Why plan decompression with a Gf (lo)?

[Storker]

GF 0/0 is the ambient pressure line (not the on/off gas line). Therefore, the safest ascent, would be one with no supersaturation...

With no supersaturation, there can't be any offgassing. If tissue pressure = ambient pressure, you are at equilibrium and there won't be any mass (gas) transport.

To offgas, your tissue pressure has to be higher than the ambient pressure.

 

[rossh]

With no supersaturation, there can't be any offgassing. If tissue pressure = ambient pressure, you are at equilibrium and there won't be any mass (gas) transport.

To offgas, your tissue pressure has to be higher than the ambient pressure.

No. That's wrong. We on/off gas based on the difference between inspired gas pressure, vs tissue gas pressure. ... the tissue gradient.

You don't breath 100% N2 .. right?.


You can actually deco in the below ambient pressure region, but it will forever.

 

[Storker]

Sorry, Ross, but you're unintelligible again. What are you arguing against? What I'm saying is that if you're at GF0, your tissue pPN2 = ambient pPN2, and it's physically impossible to offgas. There. Is. No. Driving. Force. Period.

This isn't rocket science (EDIT: or even hyperbaric science), it's phys, phys.chem, chem and chem.eng 101.

 

[rossh]

Sorry, Ross, but you're unintelligible again. What are you arguing against? What I'm saying is that if you're at GF0, your tissue pPN2 = ambient pPN2, and it's physically impossible to offgas. There. Is. No. Driving. Force. Period.

This isn't rocket science (EDIT: or even hyperbaric science), it's phys, phys.chem, chem and chem.eng 101.

I'll reply just to capture this.... and let you think about it for a bit....


Hint: GF 0 is the ambient water pressure line, while tissue N2 pressure equilibrium is at some other position.

 

[EFX]

Really? I thought that microbubbles had been measured in people who have never even been scuba diving. If so, how do they get them in the first place if they can't grow without supersaturation? Maybe I'm misremembering reading something that talked about finding microbubbles in people who had not been diving.

The key word is growth. You're right that microbubbles exist in non-divers. They are spontaneously created and destroyed by perhaps muscles moving against one another in localized cavities of low pressure relative to surrounding tissue among other causes. The net quantity and size remains relatively constant. Hence, no growth. In diving the microbubbles can grow beyond these initial sizes from supersaturated tissues providing the source gas.

 

[Diver0001]

@rossh @Storker

Actually, you're both saying the same thing but with different words. Technically, Ross is right to say that the tissue gradient is relevant. Storker, you are also right to point out that supersaturation is necessary to achieve that, particularly when using one gas.

Storker, what Ross is saying is that if you change gasses, say from EAN32 to something like Heliox that you will start offgassing N2 even without ascending. This is the basis of accelerated deco.

That said, while you could offgass N2 like that the obvious fact is that if you don't ascend then you won't get back to the surface. The point of staged decompression isn't necessarily to create supersaturation for the sake of supersaturation. In a way it's just part of the game because in a practical/logistical sense we can only carry so much gas of various sorts and we do actually need to return to the surface at some point.

R..

 

[KenGordon]

Sorry, Ross, but you're unintelligible again. What are you arguing against? What I'm saying is that if you're at GF0, your tissue pPN2 = ambient pPN2, and it's physically impossible to offgas. There. Is. No. Driving. Force. Period.

This isn't rocket science (EDIT: or even hyperbaric science), it's phys, phys.chem, chem and chem.eng 101.

So if I arrive at 6m, wait for 1 minute then swap from breathing air to 100% o2 does my GF change instantly?

No it doesn’t, GF has nothing to do with the ambient ppN2 or what you are breathing at that moment, just absolute (ambient) pressure vs tissue ppN2. GF is the risk measure, not the driver of gas exchange measure.

An addition factor is water vapour in the lungs. So air in the lungs is not even 79% N2.

 

[Storker]

what Ross is saying is that if you change gasses, say from EAN32 to something like Heliox that you will start offgassing N2 even without ascending.

Thanks for clarifying. Because no matter how carefully I read his posts, I'm unable to find that info. And I like to believe that I have a fairly decent reading ability.

 

[EFX]

I'll reply just to capture this.... and let you think about it for a bit....
Hint: GF 0 is the ambient water pressure line, while tissue N2 pressure equilibrium is at some other position.

What is that other position? If the N2 in the tissue is in equilibrium, what is it in equilibrium with?

 

[Diver0001]

What is that other position? If the N2 in the tissue is in equilibrium, what is it in equilibrium with?

Well... obviously the pprN2 in the lungs.... ie... the ambient pressure. There can be nothing else on the other side of that equation to my way of thinking.

The key thing is the tissue gradient. A tissue/compartment can continue to on-gas during ascent and it can start off gassing with a slight supersaturation. Context is everything here. Eric, you know the model better than most so I'm sure you are aware that some compartments are off gassing while others are still on-gassing. It's exactly THAT map that we're trying to navigate during an ascent.

R..