From the HBD Archive
From: griggs@benjamin.tch.harvard.edu (griggs)
Subject: co2 tank pressure
Date: 1992-05-12 19:56:56 GMT

Just as a point of information, the tank pressure reading on CO2 tanks
has little to do with the actual amount of gas left in the tank. This
results from the CO2 being present in a liquid form at the bottom of the
tank, evaporating to replace gas which is let out of the tank. The observed
pressure is purely a function of the temperature and can vary from 500 to
over 900 psi, depending on the tank temp. For this reason, suppliers of
the gas sell it by the lb (literally, they weigh the tank before and during
filling). In a 72 (F) degree room the pressure will remain at ~900 psi
until all the liquid is gone, then the pressure will drop at a linear rate
as the gas is used. The dropping pressure is a sign that the tank is almost
empty and should be refilled. The only reason I can think of for not using
the tank until it's completely empty is the danger of backpressure. The
beer, exerting pressure of it's own, could back up through the regulator and
into the tank. This would be bad for the regulator, the tank, and the beer ;).
This actually happened to a gas pressure system in our lab, so I know it's
possible. Keep on brewing...
-Chauncey "wish I had a kegging setup" Griggs


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