Nitrogen off gas on EANx

[Dive-aholic]

In the DSAT tec deep book it states that if you switch to EANx50 at 30', you EAD is 7'. It goes on to say that nitrogen is leaving your body as if you were only 7' deep breathing air, but actually ascending that shallow would probably cause DCS.

I agree with the ascending part, but I don't get the middle part. It seems to me that nitrogen would be ENTERING your body as if you were only 7' deep. You're still exposed to the same pressure at 30' (~ 2 ATA). All you're doing by breathing 50% oxygen is reducing the nitrogen you're breathing to 50% at 2 ATA which is equivalent to 79% at 7'. I understand Henry's law and solubility, but I still don't agree that nitrogen is leaving your body as if you were only 7' deep.

Am I missing something here, or did they get it wrong?

 

[miketsp]

In the DSAT tec deep book it states that if you switch to EANx50 at 30', you EAD is 7'. It goes on to say that nitrogen is leaving your body as if you were only 7' deep breathing air, but actually ascending that shallow would probably cause DCS.

I agree with the ascending part, but I don't get the middle part. It seems to me that nitrogen would be ENTERING your body as if you were only 7' deep. You're still exposed to the same pressure at 30' (~ 2 ATA). All you're doing by breathing 50% oxygen is reducing the nitrogen you're breathing to 50% at 2 ATA which is equivalent to 79% at 7'. I understand Henry's law and solubility, but I still don't agree that nitrogen is leaving your body as if you were only 7' deep.

Am I missing something here, or did they get it wrong?

It's all relative.
If you were at the surface and then descended to 30' breathing EAN50 then yes, you would be on-gassing as you were previously stable at a lower N2 PP (surface) and you increased to a PP of 1 (roughly) at depth.
The book is assuming that you were previously much deeper, on-gassing at a higher N2 PP and you have now reduced to 1, so at least your fast compartments will now be off-gassing and depending on how long you were down, your slow ones too.

 

[DepartureDiver]

In the DSAT tec deep book it states that if you switch to EANx50 at 30', you EAD is 7'. It goes on to say that nitrogen is leaving your body as if you were only 7' deep breathing air, but actually ascending that shallow would probably cause DCS.

I agree with the ascending part, but I don't get the middle part. It seems to me that nitrogen would be ENTERING your body as if you were only 7' deep. You're still exposed to the same pressure at 30' (~ 2 ATA).

While you're still at the greater pressure, the nitrogen psi is less ... so both ongassing and offgassing would occur and would be the same as if breathing air at 7'. After doing a stop at 30' on EAN50, the "faster compartments" are offgassing (since they took up nitrogen in excess of 22.2 psi of nitrogen, i.e. 30' of water pressure) and the slower ones are still ongassing since they took up less. The point the book was trying to make was that you will have a large gradient for offgassing but still be under pressure keeping nuclei and bubbles in check ... if I understood your question right.

 

[Dive-aholic]

I understand that because the oxygen concentration is higher, you're taking in less nitrogen (PP of 1 instead of 1.58, roughly), but you're still hanging at ~2 ATA pressure which should be keeping the bubbles in check at that pressure, not the ~1.21 ATA pressure you'll have at 7'. That's why you can't ascend to 7' without increasing the risk for DCS.

I understand the point the book is trying to make. But the statement seems wrong to me. It doesn't make sense that you're going to offgas any faster because of higher oxygen (you're still at a 1 ATA pressure), just that you'll ongas slower because of the lower PP, and this will make it so your tissues have less to offgas.

 

[DA Aquamaster]

The point you are missing I think is that you are also offgassing nitrogen faster because of the higher PPO2 and the higher gradient.

 

[Snowbear]

It doesn't make sense that you're going to offgas any faster because of higher oxygen ...

No - you'll offgas faster because of the decreased partial pressure of N2. The O2 is just replacing some of the inert gas, which serves to decrease that PP and make for more efficient off-gassing (works for Helium as well, BTW).

 

[Dive-aholic]

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?

 

[DepartureDiver]

I understand the point the book is trying to make. But the statement seems wrong to me. It doesn't make sense that you're going to offgas any faster because of higher oxygen (you're still at a 1 ATA pressure), just that you'll ongas slower because of the lower PP, and this will make it so your tissues have less to offgas.

Okay, let's assume a dive to 66' on air for let's say 50 minutes which will saturate the fast compartments such as a 5 minute compartment. This compartment has a nitrogen pressure of 2.37 ata (3 ata * .79 = 2.37 ata). If the diver then goes to 30 ' and does a stop on air (nitrogen pressure of 1.51 ata), the gradient for offgassing is .86 ata (2.37 - 1.51 = .86) and so after a 5 minute stay at 30', the 5 minute compartment would now have a nitrogen pressure of 1.94 ata. If on the other hand the diver was now breathing EAN50, the nitrogen pressure at 30' is .95 ata. (the same as at 7'). This results in a higher gradient between the tension in the 5 minute compartment and that being breathed. So in the same 5 minutes, the 5 minute compartment would go from a nitrogen pressure of 2.37 ata to 1.66 ata ([2.37 - .95]/2) + .95 = 1.66. Does that help some more?

 

[DepartureDiver]

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?

The driving force for nitrogen (or helium) elimination is the difference between the nitrogen (or helium) pressure in the tissue and the nitrogen (or helium) being breathed. So the greater the difference, the greater the driving force for elimination.

 

[Rick Murchison]

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?

No.
The rate of diffusion of a gas across a membrane - in this case from blood to gas in the lungs across the aveolar wall - is determined almost exclusively by the relative partial pressure of the gas in question on either side of the membrane.
For the sake of this example lets assume that your blood PPN2 is 1.5ATM. You get the same gradient for Nitrogen of 1.5 to .95 - and therefore the same diffusion rate - at 30' breathing EAN50 as you would breathing air at 7'.
Rick