# Re: 5lb. CO2: How long should it last?

```>  "George Booth" <booth at frii_com> wrote:
> > The guys doing the filling must be real lame-brains. They are supposed
> to
> > fill the tank by weight, not pressure. If there is ANY liquid CO2 in the
> > tank, the pressure will be constant - 850 PSI or so depending on temp.
> Since
> > the tank will be cold when it is filled, the pressure may read 600-700
> PSI
> > at that point. If they stop when they first see 700 PSI, you won't be
> > getting much at all. Maybe you should check with them to see just what
> the
> > heck they are talking about. It sounds like you went sonewhere else.
> Good
> > thing!
>
I agree with George.  In clarification (or reiteration), the issue is
that gas compresses more than does a liquid.  If any liquid
is present, then pressure exists to compress gas to a liquid.
The energy required to do this varies somewhat with temperature
(recall Boyle's gas Law), so you have to measure weight
so you know how much CO2 you're getting (the pressure should
be constant at any liquid/gas ratio at a given temperature.)
Of course, if you have no more gas in the bottle (everything
is liquid), then pressure will rise as you add more becuase
liquid doesn't compress very well.  This might be what they
are talking about in filling-to-a-pressure; but it places assumptions
on what "room temperature" is.  Also, the bottle should warm
a little as you add to it because of the energy for compression
(recall the bottles freeze fast when it quickly loses pressure.)
So, weight is still a better indication.  ;-))

As a side note, CocaCola did some experiments in the 70's
with packaging their beverage for astronauts (not kidding.)
There was a CO2 cartridge inside the can, and when you
opened the can, the capsule was opened, and capsule
rapidly lost pressure.  Because the can was stored at
(ambient) room temperature, opening the can would actually
chill the can (decrease in pressure, loss of energy, temperature
drop.)  It really worked.  It never made it to our litigious public
market place because of concerns over exploding cans,
and the chance that some kid would cut off the bottom of
the can to chew on the CO2 capsule.

FYI--Calling around to our gas suppliers here, they say all
bottle sizes are rated to  2000 psi (5lb, 10lb, 20lb, 50lb,
> 100lb, etc.)
>
We fill our bottles all the way and then store them by the
furnace.  <JUST KIDDING>.  ;-))

--charley
charleyb at cytomation_com

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