Hypothetical question

[Dan]

Is it? According to everything I've read, and, incidentally, also according to Henry's law, it's the drop in partial pressure that matters.

Theoretically, one might be concerned about O2 bubbles forming, though. At 9m on EAN60, the pPO2 is 1.14 bar, compared to 0.21 at the surface on air.

Isn’t the body metabolism consumes the oxygen and keeps the gas from coming out of solution (blood)?

In regards to nitrogen, I agree with you on the Henry’s Law. The nitrogen partial pressure is still lower than 0.79 atm, therefore the nitrogen gas would still be soluble in the blood.

 

[Storker]

Isn’t the body metabolism consumes the oxygen and keeps the gas from coming out of solution (blood)?

I believe so, at least to some extent, and there are reasons for me believing so. Don't you ever quote me on that, though.

 

[Dan]

Here is a link to EAD calculator:

Equivalent Air Depth (EAD) Calculator

If you enter:
Fraction of oxygen = 0.6
Planned depth = 9 m

Tap “Submit” button, you’ll get:
EAD = -0.4m

 

[doctormike]

Is it? According to everything I've read, and, incidentally, also according to Henry's law, it's the drop in partial pressure that matters.

Weren't you in that thread about GFs when the Shearwater people were working on revising their graph, etc...? I used to think that too. But it's not that simple.

The best way I have understood it is that a drop in ambient pressure is what causes bubble formation, while anything that increases the PPN2 gradient drives decompression. Those are two different things.

That's why you can offgass your tissue compartments either by ascending OR by switching to a richer mix. But switching to a richer mix while staying at the same depth doesn't cause bubble formation and hence doesn't make you at risk for DCS. If it was just the gradient that was the risk factor, switching to your 100% O2 deco bottle at 20 feet would cause DCS which it doesn't - even though it causes a sudden large PPN2 gradient.

Now there may be some limit to this, but since you can only have so much N2 loading by the time you switch to 100% O2 (i.e. not below 20 feet), there is a limit to how big of a gradient you can create with just a gas switch. I don't know what would happen as far as bubbles if you switched to O2 on a very deep dive in the short time before a seizure.

But I think that this is why GFs are calculated as the difference between the ambient pressure line and the M-value line, and not between any other line below the ambient pressure line that would correspond to a specific mix. The ambient pressure line is "breathing 100% N2", meaning that PPN2=ambient, with a slope of 1.0.

 

[Dan]

Doesn’t the bubble formation occur when the dissolved nitrogen concentration in the blood exceeds its solubility in the blood?

According to Henry’s Law, the amount of dissolved gas is proportional to its partial pressure in the gas phase. For nitrogen, as long as the nitrogen partial pressure is < 079 atmosphere (0.4 x 1.9 atm = 0.76 atm), nitrogen bubbles shouldn’t form in the blood since the dissolved nitrogen is still below saturation.

 

[doctormike]

Doesn’t the bubble formation occur when the dissolved nitrogen concentration in the blood exceeds its solubility in the blood?

According to Henry’s Law, the amount of dissolved gas is proportional to its partial pressure in the gas phase. For nitrogen, as long as the nitrogen partial pressure is < 079 atmosphere (0.4 x 1.9 atm = 0.76 atm), nitrogen bubbles shouldn’t form in the blood since the dissolved nitrogen is still below saturation.

I'm not really an expert in this field, but the way I have come to understand it, offgassing and bubble formation are not identical processes.

They are both driven by partial pressure gradients (bubble growth is driven by gas moving from it's dissolved state to it's free state), but bubble formation requires an ambient pressure differential, not just a partial pressure differential. As I mentioned, that's why suddenly creating a huge PPN2 gradient by switching to O2 doesn't cause bubbles to spontaneously form if ambient pressure is unchanged.

I could be misstating this, maybe someone better informed than me can chime in.

 

[Storker]

Weren't you in that thread about GFs when the Shearwater people were working on revising their graph, etc...?

Not as far as I can remember. But as usual, if you can prove me wrong...

switching to a richer mix while staying at the same depth doesn't cause bubble formation and hence doesn't make you at risk for DCS. If it was just the gradient that was the risk factor, switching to your 100% O2 deco bottle at 20 feet would cause DCS which it doesn't - even though it causes a sudden large PPN2 gradient.

I have to think about that at a better time of the day. It's getting late(ish) on our side of the pond.

 

[doctormike]

Not as far as I can remember. But as usual, if you can prove me wrong...

This is actually a really helpful thread, it helped clarify my thinking about a lot of this stuff.

ceiling/GF

 

[Dan]

Just from physics point of view, the nitrogen Henry’s law constant in water (don’t know the value in the blood) is 1600 atm / (mol/litre) at 25C. For discussion point of view, let’s assume the nitrogen Henry’s Law constant in the water is the same as in the blood. Then nitrogen solubility in the blood at the surface would be about 13.8 ppm (0.79 x 28 / 1600 / 1000). At 9 m depth with EAN60 the amount of soluble nitrogen in the blood would be 13.3 ppm, still below saturation. As you ascend to the surface, you will load less nitrogen (5.52 ppm). At any point during the dive you have not pushed your body beyond the maximum nitrogen loading that your body can take. So, how would the bubbles form if all of the dissolved nitrogen is still below saturation? I’m confused. Am I missing something?

 

[tomfcrist]

Hypothetically as an instructor I’d stick to the industry standard.