Air consumptions effect on nitrogen absorbtion

[Knudsenjunior]

Hello

I'm doing a DMT, and haven't been able to figure this one out. The two instructors guiding me disagree on it (although in a very civilized fashion,) and I haven't been able to find it in any of my books. I suggested I asked on YD, as I've been reading from the closet for a while and think rather highly of the qualified answers I often find here.

So here goes. Often we've got customers on our boat who are good on air comming up with 100 bar left in the tank after a rather long dive. I've often heard divemasters say, to cull the temper of customers who want to empty their tank some more, that they should be glad they're good on air, as it means they've got less nitrogen loaded from the dive.

But I rather thought nitrogen absorption was dependant on PPN, not amount of nitrogen inhaled. Put another way, the way a science teacher would, I rather imagine. Two identical glass jars are half filled with a liquid, and the remaining volume is then filled with air at three bars of pressure. Both jars are also fitted with two valves, one in, one out. The jars are left pressurized for, say, an hour, but here's the thing: During that hour one jar has 5 liters of air passing though it, in one valve, out the other. The other jar has 10 liters passing through. The pressure inside the jars remain at 3 bars at all times.

There would, I assume, be an equal amount of nitrogen in the liquid, or? This theoritical model, can it also be applied to human physiology, so that one could say, air consumption has no bearing on nitrogen absorption? (assuming of course that the two jars, or persons as it would be, actually breathe.)

I hope you can sort us out

sincerely and eagerly awaiting any help, Knudsenjunior


edit: Few spelling oops sorted

[nigelH]

Originally Posted by Knudsenjunior

But I rather thought nitrogen absorption was dependant on PPN, not amount of nitrogen inhaled.

I would agree.

Now something like carbon-monoxide where, unfortunately, you body soaks up virtually every trace inhaled breathing rate would matter but you take so little nitrogen the ppN2 will not drop in your lungs during the breathing cycle.

I think Bühlmann discusses this in Tauchmedizin but from the other end and discovers that you don't off-gas faster for breathing more volume.

[Red Sea Shadow]

IMO, nitrogen absorption is dependent only on the PPN and the time of the dive. The amount of consumed air has nothing to do with this.

[jorawley]

i agree that dive time and PPN are factors but surely amount consumed is also a factor.
1 breath with 79% would surely have a different affect than say 200 breaths at 79% as there is more nitrogen being loaded into the system?

[Lazlo]

Originally Posted by jorawley

i agree that dive time and PPN are factors but surely amount consumed is also a factor.
1 breath with 79% would surely have a different affect than say 200 breaths at 79% as there is more nitrogen being loaded into the system?

But you'd have to hold that one breath for the whole dive ...

I've really no idea. Is there a physicist in the house?

[gordon]

You could argue that breathing more frequently will replenish the Nitrogen gradient, so it will be absorbed faster if you breath more.
However, the effect would be minimal.

[jorawley]

Originally Posted by Lazlo

But you'd have to hold that one breath for the whole dive ...

whats wrong with a 30min dive at 30metres on just one breath? thought that was the entry exam into diving....hehe

[Lou]

Originally Posted by jorawley

i agree that dive time and PPN are factors but surely amount consumed is also a factor.
1 breath with 79% would surely have a different affect than say 200 breaths at 79% as there is more nitrogen being loaded into the system?

But there isn't more nitrogen being loaded into the system, that is the key point. You take in a very, very, very tiny fraction of the nitrogen in the gas that enters your lungs. The size of that fraction is determined by the PPN, and this is not changed by the tiny fraction that disappears from the gas into your body whilst it is in your lungs. So effectively your body sees the same PPN at the start of a breath and at the end of the breath. So, I would guess that for all intents and purposes, as you don't completely empty your lungs with each breath, for the whole time you are at a particular depth your body sees the same PPN and absorbs constantly.

[smudger]

Yes IIRC the absorbtion rate of Nitrogen is constant even when at work..
If you up the partial pressure then more will be absorbed though gas flow through the lungs does not affect the PPN therefore the absorbtion rate does not go up.

[Lazlo]

This gets more interesting.... Is exposed surface area a big issue, in which case....:

Do you take on more nitrogen if you're wearing a drysuit, as you've got a larger surface area exposed to air?

If you have large lungs, is that a bad thing too (for the same reason)?

Just thinking out loud