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

[ScubaSamScubaStevesFather]

I tried looking on google scholar, as well as other posts on this forum. I can't find any evidence based articles to support whether nitrogen off-gassing is more efficient at 10ft or 20 ft when decompressing on 100% oxygen. I'm looking for actual evidence based studies that can help decompression divers decide on breathing oxygen at 10 feet or 20 feet.

One argument is that at 10ft, the ambient pressure difference is higher, therefore this should accelerate nitrogen off-gassing.
The opposite argument is that at 20ft, the partial pressure of pure oxygen is higher, therefore this should also accelerate nitrogen off-gassing.

So the solid question is: For pure oxygen, is 10ft 1.4 PPo2 or 20 ft 1.6 PPo2 more efficient for nitrogen off-gassing?

Some interesting thoughts that might influence this:
1) The pressure difference between 10 feet and 20 feet seems to be a significant difference when compared to a deeper depth, whereas the difference between 1.4 and 1.6 PPo2 is the same regardless of depth (its possible this is not true) <- this might help support the argument for 10 feet

2) Since pure oxygen is being breathed, there shouldn't be any nitrogen on-gassing at either depth, for any of the tissue compartments <- this might help support the argument for 20 ft

 

[inquis]

One argument is that at 10ft, the ambient pressure difference is higher, therefore this should accelerate nitrogen off-gassing

This is incorrect. The inspired/ambient partial pressure of nitrogen is zero at both 10 and 20 ft, so the rate of off-gassing is identical. From a CNS standpoint, it's preferable to be shallower (lower ppO2) if you're going to be there a while. OTOH, it may be preferable to stay away from rough surface conditions. Your call on these other aspects.

 

[lowwall]

3m will result in faster offgassing due to the increased differential between the pressure of the inert gases (nitrogen in our case) dissolved in tissues and ambient.

But that doesn't mean it's necessarily the better choice. That extra .3atm could be enough to tip incipient or small bubbles into something damaging. Also surface conditions need to be considered.

 

[boulderjohn]

3m will result in faster offgassing due to the increased differential between the pressure of the inert gases (nitrogen in our case) dissolved in tissues and ambient.

How is the partial pressure of nitrogen at any depth different if you are breathing 100% oxygen?

 

[BRT]

The partial pressure of nitrogen in your body is higher in comparison to ambient. If this were not true why not just go to the surface and breath oxygen?

 

[boulderjohn]

he opposite argument is that at 20ft, the partial pressure of pure oxygen is higher, therefore this should also accelerate nitrogen off-gassing.

So the solid question is: For pure oxygen, is 10ft 1.4 PPo2 or 20 ft 1.6 PPo2 more efficient for nitrogen off-gassing?

The idea that a higher PPO2 increases the rate at which nitrogen leaves the body came from a paper from a couple of decades ago, and that belief has since gone away, largely because it is a violation of Dalton's Law. If you read the actual paper in which the idea appears, you will see that the conclusion comes out of nowhere. It appears in one paragraph, with no reference to the data leading to that conclusion.

Mark Powell's Deco for Divers pull no punches, saying the idea is just wrong.

That belief was part of the DIR ascent philosophy more than a decade ago. They created what they called the "S-curve" ascent profile, with more time spent at the depths near a gas switch than on shallower stops. Roughly a decade ago, GUE dropped the idea, realizing that the concept was flawed. If I worked, I could probably find the email in which Jarrod Jablonski told me that they knew the science did not work, but they were keeping it for a while because it had worked in the past. They abandoned it not long after that. As for UTD, when I took Ratio Deco from Andrew Georgitsis, he said he knew the science was wrong, but he believed in the S-curve for reasons that escape my memory. I believe they have abandoned it now as well.

 

[lowwall]

How is the partial pressure of nitrogen at any depth different if you are breathing 100% oxygen?

Hmm, how else can I put this? When I said the "differential between the pressure of the inert gases (nitrogen in our case) dissolved in tissues and ambient" I was talking about absolute pressures, not partial.

Maybe it will help if I use the terms "tissue tensions" and "gradient" since that's how this concept is expressed by some of the technical dive agencies, thus "3m will result in faster offgassing due to the increased gradient between ambient and the inert gas tension tissue."

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.

 

[inquis]

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.

What matters is the "inert gas pressure" (both in tissues and in the breathing mix), which is innately a partial pressure since it's specific to that gas. If you run two O2-based deco plans in Subsurface/Multi-Deco/similar, that differ in the final stop depth, you'll see identical deco times. This is due to an inspired N2 partial pressure of 0 atm regardless of depth.

 

[boulderjohn]

Think about the process of diffusion and why we have tissue halftimes.

Through the processes of respiration and perfusion, nitrogen enters and leaves our body. It comes in when we breathe, enters the bloodstream, and is carried through perfusion through the tissues. It leaves the bloodstream and enters the tissues randomly as it goes. At the same time, nitrogen in the tissues randomly enters the bloodstream and is carried out to the lungs, where it is exhaled. When we are at the surface after a prolonged stay, through the law of averages, the same number of molecules are entering our body and leaving it at the same time. We are at equilibrium.

If we then descend breathing air to 99 FSW, we suddenly have 4 times as many nitrogen molecules entering the tissues as leaving them. This "pressure gradient" causes us to ongas. As the tissues gain nitrogen, though, the ratio of molecules coming in and molecules going out lessens. Eventually there are only twice as many molecules going in as coming out. The amount of time that takes varies from one tissue to another. We call the time it takes for a tissue to reach that 2-1 ratio a halftime. With the difference so reduced, it takes roughly that same amount of time to get to the next half of the difference.

Eventually a tissue will reach equilibrium at our particular depth, meaning roughly the same number of molecules are entering that tissue as there are leaving it. When we ascend, the number of molecules in the air we are breathing drops, so we now have more molecules leaving the tissues that were at equilibrium than entering. We do not want to ascend too fast, because the gradient between our tissues and ambient water pressure cannot be too great. We can have to give enough time for the amount of nitrogen in our body to drop to a safe level relative to water pressure.

We can lessen the amount of time it takes for the nitrogen levels in our tissues to drop to a safe level by breathing gases with less nitrogen as we ascend. That way the number of new nitrogen molecules entering the body to replace the ones that are leaving is less. By breathing gases with less nitrogen, we can therefore decrease the amount of nitrogen in our system without having to ascend to a dangerous ambient pressure to do so.

When we breathe oxygen, no nitrogen molecules enter our tissues to replace the ones that are leaving.

 

[L13]

I don't have any empirical sources, but my understanding is that all current models for off gassing are all(mostly?) partial pressure based. So, the off gas rate would be the same at both depths on 100% O2. However, all (most?) modern models for DCS/bubble formation/bubble growth ("tissue tensions" and "gradient factors", etc.) depend on absolute pressures. So, deco on 100% O2 at 20' would present less risk of DCS than at 10'.

Essentially, it boils down to the the relative risk of DCS (~Gradient Factor) at 10' vs. the risk of CNS/O2 exposure risk from continued PO2 exposure at 1.6.

I haven't seen the evidence/analysis that explicitly shows when the greater risk transitions from one to the other. But I think (most?) deco planning algorithms move deco from 20' to 10' when the DCS risk at 10' drops to an acceptable level determined by the same algorithm as used at all other stops and the surface.

Implicit in that is the idea that once DCS risk drops to an algorithmically acceptable level, CNS/O2 exposure risk dominates.

P.S.
Please! Someone correct me if any of these ideas are wrong!