Nitrogen off gas on EANx

[Dive-aholic]

Okay, makes sense now. It would've been nice if they had gone into details in the book rather than just state "this is the way it is." Thanks for all the help on this.

 

[Snowbear]

Okay, so, by breathing more O2, you're, in effect, pushing out more N2 than would normally come out on its own. Is that right?

Not quite....
The O2 is replacing some of the N2, reducing it's partial pressure in relation to what's in your body. This reduced PP in relation to the PP in your body (blood), is what is increasing the off gassing. The O2 itself does not "push out" the N2. The O2 molecules, in effect, serve to displace some of the N2 molecules, reducing the amount of N2 in the pressure gradient between your body and the ambient gas you are breathing....

 

[Dive-aholic]

Not quite....
The O2 is replacing some of the N2, reducing it's partial pressure in relation to what's in your body. This reduced PP in relation to the PP in your body (blood), is what is increasing the off gassing. The O2 itself does not "push out" the N2. The O2 molecules, in effect, serve to displace some of the N2 molecules, reducing the amount of N2 in the pressure gradient between your body and the ambient gas you are breathing....

Each explanation makes it even more clear. Thanks.

 

[pete340]

Okay, so, by breathing more O2, you're, in effect, pushing out more N2 than would normally come out on its own. Is that right?

Depends on what you mean by "pushing out". <g> The physics of the thing is that the rate of transfer of any gas depends on the difference in partial pressures of that gas between your breathing gas and your body tissues (that's a bit of an oversimplification, but it's good enough for now). With Nitrox, higher PPO2 means lower PPN2 (they have to add up to 100%). When you're descending, the reduced PPN2 in the gas you're breathing means you take on N2 more slowly, so for the same dive profile you end up with less N2 in your body tissues when you dive Nitrox. When you're ascending, if you started the ascent with the same PPN2 in your body tissues you'd offgas N2 more quickly with Nitrox because of the reduced PPN2 in your breathing mix.

In short, the PPO2 in itself doesn't matter; it's the PPN2 that's important. You can replace N2 with anything else (Hydrogen, Helium, Argon, Oxygen, etc.) and get the same effect on nitrogen absorption. Of course, you also have to consider the side effects of the replacement gase. Replacing N2 with Chlorine would not be a good idea.

 

[DepartureDiver]

. . . it's the PPN2 that's important. You can replace N2 with anything else (Hydrogen, Helium, Argon, Oxygen, etc.) and get the same effect on nitrogen absorption.

Just to make sure it is clear to Rob, each different inert gas behaves independently, so nitrogen offgassing (and ingassing) depends on the difference between the nitrogen tension in the tissue and the ppNitrogen being breathed, but not on any other gas such as oxygen, helium etc But if helium was used, it too is an inert gas that must be considered as it's presence will add to the total gas tension in a tissue, but its offgassing depends only on the helium in the tissue and that being breathed. The nitrogen present in the tissue and that being breathed will not affect the helium in or offgassing. So if you made a trimix dive and then were decompressing on EAN50, the driving force for helium offgassing is always at it's greatest since no helium is now being breathed and the nitrogen offgassing will depend on the nitrogen tissue tension and the ppNitrogen being breathed ... in this example a 50% blend … which results in faster nitrogen offgassing than breathing air … but which will not affect the helium elimination at all due to the lack of helium in both air and EAN50. Once on pure oxygen, all inert gasses will have the largest driving force for elimination since no inert gas is being breathed.

 

[DA Aquamaster]

PPO2 is important from the standpoint of oxygen toxicity. Off gassing at stops below 20' on pure O2 would be rapid given the presence of no inert gases, but the effects of O2 toxicity have to be considered.

In practice two or more deco gases are used with the diver switching to higher O2 mixes as he ascends to keep the PPO2 as high as practical (as close to 1.6 as possible) to minimize the PPN2 and maximize the gradient.

 

[WarmWaterDiver]

Perhaps PADI should have phrased it more along the lines of "Your nitrogen gas transfer will equilibrate at 30' on EANX 50 in similar manner as the EAD of 7', but overall pressure is still 30' to keep gas bubbles in check, which would not occur if breathing air at 7'."

 

[Dive-aholic]

Thanks everyone. I understand the physics now. PADI's wording threw me off. They really oversimplified it. In fact, the whole book is rather simplified. And that's not good enough for me. I want to know the physics of this stuff, but they really don't go into it. Any recommendations of other books that provide better explanations would be appreciated.

 

[pete340]

Thanks everyone. I understand the physics now. PADI's wording threw me off. They really oversimplified it. In fact, the whole book is rather simplified. And that's not good enough for me. I want to know the physics of this stuff, but they really don't go into it. Any recommendations of other books that provide better explanations would be appreciated.

Unfortunately, there's a lot of nonsense that's been written. The problem is that this is, fundamentally, a highly technical topic, and writing about technical subjects for non-technical audiences is very difficult. PADI's stuff is pretty good. One of theirs that was very helpful for me is "The Recreational Diver's Guide to Decompression Theory, Dive Tables, and Dive Computers." It's basic, but it talks about compartments, transfer rates, and how they fit in to the overall theory. At the other extreme is just about anything by Bruce Weinke. His titles usually have the word "basic" in them, but he's got a PhD in Physics, so what he considers "basic" is what most of us would consider advanced. I majored in Physics in college, but that was a long time ago. I recognize the things he's talking about, and remember being able to apply them, but there's a lot of what he says that I don't follow. (If you're not comfotable with gradients and curls you'll have trouble with the details) In between, there's a bunch of stuff by people who don't know enough physics to get the details right, but have read enough to get the general gist of the theory. When they talk about details they can be dangerous. Know your source.

 

[jbd]

Thanks everyone. I understand the physics now. PADI's wording threw me off. They really oversimplified it. In fact, the whole book is rather simplified. And that's not good enough for me. I want to know the physics of this stuff, but they really don't go into it. Any recommendations of other books that provide better explanations would be appreciated.

Do you have Basic Decompression, Theory and Application by Bruce Wienke?
It is far more in depth than the typical dive texts. There is a lot of math and physics, but as the author told me there is enough text to understand the material without having to do the math. I understand the physics but it has been way too long since I have done that kind of math. I think you would like the book.