From: mango@nova.esd.sgi.com (Eric Manghise)
Newsgroups: rec.scuba
Subject: Re: Nitrox (Was Re: Thermodynamic Decompression)
Date: 22 Aug 92 01:23:12 GMT
Reply-To: mango@nova.esd.sgi.com (Eric Manghise)
Organization: Silicon Graphics, Inc., Mountain View, CA, USA

In article <1992Aug21.220829.12246@mailer.cc.fsu.edu>,
mayne@rho.cs.fsu.edu (William Mayne) writes:
|>
|> >1) My understanding is that when diving on Nitrox, say 32% O_2, 68% N_2,
|> >   the partial pressure of N_2 is reduced 68/79 = 0.86, so being at depth
|> >   d on Nitrox is like being on depth D = d*0.86-1.4 on normal air.
|> 
|> You've got the idea right. I don't know how you got the 1.4. That term is
|> a correction for the difference in partial pressure of N_2 at the surface
|> when using nitrox. The actual formula, with .68 substituted for the fraction
|> of N_2 is:
|> 
|> IAD=d*(.68/.79)-33*(1-.68/.79)
|>    =d*0.86-4.62
|> 
|> Hence, for example, the IAD at 100 feet on this mix is 90.62 feet,
|> close enough for me to call it a 90 foot dive.

	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 )


		Where O2 is the percentage of O2 in the mix.
		Where d is the depth.


	Hence for the dive you stated the EAD and P02 at 100ft for this mix
	of 32/68 (02/N2) would be.
	
		EAD = 81.48
		PO2 = 1.29 atm

	This would be consistent with the 2 saftey stop margin this
	mix would give you. This mix reaches partial O2 pressure of 1.58 atm
	at about 130 ft. Typically 1.6 atm is considered to be toxic after about
	a 45 minutes exposure or you can calculate the OTU's by another method.
		
|> 
|> No. Claim (3) is explained entirely by conventional theory. 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 do agree that
	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.
|> 
|> >2) Please explain the term Nitrox decompression. Does it "just" mean diving
|> >   on a Nitrox blend (thereby allowing you to use the reduced equivalent
|> >   depth described above when doing decompression). Or does it, as I
|> >   have been (mis)lead to understand mean diving on air and then switch
|> >   to Nitrox blend when doing decompression stop. Because unless my 
|> >   understanding is wrong that seems to be the completely wrong thing
|> >   to do - though it sounds a little like claim (3).
|> 
|> You are correct that it is exactly like claim (3) in most cases.
|> 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? You certainly may
|> get better results using nitrox for the whole dive, provided you can
|> do that safely. But switching to nitrox, or to a more enriched blend, as
|> your depth allows during ascent won't hurt and may help. 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. This isn't just my speculation. Dick
|> Rutkowski (sp?), the IAND nitrox guru, lists decompression from deep air
|> dives as an application. (Ref. IAND Nitrox manual.) Of course if you
|> aren't spending a significant amount of time in decompression the benefit
|> to switching will be negligible. Another scenario is that even if you dive
|> is shallow enough you may not have enough nitrox for the whole dive, and
|> choose to use your limited supply only in a decompression bottle.

	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. Most people still do
	the reccomended deco time but the added O2 content gives you a greater
	saftey margin. I personally decompress on 50/50 mix like this.

|> oxygen rich environment it is easier to get oxygen where it needs to
|> go, to your cells. Supposedly breathing a gas richer in oxygen is also
|> good for CO_2 headaches and hangovers. I couldn't say since I'm not sure
|> whether or not I've experienced the former.
|> 
	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.

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