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Re: Silicone tubing and CO2

> From: Dgrim62 at cs_com
> Subject: Silicone tubing and CO2
> Hey everyone,
> Can anyone give me information about using silicone tubing (100%) to deliver 
> CO2 to the tank? My main concern is wether or not the gas will make the 
> tubing brittle over time, or if there is a significant loss factor thru the 
> tubing (gas permeability)? Thanks for any help.

	I've never had a problem with Tygon tubing, which is a silicone.
It does have "relatively high gas permeability" (manufacturer's web page),
and is not suitable for use with acids (from a chemical point of view I
can see why), but any loss is pretty small, and the tube is just fine
after getting on for five years.

	The quoted permeability for CO2 is such that if the tube wall is
1 mm thick, and the pressure across it is 1 cm of mercury, then you will
lose about 2.5 x 10^-6 millilitres per second through each square centimetre
of tube wall.  The partial pressure difference across mine is about 76 cm
of mercury, so I may be losing a few mL per hour.  I suppose that is 
significant - I could shorten the tube.  It's the partial pressure of CO2,
not the overall pressure difference that matters - nitrogen and oxygen
are diffusing _in_ as well.

Paul Sears        Ottawa, Canada