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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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