Frank,

Coming into this late so I apologize if I missed the point.  I think the
real point of the initial question was why is NITROX mixing more
dangerous than Tri-Mix filling?  I think most people know the dangers of
mixing yourself if you don't know what you're doing, are careless, don't
service equipment, etc.  Unless you're doing poor man's tri-mix
(heli-air) then you're still adding O2 to the initial mix prior to
toping with air.  I think in this instance the risks in filling are the
same in regard to the O2.

Art.


		-----Original Message-----
		From:	Frank Riffel [mailto:frank.riffel@en*.co*]
		Sent:	Wednesday, April 07, 1999 1:52 PM
		To:	S I L E N T I M M E R S I O N
		Cc:	techdiver@aquanaut.com
		Subject:	Re: Trimix Question

		S I L E N T I M M E R S I O N wrote:

		> Frank,
		>
		> >I can understand that someone doesnt want to do the
more dangerous Nitrox fills, but with >Trimix...
		>
		> What's the dangerous part?????
		> Care to explain?

		There are several ways to produce Nitrox these are
continous blending, molecular weight mixing,
		nitrox membrane and partial pressure filling. The
typical home brewer will use partial pressure
		filling which is the least expensive.

		The main hazard of Nitrox filling results from the
oxidizing properties of oxygen and its abiltiy to
		accelerate combustion. Oxygen will lower the ignitation
temperature of any fuel and increase the
		combustion rate dramaticaly. The danger is that some
fuel (gaseous hydrocarbons, oil, grease,
		lubricants) inside the tank or filling setup will
explode. You can imagine that an exploding tank is
		not healthy (btw. explosion pressure of a hand granade
is ~ 1200psi).

		How does partial pressure filling work?
		 - Transfill some amount of Oxygen with a whip to the
scuba tank
		 - Fill the tank with air from a compressor

		So where are the problems?
		 - All parts that will see high pressure oxygen must be
oxygen serviced, this includes tanks, valves,
		  regulators, filling whip, pressure gauges
		 - The air which is used for topping must be free of
hydro cabon fumes and oil. A typical requirement
		  is less than 25ppm hydrocarbons and 0.1mg oil mist per
m3.

		Common failures
		  - A piece of equipment is not oxygen serviced
		  - The filling air contains to much oil mist
		  - Oxygen tranfilling is done to fast (this will warm
up the scuba tank due to adiabatic compression)

		For partial pressure mixing it is vital that you use an
extra nitrox filter stage after the compressor.
		This filter should be equiped with a check valve that
avoids back flow of nitrox/oxygen into the
		filter and compressor system.
		Filling an oxygen serviced tank only ONCE on a
compressor without the extra nitrox filter may put
		enough oil into it to create a serious DANGER next time.


		Regards,

		Frank

		P.S. You should have heard about that in your Nitrox
course.


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