Where is the membrane?

I missed the initial thread of this message but if a vessel is truly sealed,
once the partial pressure of the dissolved gas (in this case CO2) reaches
equilibrium with the head gas (air), the amount of the CO2 leaving the
liquid should equal the amount of gas re-entering the liquid.  The trouble
with soda bottles is as the volume of the liquid goes down, the head gas
space goes up and the amount of dissolved gas (which isn't high to begin
with ) needed enter the head space to reach equilibrium renders the
carbonated liquid flat.  

The process works in near full, well sealed vessels, though. 

Rich  

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Howard Winter
Sent: Thursday, March 18, 2010 1:15 PM
To: Microcontroller discussion list - Public.
Subject: Re: [TECH]:: Factors affecting gas flow through a porous membrane
/Help!

Russell,

On Fri, 12 Mar 2010 04:13:32 +1300, Russell McMahon wrote:

>... 
> The important implication for me is the (utterly obvious now) fact
> that you CANNOT stop a given gas diffusing by "balancing" it with
> anything on the other side.

Indeed, and I've observed this with CO2 dissolved in liquid vs. present in
the gas above it (ie a bottle of Coke which isn't full).  You can buy a
little pump fitted to a 
lid which you put onto the bottle, and give a number of squeezes, increasing
the pressure in the bottle by adding air.  It's sold as preventing
part-empty bottles 
losing their fizz, but the result is as expected from what you say above -
it's completely useless!  Note that there is no membrane, just the liquid's
meniscus, but I 
think the principle at work is the same.

I wonder if I could bring a case under the Trades Descriptions Act, or the
Sale of Goods Act, because the the device doesn't do what it's being sold
for?

Cheers,


Howard Winter
St.Albans, England


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