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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