Partial Pressures and Depth Question

[rongoodman]

But that was a result of the small connection size. Compare that to the vast surface area available in your lungs.

 

[lowwall]

LanceRiley is correct on this. I once had two tanks with differnt belnds, and I wanted them to be alike. I connected them with a transfill whip and left them connected for 3-4 days. When I tested them after that, they each had the same blends they started with. Whatever mixing had happened was undetectable.

But that was a result of the small connection size. Compare that to the vast surface area available in your lungs.

Assuming both valves were open ...

If I'm interpreting the attached correctly, I think it's actually the high pressure (and the small area). Diffusion rates are slowed as pressure increases. It's not something we need to consider at the single digit bars of pressure we experience, but it's definitely going to make a difference at hundreds of bars.

see DIFFUSION COEFFICIENT

 

[rongoodman]

It's amazing the twists and turns these discussion can take. I made the mistake of clicking on that Wiki link about Fick's Law and was confronted by a bunch of partial differential equations. Too many years since that kind of math for me.

 

[lowwall]

It's amazing the twists and turns these discussion can take. I made the mistake of clicking on that Wiki link about Fick's Law and was confronted by a bunch of partial differential equations. Too many years since that kind of math for me.

My math was never at the point where I could solve these. But looking at the equations in my last post, I can at least understand what "p" showing up in the denominator is going to do it you take it from 1 to 200.

 

[LanceRiley]

This is highly illustrative. There is zero total pressure difference, but a huge partial pressure difference. It will not take forever for molecules to even out the concentrations between the 2 tanks by diffusion. It will mostly be done in a matter of hours.

Ok exaggerated…. But in the context of decompression…. Thats like forever

 

[LanceRiley]

What you're saying is mostly correct, but the distinction others are trying to make has to do with that word "pressure". Gas transfer is actually driven by PARTIAL pressure. That extra word is very important. By definition, that is the product of ambient pressure and fraction of a particular molecule (e.g., 21% in air). The people talking about "concentration" are also correct, as partial pressure can also be viewed as a density (# of molecules per some volume).

Up thread, I believe the term "partial pressure" was intentionally not used because it's a bit confusing. It's not ambient pressure (alone). In fact, if you had said "partial pressure", I don't think there would have been disagreement.

Pardon my use of words guys..


I noticed the OP is not a technical diver . I mentioned earlier that i wanted to explain it simply as he asked a very simple question with simple words.

And i had a disclaimer that if he wants to learn more. He might want to take a technical class

 

[LanceRiley]

LanceRiley is correct on this. I once had two tanks with differnt belnds, and I wanted them to be alike. I connected them with a transfill whip and left them connected for 3-4 days. When I tested them after that, they each had the same blends they started with. Whatever mixing had happened was undetectable.

I learned this whill making Nitrox. Earlier on .

1 tank was 31% the other was 33%

Both were approx 200bar. I connected both with a transfill. Hopping that they would average out to 32% after a day nothing happened lol. Or maybe there was transfer. But my transfill whip had an analog guage. Cant really be that precise.


Ended up draining 10 bar off the other one. Leaving me with a 190 and 200bar.

Then balanced botht 195. Then the concentration moved a wee bit

 

[LanceRiley]

But….. difference in ambient pressure is difference.

Now that fick’s law is being mentioned

Higher pressure to lower pressure. Even if its little… there will be movement.

For those diving perdix.

A gf99 of 5% would mean just that 5% of ambient to m value pressure. Thats still offgassing. But at that rate …. Its taking forever ……. ok minutes. Which could be well spent moving shallower or… if near the surface …. Just surface.

But were really hijacking the OP’s thread