> > 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 i=
n the gas above it (ie a bottle of Coke which isn't full). =A0You can buy a=
 little pump fitted to a
> lid which you put onto the bottle, and give a number of squeezes, increas=
ing the pressure in the bottle by adding air. =A0It's sold as preventing pa=
rt-empty bottles
> losing their fizz, but the result is as expected from what you say above =
- it's completely useless! =A0Note that there is no membrane, just the liqu=
id's meniscus, but I
> think the principle at work is the same.

Agh. So obvious!
Yes. I played with such schemes some years ago with generally poor results.
You will get some effect - but not necessarily what you want. I
haven't thought this through but, in the pressurised space above the
Coke etc you get CO2 in proportion to the molar ratio of the gases. If
the space above the liquid is well sealed to ambient, Long term CO2
will come out =A0of solution until CO2 is about 40% of the total in the
gas space. (Molecular weights ~=3D N2 - 28, O2 - 32, CO2 - 44). E&OE. If
this space is at atmospheric then the partial pressure of the CO2 will
be about 44/104 x 14.5 =3D~~~ 6 psi. That seems low compared to
pressurisation pressures. Putting compressed air there ay say 2
atmosphere will double equilibrium CO2 to say 12 psi =3D still too low.
*BUT* you have all sorts of interesting issues with solubilities,
relative diffusion rates out of bottle pump seal to ambient etc. The
real result seems confused.

AND inm the first case you need X grams of CO2 to leave the liquid to
achieve equilibrium. If you double the PPs then you double the mass of
CO2 that has to come from somewhere.

It seems that pumping it up with CO2 may be useful :-).

Also, expelling all air AND then folding bottle and putting a weight
on it may also be useful. Which I've tried. It wasn't obvious that it
was overly effective.


Russell






> I wonder if I could bring a case under the Trades Descriptions Act, or th=
e 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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