Hi Matt and volks,

I think the vapor pressure of Propane/LP gas depends only on temperature and
not on volume of liquid in the tank.  The propane you buy at the local
supplier goes in as a liquid under pressure, kept in liquid form by the
vapor pressure at the temperature of the tank.  That's kinda why you NEVER
fill a propane tank FULL of liquid.  That relieve valve or vent valve spits
liquid propane when the tank is full for safe use.  You could force more LPG
in there but you risk blowing the tank as the pressure compresses the gas
"head" to zero and the thermal expansion of the incompressible LPG ruptures
the tank.

For refilling the 16oz cannisters.  NEVER fill 'em full.  That schrader
valve thing has a tube inside the tank that will determine the amount of
propane the vessel can safely handle.  When you refill 'em sensibly you
monitor the amount of liquid in the vessel by relieving the pressure at the
valve and watching for liquid propane to escape.  When it does, the vessel
is "full".  Or so I've heard some old wives say.

I personally feel that Willie the Shake was right.  Far too many sharks to
be filling your own propane tanks, or using scissors with sharp ends, or
buying paint under age or....

Yes there are tools to do  the job right.  No, I don't recommend any of 'em.
But be sure to have sufficient headroom in any pressure vessel that you fill
with an incompressible liquid that vaporizes at ambient.  LOX will make a
truly nasty mess if you bust a tank.  LN is also a problem.  I don't think
beer does that.

But lemme check...