Propane is always sold by weight but often converted to gallons…
A full 20# tank is full when it has 20# of propane and is 80% full by volume…
The only true measure of propane is by weight or at a temperature of 60* if by volume … obviously we don’t always have those controlled conditions…
A pound of propane has the same BTU content at any temp, a gallon would not…
Propane has a high expansion rate and the tanks, bottles or cylinders (your choice) can be overfilled if filled by volume only without a temperature compensation device…
all home delivery trucks are required to have these devices for home heating propane but unfortunately it isn’t required at all small tank filling stations…
It is never a good idea to fill you tanks in a cold climate when you will be immediately heading to a warm or a hot climate… expansion can cause the tanks to vent off the excess gas into the atmosphere and be dangerous at times…
It is not always a good idea in the southwest to fill the tanks when going to cold high altitude locations… some propane sold in the SW has a higher concentration of butane in the propane that is less suitable for use in cold temperatures…
Best bet is to fill the tanks near the area where they will be used, although that isn’t always possible…
I would like to know how a 20 pound cylinder can be filled to 18 or 20 pounds of liquid from a bulk vapor delivery system…