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> As Scott pointed out vapour
> pressure might have something to do with it as well.
The vapour pressure of butane will determine the pressure of the refill
can. When you fill a totally empty can with butane the pressure will
build until it reaches the vapour pressure (about 2 bar absolute
pressure at room temperature for butane), at this point any more butane
you push in will condense out as liquid and the pressure will not
increase any further.
The reverse happens if you were to slowly release the butane gas, this
causes the pressure to momentarily drop in the can which allows some of
the liquid to evaporate to restore the vapour pressure.
What you are doing in the refill situation though is to use the pressure
in the can to force the liquid up into an "empty" lighter (due to the
dip tube inside the refill can).
When you connect the lighter and refill can together the pressure in the
can will force the liquid up the dip tube and into the lighter.
Initially the pressure in the lighter is below the vapour pressure so
the liquid will evaporate rapidly as it enters the lighter (the same as
if you just sprayed it into the atmosphere). This will continue until
the pressure is equalised and both the lighter and the refill can are at
the vapour pressure of butane.
Based on that simple explanation there should never be any liquid
flowing into the lighter. If the pressure in the lighter is below the
vapour pressure then any liquid coming in will evaporate, and once it's
at the vapour pressure then no more liquid will flow in due to the
pressures being equal. But what I think happens is that when the first
bit of liquid evaporates inside the lighter it reduces the temperature
(eg it drops to 1.5 bar after a 10 degress C drop in temperature). What
this means is that once the pressure in the lighter has reached 1.5 bar,
the liquid entering it will no longer evaporate and will remain liquid.
At this point liquid will continue to flow until the pressures (and
temperatures?) have equalised.
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