Nitrox and Accelerated Deco

[tootricky]

Help me out here please. I stumbled on to Tangles the Cave Diver's (Kenny Everiss ) website in a fit of boredom and came across an article about Nitrox, I won't post the link as I don't wish to promote his site. I have a problem with the following statement

Originally Posted by Tangles

However there is a second benefit. The lower nitrogen PP in your lungs on ascent mean that you have the effect of accelerated decompression. There is a bigger gradient between your nitrogen tissue tension and your lung PP so the nitrogen is sucked out of you.

Surely this is wrong? If you breathe the same mix throughout the dive then gradient is determined by the ambient pressure alone and, therefore, will be the same regardless of mix. Or am I missing something? If I remember it correctly it should be something like this:

Let's look at 2 dives to 30m, one on air and one on 32%. Keeping it simple, let's assume full tissue saturation. Again for simplicity, let's look at what's going on at 10m.

Air: PPN2 @ 30m = 3.16, PPN2 @ 10m = 1.58 => gradient = 2
32%: PPN2 @ 30m = 2.72, PPN2 @ 10m = 1.36 => gradient = 2

Any help appreciated.

Cheers/Nic

[gibbon]

Think about it in terms of EAD, on a richer nitrox mix the pp of N2 will be lower compared to Air or a lower nitrox mix.

You are right about the gradient between depth and mix, but the gradient between N2 pp at the lung surface is a different thing, if you are loaded up on N2 then changing from 21% to 50% makes a difference, I think that is what is ment.

Originally Posted by tootricky

Help me out here please. I stumbled on to Tangles the Cave Diver's (Kenny Everiss ) website in a fit of boredom and came across an article about Nitrox, I won't post the link as I don't wish to promote his site. I have a problem with the following statement


Surely this is wrong? If you breathe the same mix throughout the dive then gradient is determined by the ambient pressure alone and, therefore, will be the same regardless of mix. Or am I missing something? If I remember it correctly it should be something like this:

Let's look at 2 dives to 30m, one on air and one on 32%. Keeping it simple, let's assume full tissue saturation. Again for simplicity, let's look at what's going on at 10m.

Air: PPN2 @ 30m = 3.16, PPN2 @ 10m = 1.58 => gradient = 2
32%: PPN2 @ 30m = 2.72, PPN2 @ 10m = 1.36 => gradient = 2

Any help appreciated.

Cheers/Nic

[chrisch]

Originally Posted by tootricky

...
Surely this is wrong? ...

Yes it is.

But hey what did you expect?

Chris

[Lazlo]

Gradient is the absolute difference between two partial pressure values.

If your tissues are saturated with nitrogen, it doesn't matter what you were breathing, they are "full up". So the gradient would be determined by the saturation partial pressure minus the ambient partial pressure. Less partial pressure of nitrogen in the lungs would mean a higher gradient.

Not sure about the term "sucked out" though.

Correct me if I'm wrong someone...

[tootricky]

Originally Posted by gibbon

Think about it in terms of EAD, on a richer nitrox mix the pp of N2 will be lower compared to Air or a lower nitrox mix.

You are right about the gradient between depth and mix, but the gradient between N2 pp at the lung surface is a different thing, if you are loaded up on N2 then changing from 21% to 50% makes a difference, I think that is what is ment.

You are right in respect to switching to a richer mix but the original quote refers to remaining on backgas.

Cheers/Nic

[gibbon]

Originally Posted by tootricky

You are right in respect to switching to a richer mix but the original quote refers to remaining on backgas.

Cheers/Nic

Maybe they use the mystic powers of the digitised wolf jumper to suck out the N2.

[nigelH]

Originally Posted by tootricky

Air: PPN2 @ 30m = 3.16, PPN2 @ 10m = 1.58 => gradient = 2
32%: PPN2 @ 30m = 2.72, PPN2 @ 10m = 1.36 => gradient = 2

Gas flows depend on the difference of ppInert not the factor.

Let's pull an example for a saturated 18.5 minute compartment on Professor Bühlmann's tables (a=0.7562 b=0.8126) so the safe pressures for your diver saturated to 30m are:
air (3.16-0.7562)*0.7825 = 1.88bar
32% (2.72-0.7562)*0.7825 = 1.53bar
So the Air diver is heading for a 9m stop and the Nitrox diver to a 6m one.
The air diver won't be released to go to 6m until his ppInert is down to about 2.75 (virtually where nitrox diver started) and then he is breathing 1.26bar N2 while the nitrox diver was breathing 1.09bar at the same point.

Nitrox diver will be back on the boat and have finished the biscuits before air diver even gets to smell the tea brewing.

(OK I know there are lots more compartments but the 18.5min one is probably the limit on most recreational 30m dives and doing a big table and selecting the worst case will make this unreadable.)

[chrisch]

Bruce isn't sure either - he's now asking on The Deco Stop

The Deco Stop

Chris

[tootricky]

Originally Posted by nigelH

Gas flows depend on the difference of ppInert not the factor.

Let's pull an example for a saturated 18.5 minute compartment on Professor Bühlmann's tables (a=0.7562 b=0.8126) so the safe pressures for your diver saturated to 30m are:
air (3.16-0.7562)*0.7825 = 1.88bar
32% (2.72-0.7562)*0.7825 = 1.53bar
So the Air diver is heading for a 9m stop and the Nitrox diver to a 6m one.
The air diver won't be released to go to 6m until his ppInert is down to about 2.75 (virtually where nitrox diver started) and then he is breathing 1.26bar N2 while the nitrox diver was breathing 1.09bar at the same point.

Nitrox diver will be back on the boat and have finished the biscuits before air diver even gets to smell the tea brewing.

(OK I know there are lots more compartments but the 18.5min one is probably the limit on most recreational 30m dives and doing a big table and selecting the worst case will make this unreadable.)

OK, if I'm understood this at all then what you are saying in the above example is the the diver will have on-gassed less at the same point in the dive breathing 32% rather than air? i.e. you have on-gassed more slowly? My question is, is the rate of off-gassing increased on the ascent? Can I see some figures as I'm struggling to get my head around this.

Cheers/Nic

[Janos]

Originally Posted by tootricky

OK, if I'm understood this at all then what you are saying in the above example is the the diver will have on-gassed less at the same point in the dive breathing 32% rather than air? i.e. you have on-gassed more slowly? My question is, is the rate of off-gassing increased on the ascent? Can I see some figures as I'm struggling to get my head around this.

Yes. The rate of off-gassing is increased on the ascent. (ie a diver diving on 32%, and then switching to air will take longer to decompress than a diver staying on 32% for the whole time). However, for the vast majority of dives, the biggest benefit of diving Nitrox is that less Nitrogen is absorbed on the bottom phase of the dive.

Janos