From: mayne@pipe.cs.fsu.edu (William Mayne)
Subject: Re: Nitrox (Was Re: Thermodynamic Decompression)
Reply-To: mayne@cs.fsu.edu
Organization: Florida State University Computer Science Department
Date: Sat, 22 Aug 92 21:47:53 GMT

In article <1992Aug22.012312.8289@odin.corp.sgi.com> mango@nova.esd.sgi.com (Eric Manghise) writes:
>In article <1992Aug21.220829.12246@mailer.cc.fsu.edu>,
>mayne@rho.cs.fsu.edu (William Mayne) writes:
>|>...
>|> IAD=d*(.68/.79)-33*(1-.68/.79)
>|>    =d*0.86-4.62
>
>	Its not IAD it EAD (equivalent air depth) and I think the
>	formula you want to use is the following:
>
>		EAD = ((((1.0 - O2) * (d + 33.0) ) / .79) - 33.0 )

You are of course correct on all points. IAD versus EAD I can blame
on carelessness like a typo. I was giving the formula off the top of
my head and with an algebraic manipulation which I messed up. I assure
myself I'd be more careful and check it for a dive. I apologize for
not being equally careful on the net. I was thinking more about the rest
of my post.

>|> Tables
>|> assume you are breathing air at 10 feet = 43 fsw total pressure =
>|> 43*.79 fsw N_2 pressure. If your actual breathing mix is only 68% N_2
>|> the actual N_2 pressure is 43*.68. Lower ambient partial pressure of
>|> N_2 yields faster elimination of excess N_2. In almost any theory,
>|> other things being equal, a lower ambient partial pressure of N_2
>|> means slower absorption or faster elimination of N_2 from tissues.
>|> In fact the benefit may be less than predicted by the simple
>|> conventional theory, but it is still an advantage.
>
>	I am not sure what you are saying but tables assume you are
>	breathing air at sea level at the partial pressure of N2 at
>	sea level is about .79 atm and .21 atm for O2.

I'm saying they also assume that you are breathing .79*(d+33) fsw N2
at depth d. 1 fsw = 1/33 atm. Hence if your actual fraction of N2 is
lower during a decompression stop you off gas faster and get a little
extra margin of safety. The tables are computed assuming the slower
off-gassing you get with air during decompression, even if they are
nitrox tables which adjust your assumed N2 loading for slower absorption
at depth.

>	lower partial pressure of N2 does mean less nitrogen is absorbed
>	into your tissues, not that it is absorbed slower but that the
>	pressure gradient is less which means a smaller amount of nitrogen
>	is absorbed.

I don't get this. It appears to me that we are saying the same thing.
The reason you absorb less is because the pressure gradient is less,
which means slower absorption. Slower absorption for the same time means
less absorbed. or vice versa. Nothing (pardon the expression) deep there.

>|> >2) Please explain the term Nitrox decompression...
>|> 
>|> Conventional theory would say that you could actually shorten
>|> decompression using nitrox. NOAA nitrox tables don't do this. It would
>|> be risky unless you had custom tables computed by experts.
>|> 
>|> Why do you believe that switching to nitrox during decompression from
>|> an air dive would be the completely wrong thing to do?... A common
>|> application is doing a deep dive on air, where nitrox could not be used
>|> because of the risk of oxygen toxicity, but switching to nitrox after
>|> ascending to nitrox-safe depths.
>
>	The reason people decompress on nitrox mixes most typically 50/50
>	at 30ft and above is to take advantage of the higher partial pressure
>	of O2 in the mix. From a physiological standpoint the high O2 content
>	causes a faster elimination of N2 from your tissue because of a change
>	in the tissues pressure gradient due to the O2.

Again we saying the same thing, except that I would describe the primary
cause of faster N2 elimination as the reduced fraction and hence partial
pressure of N2, yielding a higher N2 pressure gradient. For this effect
the role of the extra O2 is simply to replace N2. As I understand it the
role of elevated levels of O2 itself actually slows total gas transfer
a bit resulting in LESS benefit than simple Haldane theory would predict
for the lowered fraction of N2. This is not a problem since the tables
are figured without taking advantage of the reduced N2 during decompression
anyway.

Around here the practice, for hard core deep divers, is to use trimix for
the dive, then switch to air (21% nitrox) at about 190 feet on the way up,
then go through a couple of richer blends until they are able to go on
O2, at 20 feet or less. They are using custom tables. Caution is required
using O2 under water.

>       [Most people stay for]
>	the reccomended deco time but the added O2 content gives you a greater
>	saftey margin. I personally decompress on 50/50 mix like this.

I agree. I said it wasn't safe to naively reduce deco time on the basis
of theory, though some custom tables do it. I wouldn't do this on my own,
either.

Question: Do you use, and/or feel it necessary to use, O2 cleaned regulators
for the 50/50 mix? My understanding is that this is recommended above 40/60,
though some people tell me they use regular O rings even with pure O2.
From what you say below I take it that you have oxygen compatible regs.
I'm just moving from theory to practice with this and I'm sticking with
40/60 for now since I haven't modified my regulators. What I'd really like
to do when I can afford it is have a dedicated O2 bottle and regulator.

>	I usually will breathe surface oxygen after a deco dive for 10-15 minutes
>	depending on the exposure to further hasten the elimination of N2 from
>	my system. Also it tends to make me fell more alert and better. Oxygen
>	at sealevel cannot hurt you under most normal exposures, you body is
>	designed to metablolize O2 quickly. It does work for hangovers by the
>	way.

I breathe O2 at 10 feet for 20-25 minute intervals when available, with 5
minute air breaks. One of my diving buddies and mentors provides the gear
when we dive together. Surface O2 is also a great idea, especially if you
haven't already started it at 10 feet. I doubt that the immediately felt
benefit is from the faster elimination of N2 as much as the direct benefit
of O2. I also feel these benefits. I just question the cause. I wonder if
it is partly mental, as well. I haven't tried surface O2 or dove with a
serious hangover.

Bill Mayne
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