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

[PfcAJ]

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.

I use 40/85. It's like a minute difference.

 

[tbone1004]

I use 40/85. It's like a minute difference.

interesting, I figured you would have been on 20/85 hence the comment

 

[PfcAJ]

interesting, I figured you would have been on 20/85 hence the comment

No wai, not after NEDU study and hangin with Doolette for the past decade.

 

[The Chairman]

Long way from 150' to 20'.

Don't dally then.

 

[Dr Simon Mitchell]

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.

Hello tbone.

I know this issue has been talked about a lot over the years, but I have never been able to find any evidence that it is a pragmatically relevant risk. We presented what we understand is known about the effect of prolonged oxygen exposure on lung diffusing capacity for gas in the Comprehensive Physiology paper that Ryan links to above (pages 180 - 181). A reasonable synthesis of that evidence would be that extreme oxygen exposures (beyond what is likely to be encountered in 99% of technical diving scenarios) can reduce lung diffusing capacity for gas, but that the changes seen even in technical dives that would be considered extreme are unlikely to disturb gas transfer to an extent that would materially affect the risk of DCS. For example, in the most comprehensive human experiment we cite, after exposure to 1.5 ATA of oxygen for about 18 hours, the lung's ability to diffuse carbon monoxide was only reduced by about 5%. After a technical dive where the exposure is typically much shorter (even for extreme dives) this effect would be smaller and there would be very little effect on DCS risk.

If you are aware of any relevant evidence we missed in that review I would be very interested to hear about it.

Simon M

 

[tbone1004]

Hello tbone.

I know this issue has been talked about a lot over the years, but I have never been able to find any evidence that it is a pragmatically relevant risk. We presented what we understand is known about the effect of prolonged oxygen exposure on lung diffusing capacity for gas in the Comprehensive Physiology paper that Ryan links to above (pages 180 - 181). A reasonable synthesis of that evidence would be that extreme oxygen exposures (beyond what is likely to be encountered in 99% of technical diving scenarios) can reduce lung diffusing capacity for gas, but that the changes seen even in technical dives that would be considered extreme are unlikely to disturb gas transfer to an extent that would materially affect the risk of DCS. For example, in the most comprehensive human experiment we cite, after exposure to 1.5 ATA of oxygen for about 18 hours, the lung's ability to diffuse carbon monoxide was only reduced by about 5%. After a technical dive where the exposure is typically much shorter (even for extreme dives) this effect would be smaller and there would be very little effect on DCS risk.

If you are aware of any relevant evidence we missed in that review I would be very interested to hear about it.

Simon M

thanks, i'll go back and reread, what I had been told may well have been dated based on the current schools of thought

 

[kensuf]

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

Umm, that is not the reason most of the CCR divers I know run PO2's in the 1.0-1.2 range. But with only 340 hours on the loop I'm but a a newb.

 

[sportxlh]

My TDI Deco Procedures instructor had me run several V-Planner dive plans using 50% and 100% O2 deco mixes (which start at different depths: 20ft and 70ft) with similar bottom times. The plans that used 100% oxygen at 20 feet for deco always had shorter total run times. But not by much: only a few minutes when bottom time was 150 feet (which is cert depth for the class). He was trying to make the point that you need to choose risk and rewards in your deco mix that fit your mentality: the PO2 of pure O2 is unforgiving at 20 feet which is where you'll spend much of you deco time: you can't accidently go any deeper during your deco stop but you'll have less overall run time. With 50%, you might start at 60 or 70 feet but are quickly moving to shallower depths where PO2 is more forgiving of a diver mistake at say 50 and 40 feet. His question, which everyone answers for themselves, is the shorter run time worth the PO2 risk if you screw up or not?

I'm sure the calculus changes at dives below 150 feet and I've never tried to run a plan on V-Planner below 150.

 

[211ratsbud]

1 min difference for 150, 25minute, air dive deco on 50 vs 100. 100 favors the minute.

It seems that risk would be "risky" depending on the environment.

 

[PfcAJ]

1 min difference for 150, 25minute, air dive deco on 50 vs 100. 100 favors the minute.

It seems that risk would be "risky" depending on the environment.

The difference in deco is minimal. The difference in minimum gas is not.