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I'm certainly not an expert on this.  As George has pointed it, my statement
is only correct if you believe the algorithm.  I think the reason is that
your bottom mix is high in Helium and lower in Nitrogen.  Switching to 50/50
deep allows you to get on a gas with no Helium in it.  The gradient is high
so allows the absorbed helium to exit faster.  No more helium on-gassing
after the 80 foot stop, only off-gassing.  Naturally you are on gassing the
nitrogen but the model is biased I think that Nitrogen is you friend and
Helium is taboo.   In the real world physiology this is probably not the
case.  Not a doctor here so I have to step back and let the experts jump in.

Art.


	-----Original Message-----
	From:	James Dibbs [SMTP:JDibbs@ma*.co*.au*]
	Sent:	Wednesday, January 12, 2000 6:44 PM
	To:	Paltz, Art; 'Oystein Mehus'; Manos Manoli
	Cc:	Tech Diver
	Subject:	RE: I think I don't understand

	I have a question.

	Does this have anything to do with the fact that although the ppO2
on 50% at 22m is the same as 100% at 6m, your nitrogen gradient is not as
you are still breathing N2 in the 50% mix?

	I thought about this when I read the "why we don't use 80%" post. 

	"nor does it work as well as pure oxygen without an inert gas at any
depth"

	Thoughts?

	James Dibbs
	Sydney, Australia
	jdibbs@ma*.co*.au*

	 

	-----Original Message-----
	From: Paltz, Art [mailto:Art.Paltz@R2*.CO*]
	Sent: Thursday, 13 January 2000 5:42 AM
	To: 'Oystein Mehus'; Manos Manoli
	Cc: Tech Diver
	Subject: RE: I think I don't understand


	I don't think it's that cut and dry.  I think it depends on the
amount of
	nitrogen in the bottom mix.  According to Buhlman (since it was
referenced),
	if you've got a lot of nitrogen in the bottom mix then the O2 is
going to
	get your our faster (greater N2 difference in the bottom mix).  Less
	nitrogen in the bottom mix and the 50/50 is going to get you out
faster (the
	He difference is 100% and it comes out faster then N2).  According
to
	straight Buhlman (using Decom with N2/He M's set to 0%) I get the
following.

	Same simple dive 130 feet for 45 minutes.

	Using air you'll get out of the water in the following times.
	Deep stops, O2 deco - 87 minutes
	Deep Stops, 50/50 deco - 90 minutes
	No deep stops, O2 deco - 85 minutes
	No deep stops, 50/50 deco - 89 minutes.

	Using air O2 wins both times by 3-4 minutes.

	Say you are using a helium rich mixture, say 50% He, 21% O2 (want to
keep
	the O2 the same when comparing.  What you would be doing with this
mix in
	your tanks I don't know.

	130 ft, 45 minutes, 50% He, 21% O2.
	Deep stops, O2 deco - 130 minutes
	Deep stops, 50/50 deco - 117 minutes
	No deep stops, O2 deco - 128 minutes
	No deep stops, 50/50 deco - 114 minutes

	50/50 wins both times by 13-14 minutes.

	With a lower Nitrogen bottom mix you're better off with 50/50 then
the O2.
	I think it naturally comes back to picking the right gases for the
dive.
	Naturally this is Buhlman's interpretation.

	Hope this helps,
	Art.


		-----Original Message-----
		From:	Oystein Mehus [SMTP:oymehus@on*.no*]
		Sent:	Wednesday, January 12, 2000 8:50 AM
		To:	Manos Manoli
		Cc:	Tech Diver
		Subject:	Re: I think I don't understand

		Manos Manoli wrote:
		> 
		> Jeff i made many many tests with diferent Ean mixtures
belive me
		> 100% oxygen on 6 meters does a hell of a lot diference
than 50% on
	all stops.
		>

		This subject interests me. Do you have any references to
		why this might be so? And why does not deco-programs
		reflect this knowledge? According to the algorithms (at
		least Buhlman) you'll get out sooner if you use 50% from
		21m than if you go straight to O2 on 6m.

		Oystein
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