Increased nitrogen off-gassing 10ft/3m VS 20ft/6m on 100% oxygen

[lowwall]

Seams like a reasonable summary to me. Calling the non-blood a "liquid" is a little weird, but it shouldn't change the answer.

Muscles and internal organs are almost 80% water so it's a reasonable first approximation.

BTW, we have the first response.

Welcome to Physics SE! Note that the concentration difference provides only the driving force. The flux, or rate at which the process occurs, depends also on the mobility of the nitrogen, which presumably carries its own pressure dependence.

 

[Merida Cave Diver]

Interesting discussion, I appreciated reading it.

 

[ScubaSamScubaStevesFather]

Sounds like there’s no clear evidence for one way or the other. Maybe the best approach is to aim specifically for 15 feet?

 

[lowwall]

Sounds like there’s no clear evidence for one way or the other. Maybe the best approach is to aim specifically for 15 feet?

That's fine for us Americans. But it means the metric peoples of the world would need to use a decimal point for their stop. I'm not sure most computers offer one. Maybe 5m (16.4ft)?

 

[EFX]

What you essentially have are two gas mixes at different pressures separated by a membrane that is permeable, but slows any flow. The higher the difference in absolute pressure between the two sides, the more quickly the gas will move from the higher pressure side to the lower pressure side.

Inert gas will move from higher to lower pressure. Going from a deeper to a shallower depth means that our tissues being at a higher pressure than our lungs at a lower pressure will cause off gassing. That pressure difference is the partial pressure difference of each individual gas and not the difference in total pressure. However, the partial pressure is derived from the ambient total pressure and the percentage of the inert gas in the mix. Here is an example (all pressures are absolute):

Bottom gas: 20/60, tissues saturated at 66 ft, total pressure: 3 atm, ppN2: 0.6 atm.
Gas switch at 20 ft to Nx32, total pressure: 1.6 atm, ppN2: 1.1 atm.

At the gas switch the tissues experience a total pressure drop of 3 - 1.6 or 1.4 atm which according to your explanation in the quote above should cause all inert gases to offgas. But, in reality while the Helium is offgassing, the nitrogen due to the gas switch is ongassing due to the higher partial pressure of 1.1 atm at ambient relative to 0.6 atm in the tissues.

Two different inert gasses may flow in opposite directions between the blood and tissues. It depends solely on the partial pressures of the individual gasses.

Getting back to topic I believe there is no difference in off gassing between 10 and 20 ft while on 100% O2 because the partial pressure of helium and nitrogen is 0. However, the starting inert gas pressure in the tissues will be slightly less at 10 ft than 20 ft because of the short time of offgassing from 20 to 10 ft prior to the gas switch. Therefore, off gassing to a safe value at 10 ft will be slightly faster than the off gassing at 20 ft. The difference in time is negligible and can be ignored.

 

[lowwall]

That is what the algorithms assume, but I still haven't seen any evidence that it is reality as opposed a simplification that is close enough for our purposes.

There has been a follow-up on the reply on physics stackexchange. The person with the original response posted the following after being asked to expand on it:

The question currently focuses on concentration differences, but even at a set concentration difference, pressure can affect the driven movement of gas through a material in complex ways. A complete analysis of the problem would need to include this factor (termed the mobility) as well.

 

[dmaziuk]

That is what the algorithms assume, but I still haven't seen any evidence that it is reality as opposed a simplification that is close enough for our purposes.

We know it's a simplification that's close enough for our purposes. In CompSci that's what a model is.

 

[lowwall]

We know it's a simplification that's close enough for our purposes. In CompSci that's what a model is.

Right. But in this case, we are looking at a very specific question (do you offgass nitrogen faster at 3m or 6m on 100% O2) and want to go beyond the model.

For example, no one will seriously argue that 16 discrete types of tissues with half lives of integer minutes accurately describe the human body. If you are interested in how (pick an organ) the spleen offgasses, you need to look beyond the model.

 

[EFX]

That is what the algorithms assume, but I still haven't seen any evidence that it is reality as opposed a simplification that is close enough for our purposes.

Read up on ICD (Isobaric Counter Diffusion). That is your proof. My example above describes the situation that allows a possible DCS hit from ICD.

 

[dmaziuk]

Right. But in this case, we are looking at a very specific question (do you offgass nitrogen faster at 3m or 6m on 100% O2) and want to go beyond the model.

Then I think you should screw the physics and look at the physiology instead: the lungs don't fully deflate. Therefore right after gas switch there is a residual air in there that mixes with inspired O2, goes into the tissues, yadda yadda yadda. So there is a non-zero fraction of N2 in there that gets smaller with every breath but is nevertheless there, and is subject to laws of our simple model. QED. (Edit: I mean: you do off-gas faster at 6m, QED.) At 3m, d'oh.

How Many Angels Can Dance on the Head of A Pin - Rey Reynoso's Bible Archive

We Christians like to get into debates that there really is no answer to. And we get into a huge huff about it too. One old debate was the one about Angels and Pin points (not pin heads as is often cited although it does result in a certain amount of pin-headishness). I found a […]

biblearchive.com