Nitrox and Accelerated Deco

[nigelH]

Originally Posted by tootricky

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.