0 300
1.1 240
1.2 210
1.3 180
1.4 150
1.5 120
1.6 45

NOAA pulmonary limits
Period (days) Dose/day (units) Total (units)
1 850 850
2 700 1400
3 620 1860
4 525 2100
5 460 2300
6 420 2520
7 380 2660

The general consensus in technical diving seems to be that the CNS toxicity "clock" is hooey. People are exceeding that by 1000% percent through use of backgas breaks. The pulmonary limits are much closer to reality and backgas breaks don't help mitigate this. Using a lower bottom ppO2 helps.

 

[DA Aquamaster]

The NOAA CNS limits don't consider any half life (while many computers and software programs consider a 90 minute half life on the CNS clock) and give no credit for back gas breaks, so I think their relevance to current technical diving practices is a little thin.

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This may not be a popular suggestion, but 80% may work better than 100% O2 for someone sensitive to 02 exposure. When you consider the cumulative effects on the 30', 20' and 10' stops, the CNS clock is greatly reduced with 80% but the impact on the total run time is no more than a minute or two.

 

[rjack321]

I met a cave diver from AL at the Millpond last year. She felt she was sensitive to O2 CNS toxicity (symptoms short of seizure) so she used 80% at 20ft and just did extra time. I don't know how much extra she budgeted. Seemed to work for her.

 

[ucfdiver]

Why not use 100% at 15ft?

 

[rjack321]

Why not use 100% at 15ft?

I dunno you'll have to ask her. I'm guessing its a wash. She does get to stay with her 100% breathing buddy at 20ft, I don't think he bothered with 80%. She might be on this board, I don't even recall her real name though.

 

[Superlyte27]

Do you actually believe that? (o2 clock that is)

SSSHHHHH! Don't tell them the myth.

 

[karstdvr]

I met a cave diver from AL at the Millpond last year. She felt she was sensitive to O2 CNS toxicity (symptoms short of seizure) so she used 80% at 20ft and just did extra time. I don't know how much extra she budgeted. Seemed to work for her.

Off topic...but should this person be doing deco dives,or would I want to buddy with them. There are many things that increase your sensitivity to toxing such as medications etc,but if you are experiencing symptoms then this should be a warning that you got out alive this time,but maybe not the next.

 

[LiteHedded]

Off topic...but should this person be doing deco dives,or would I want to buddy with them. There are many things that increase your sensitivity to toxing such as medications etc,but if you are experiencing symptoms then this should be a warning that you got out alive this time,but maybe not the next.

if I were regularly experiencing those symptoms I would have to think long and hard about my diving

 

[DA Aquamaster]

Why not use 100% at 15ft?

You'd have to get a profile that allows you to do the 20' stop at 15', but even then, the decrease in PPO2 is minimal. 100% O2 at 15 ft has a PPO2 of 1.45 rather than 1.61 and has a PPO2 of 1.3 at 10', so the PPO2 is high throughout the longest deco stops.

In comparison, 80% has a PPO2 of 1.53 at 30', 1.28 at 20' and 1.04 at 10', greatly reducing the CNS clock but with almost no effect on the deco.

For example, a 200' for 30 minute GF (GFs of 30-85) profile on 20/30 with 32% and 80% for deco has an 83 min run time while the same plan with 32% and 100% O2 for deco has the same 53 minute run time. With RGBM the the same profiles are 75 minutes and 76 minutes respectively. So in this case, there is either no difference or 1 minute less deco with 80%.

More importantly the CNS clock on the dive with 100% is 86% while in contrast with 80% the CNS clock is only 49%. So in effect 80% gives you the same deco efficiency but only 60% of the CNS clock compared to 100%.

It's an easy decision to make if the intent is to reduce the CNS clock on the dive.