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Discussion Starter #1
Help me out here please. I stumbled on to Tangles the Cave Diver's (Kenny Everiss :D) 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

Tangles said:
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
 

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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.

Help me out here please. I stumbled on to Tangles the Cave Diver's (Kenny Everiss :D) 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
 

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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...
 

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Discussion Starter #5
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
 

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Nigel Hewitt
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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.)
 
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Bruce isn't sure either - he's now asking on The Deco Stop :D

The Deco Stop

Chris
 

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Discussion Starter #9
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
 

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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
 

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Nigel Hewitt
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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.
OK. Let's pick up at the point when they both reach the 6m stop. OK nitrox diver goes straight there and air diver stopped at 9m first.

They have 2.75bar of N2 in the 18.5 minute compartment (usual caveats about compartments not actually existing but being a math model should go here but I'll do it at the end.)

Air diver is breathing 1.26bar N2 while the nitrox diver is breathing 1.09bar
What they are breathing is what the compartments are moving toward and in the half time (18.5mins) they will get half way. For air diver that is 2.00bar and for nitrox diver it is 1.92 but the N2 level the a and b constants give them to surface is 2.03bar so you can see that air diver just got there while nitrox diver is well past, well about two minutes past.

Finally they both surfaced with about 2bar of nitrogen but again nitrox diver got there first.

OK if you were working it out for real you would have to do all 16 Bühlmann compartments and do the gassing on and gassing off maths as the longer compartments take longer to gas on and then work out which compartment needs the deepest stop and do that one. Compartments are fiction. No bit of you is an 18.5 minute compartment but they are numbers set to overlap so they cover all bits of your body. If you model one compartment feeding into another compartment you just get the same gassing on and off as a compartment whose time constant is yet another value. Bühlmann's compartments run from 5 minutes (less than 2 for helium) to over 10 hours to cover all types of diving.

I hope I haven't confused matters. I could do formula and mathematics but I'm guessing that wouldn't help.

One last point: At the 6 meter stop nitrox diver could have really cheated and switched to a 100% Oxygen stage. Being 100% O2 isn't the important thing but being 0% N2 is. Hence he is going toward 0bar of N2 and in the 18.5 mins he is half way there which is 1.37bar. - Well no. He surfaced at 2bar like the others will so he gets out after about 7 minutes. Now that is accelerated deco.
 
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