On Sat, 19 Dec 2015 17:05:50 -0700, Tony Hwang wrote:

They always fill by the weight, not by the volume.

Are you sure?
Propane is a liquid when they pump it into your tank from their truck.

Also, they charge you by the gallon.
Their meter on the truck measures in gallons.

Although the meter on the tank measures in percent.

Of course, that doesn't mean they don't fill by weight.
But, it implies they measure gallons.