Who is riding trimix dil all the way to the surface and who is flushing to a nitrox dil?

[Alekseolsen]

Random question, but what is the minimum O2 percentage at the surface to not worry about passing out from hypoxia?

0.16 in PPO2, meaning 16% for OC. For ccr DIL with 16% O2 and Oxy injection off for some reason quickly drops to hypoxic levels, which I assume is the reason there's variations to the Normoxic + minimum O2 levels. (Ie MOD2+).

 

[rongoodman]

The ppO2 on the summit of Mt. Blanc is 11.4% and people aren't routinely passing out up there.

 

[broncobowsher]

The ppO2 on the summit of Mt. Blanc is 11.4% and people aren't routinely passing out up there.

They don't have the added work of breathing of breathing through a couple hoses, counterlungs, and a scrubber either. Run that in a rebreather any you are a couple breaths away from going nighty nighty.

You also state not routinely passing out, which implies there is an occasional pass out.

By no means is 11.4% considered safe in a rebreather. 0.16 is generally considered lowest acceptable level and even that is pretty sketch. That is more of an OC bailout minimum acceptable.

 

[mr_v]

The ppO2 on the summit of Mt. Blanc is 11.4% and people aren't routinely passing out up there.

I suggest that you try running or doing any physical activity at that elevation to see what that PPO2 means. FYI, I am speaking from experience: I ran a mile at my top speed loaded with gear at 11,000ft, which is lower than Mt. Blanc. In retrospect, it was a bad idea. The headache was terrible and I was about to pass out. That happened when I was 25 and in the top shape.

There is no reason to screw around with dil changes as you come up. Choose your gas mix properly, then flush with O2 at 6m or shallower.

 

[formernuke]

They kept the sub at .18. I could notice the difference in how I felt.

 

[bubblemonkey2]

Random question, but what is the minimum O2 percentage at the surface to not worry about passing out from hypoxia?

For open air or OC, gotta be in the teens %. Personal variation and level of exertion would skew -/+ 3% at least?
Who here can link some study data?

Trick question on CCR. People pre-breathe their CCRs on 4/80 diluent (at 0.7 setpoint).
A ppO2 of 0.4 (40% at the surface) is officially the minimum to not worry about passing out from hypoxia. [But most people would say it should be even higher.]

Someone could try an experiment breathing their loop down on a couch (and get rescued?), but the numbers they get for minimum % if they survived would be as irrelevant to real CCR diving as high altitude mountaineering is.

 

[broncobowsher]

I suggest that you try running or doing any physical activity at that elevation to see what that PPO2 means. FYI, I am speaking from experience: I ran a mile at my top speed loaded with gear at 11,000ft, which is lower than Mt. Blanc. In retrospect, it was a bad idea. The headache was terrible and I was about to pass out. That happened when I was 25 and in the top shape.

There is no reason to screw around with dil changes as you come up. Choose your gas mix properly, then flush with O2 at 6m or shallower.

I took a mountain bike up to a camp at 7000's elevation and tried to ride it around. Results were about the same as your 11,000 top speed run. I am sure you were in better shape than I was as well. But it isn't an issue in the 1000-2000 elevation. That low PPO2 can really screw with you.

I accidently hit a PPO2 of 0.10 while getting ready once. Checklist hit a stop between checking ADV and turning on O2. While working through that stop I breathed down the loop. I have since changed the checklist. Check ADV - Open O2. No steps to get hung up on in between anymore. The 0.10 felt completely normal. I was standing chest deep in water (was hot out and standing in water to prevent overheating while doing final checklist). No real physical exertion, no more than fiddling with a remote sitting on a couch.

 

[mr_v]

Trick question on CCR. People pre-breathe their CCRs on 4/80 diluent (at 0.7 setpoint).
A ppO2 of 0.4 (40% at the surface) is officially the minimum to not worry about passing out from hypoxia. [But most people would say it should be even higher.]

What?

 

[GF99/99]

How much time spent between 18 and 6 metres? Up to 0.75 atm of He (50/27 loop) will be free to re-enter during that time, since gas atoms don't "see" each other.

Might not matter so much until you're doing @GF99/99 's 5-hour dives

Yes I find that dil switching becomes very very effective sub 130m you can knock a lot of deco off your run time. Also keep in mind dil switching is also for bail out efficiencies. Say your diving a 180m dive with 85% He as your on board dil if you were to “ride” this all the way up to the surface you risking IBCD hit if you have to bail out in the shallows your going to have to bail out to another higher less effective He trimix. If your dil switching at 110m and 60m and 18m leaning off the He your softening that IBCD spike.

These are thing that get overlooked with dive planning, it is not as simple as just following your computer up. Also this makes your B/O plan a lot more complex to know what depths and where you can actually bail out to specific gases. And generally if you have to B/O it is an emergency and you don’t have a lot of time to think. And when your have 4-5 different mixes with you it can get complex real fast.

Some may have different thoughts and opinions from me and I am open to hearing them but personally when I dive sub 150m I always dil switch and generally even below 110m I will 99% of the time do dil switches. Only time I do not is when it adds complexity to the dive to only same 20min for example when I am diving dual/bail out rebreathers. It is quite task loading to dil switch 2 units. Or shallow dives, dil switching on an 80m dive is fairly pointless.

 

[211ratsbud]

I take 15/50ish regularly top to bottom back to the top again.

Never thought about intent to switch to He free mix but I am prepared to plug in 50% where it's appropriate to do so in case need arises.