Does 100% O2 on deco provide a greater benefit than other deco mixes?

[RyanT]

not just toxicity, depending on your dive profile and length you can also burn your lungs with the high PO2's and that reduces offgas efficiency. Huge part of why CCR divers typically run PO2's closer to 1.0 vs the 1.4 that most OC divers use

Certainly true, but even for the average tech diver, CNS toxicity is really the concern. Take a look at figure 11 in the attached paper. You really need a big oxygen dose for a fairly extended period to start seeing a loss of pulmonary vital capacity.

 

[tbone1004]

Certainly true, but even for the average tech diver, CNS toxicity is really the concern. Take a look at figure 11 in the attached paper. You really need a big oxygen dose for a fairly extended period to start seeing a loss of pulmonary vital capacity.

big difference in pulmonary vital capacity, cns toxicity, and decompression efficiency though. what we're talking about isn't seizures, loss of function etc, it's a lowered ability for the alveoli to transfer gas in both directions, the important one here being N2 and/or He out, not O2 in. Likely not significant for reasonably long dives, but it's somewhat similar to the effect of thermal comfort and on/off gassing efficiency where the algorithms can't adjust for those differences and if you're warm on the way down, you can on and off gas more efficiently than you being cold on decompression. Will give you a higher chance for DCS compared to an even keel of temperature or a cold start and warm finish. Same thing with this, you run at a PO2 of 1.4 for 3 hours and then stay at a PO2 of 1.6 through deco, you're probably going to have a higher chance of DCS than if you stayed at 1.1 and 1.4. Going to have a longer decompression obligation, but it's all about balancing those risks

 

[RyanT]

@TBone, yes I understand this completely. I probably should have been more specific. When I said "average tech diver" I really should have said "average open circuit tech diver". A lot of CCR divers (an even some open circuit) are making much longer dives these days where balancing vital capacity, CNS, and efficiency becomes a more important juggling act.
In figure 11 in the paper, they show that a PO2 exposure of 1.5 at 4 hours yields about a 3% loss of vital capacity and just barely begins to present pulmonary symptoms. For me, I don't expect to ever come close to that on my biggest trimix dives.

 

[PfcAJ]

Yeah, I rarely dove 50% (twice?) and it's even less likely now that I dive a rebreather.

So what do you bring for your first bailout deco gas on deep (150'+) dives?

 

[Kevrumbo]

not just toxicity, depending on your dive profile and length you can also burn your lungs with the high PO2's and that reduces offgas efficiency. Huge part of why CCR divers typically run PO2's closer to 1.0 vs the 1.4 that most OC divers use

The practice to ameliorate the inflammatory effects of prolonged Oxygen breathing in a Recompression Chamber is to take 5min Air Breaks for every 20min of O2 Therapy.

Similarly for OC deco, we take a 5min Air or normoxic Break for every 10min to 15min of O2 Breathing...

 

[The Chairman]

So what do you bring for your first bailout deco gas on deep (150'+) dives?

My rebreather and plenty of air and O2. I only dipped below 150 twice last year. I prefer caves and the caves I prefer keep me right around 100.

 

[PfcAJ]

My rebreather and plenty of air and O2. I only dipped below 150 twice last year. I prefer caves and the caves I prefer keep me right around 100.

Long way from 150' to 20'.

 

[tbone1004]

Long way from 150' to 20'.

how many would you take? do you take 35/25 because of the helium or 32% if not, 50%, and O2? if I'm in the ocean it's about managing the number of bottles since I can't drop them off. For that I imagine it's a tradeoff of properly sized bailout gas and if you can size the bottles well enough to get you to the O2 bottle. Obviously not ideal for decompression but it's the tradeoffs you're making for carrying less bottles

 

[PfcAJ]

how many would you take? do you take 35/25 because of the helium or 32% if not, 50%, and O2? if I'm in the ocean it's about managing the number of bottles since I can't drop them off. For that I imagine it's a tradeoff of properly sized bailout gas and if you can size the bottles well enough to get you to the O2 bottle. Obviously not ideal for decompression but it's the tradeoffs you're making for carrying less bottles

I'd take a 70 bottle instead of oxygen.

 

[tbone1004]

I'd take a 70 bottle instead of oxygen.

70% or 70ft? I'm assuming 70ft, but yeah I agree with you. Run time is quite a bit shorter with only 50% than with only 100% and not much shorter with both. Exaggerated more so with the GF's that you use vs what I use, but it's still pretty significant. Extra advantage in OW of being out of the surge as well.