Are gas stations and other ameteur tank filling facilities limited to 15 lbs by some state or federal law? Just curious…
I’m pretty sure that the person filling the tank must have propane dispensing certification. (servsafe for lpg….) I doubt that many gas stations comply. This is why the govt. started requiring the overfill protection devices in the first place. Untrained people were occasionally filling tanks past their 20 lb. capacity, so when the liquid expanded, the tank would release gas through the safety vent to prevent a tank failure but the release of the fuel could cause a fire if there was an ignition source.
I think that many places “short” their customers because they can. Unlike gasoline, you don’t pay by the gallon and cannot see how much is pumped into the cylinder. At least Blue Rhino tells you that you are not getting a “full” cylinder. But they tell you it is because of “safety” which is bunk. Just go to any gas cylinder manufacturer and you’ll see the capacity in the spec sheets.