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Re: CO2 Tank Pressure

Date: Tue, 28 Dec 1999 13:13:52 EST
From: EFPav at aol_com
Subject: CO2 Tank Pressure

    I just got a 20# CO2 bottle (full) and an O2 regulator for it.  By the
"dummy" lines on the regulator, I am below 1/3 full.  I figure this is
erroneous because CO2 must compress at a different pressure than O2.  Does
anyone know what the tank pressure should be at for a full 20# CO2 cylinder?



Ah, Eric, it's questions like yours that make me feel young again (jk).  The short
answer is that iff the pressure is around 900 psig, you still have plenty. The bad
news is that you can't tell how much (as explained below).

1) Let's assume your CO2 tank is reasonably "full".  So inside, there is a mixture
of both CO2 liquid and vapor.  As long as both liquid and vapor are present,
the pressure in the tank will depend only on the temperature (not "fullness")
as shown in the table below:

Tank Temperature        Cylinder Pressure
(F)                                (PSIA)
69                                837.8
71                                859.8
75                                905.1
77                                928.4
79                                952.2
81                                976.5

The gauge pressure (PSIG) is approximately 14.7 psi less than the values in the
table above.   Note that pressure climbs rapidly with temperature (which is why
you shouldnt leave a CO2 cyclinder in the trunk on a summer day -hopefully the
relief valve will bleed the pressure off).

2) As the CO2 is used, the pressure in the tank will "attempt to drop" and a small
amount of liquid vaporizes to restore the pressure back to equilibrium. The
temperature drops a slight amount also, and heat leaks in from the surroundings to
restore the temperature as well).  So as CO2 is used, the liquid level in the cyclinder

3) When the last drop of CO2 liquid vaporizes, the system is vapor only,
and the pressure WILL DEPEND on "Fullness" according to the gas relation

       P  =  (lbs of CO2/44)*Z*R*T/V


P=pressure (PSIA)
(the constant 44 is the molecular weight for CO2)
R=Gas constant (10.731 in this case)
T= absolute temperature in Rankine (degrees F +460)
V= Tank  volume (Ft3)
Z = compressibility factor (a "fudge" factor to correct for nonideality at high pressure)

You can tell when your cylinder has used up all the liquid, because the pressure drops.
But since most aquariums use very small amounts of CO2, several months of CO2
supply are stll available.  I generally refill at about 200 psig.  I don't recommend letting
the pressure getting below 100 psig, because a third "snow" phase can form  under the
right conditions (44 psia?) which will screw up the regulator (plus how accurate are those regulators
anyway at the end of the scale anyway).

Happy Growing