Is ppO2 change too large?

[Blackwood]

Even if this were plausible for the gases the OP mentions...
isobaric counter diffusion requires an inert to "counter diffuse"
its not a problem going onto O2 from anything

I don't completely understand ICD, but I am fairly certain that Oxygen diffuses into our tissues, and if it does so slower than the gas which is diffusing out (like Helium), you're experiencing ICD. Whether or not it's risky is another question.

(could be completely wrong about the above )

 

[rjack321]

I don't completely understand ICD, but I am fairly certain that Oxygen diffuses into our tissues, and if it does so slower than the gas which is diffusing out (like Helium), you're experiencing ICD. Whether or not it's risky is another question.

(could be completely wrong about the above )

Oxygen does dissolve into our tissues but its pretty much impossible to get an oxygen bubble in our tissues, at least on the kinds of dives we do in wet and drysuits. 1,000+ft down in a saturation situation lots of unusual gas dynamics take place.

ICD is a theoretical problem by and large although Weinke and Naui-tech think its a fairly big deal, hence their recommendations to start with O2 as a deco gas and then add something like 50% deeper. Absent an inert gas to rush into your tissues as the inert loaded for backgas rushes out, ICD can't even theoretically happen.

As you know, people dive stuff like 18/45 with only 50% for deco all the time and don't have any substantive ICD issues. He rushing out N2 rushing in. Some people say that's because the switch is "too shallow". Regardless ICD still doesn't seem to happen.

ICD has just about nothing to do with ppO2.

 

[Blackwood]

Absent an inert gas to rush into your tissues as the inert loaded for backgas rushes out, ICD can't even theoretically happen.

Why not?

Though most of the papers out there study "isobaric inert gas counterdiffusion," I can't find anything that precludes the diffusion of oxygen opposite that of nitrogen or helium. Sure, some of that Oxygen is metabolized, but with nothing nearing 100% efficiency, right? And sure, maybe the Oxygen helps negate the bubbles produced by the effects of counter-diffusion.

ICD has just about nothing to do with ppO2.

Agreed, it has to do with different diffusion rates of gases.

 

[Meng_Tze]

Absent concrete proof either way, I am not taking any chances and do not let the change in inert gas be more than 0.5 bar when switching.

It is like Pascal said: " I do not believe in a God, but I pray nevertheless just in case"

 

[rjack321]

Why not?

Though most of the papers out there study "isobaric inert gas counterdiffusion," I can't find anything that precludes the diffusion of oxygen opposite that of nitrogen or helium. Sure, some of that Oxygen is metabolized, but with nothing nearing 100% efficiency, right? And sure, maybe the Oxygen helps negate the bubbles produced by the effects of counter-diffusion.

Oxygen dissolved fairly easily in blood and other tissues. Its roughly 2x more soluble than N2. So if you are staying near or slightly below a N2 dissolved gas ceiling (we'll pretend that's at 18ft) when you are going onto O2 at 20ft, you body is already pretty much saturated with O2 already. Which is why healthy adults very easily have a 99%+ hemoglobin saturation - its easy for our bodies to do. You arrived at 20ft pretty much saturated with O2, and going onto 100% doesn't really saturate you any more. So basically there is no gradient/vacancy for O2 to rush into.

Which is the whole concept behind ICD, a "light fast" gas rushing in and a "heavy slow" gas not leaving fast enough which (net) creates a bigger gas load and bigger bubbles than you had pre-switch.

In addition, per Graham's Law, N2 is actually faster than 02 across membranes. So the switch from a nitrox or trimix backgas onto 100% O2 is a fast to slow switch and can't cause ICD from that perspective either.

 

[Blackwood]

Thanks Richard. I'm still a bit fuzzy. Looks like I have some reading to do.