Filling Propane Tanks
Most grills and small campers use standard 20 lb propane tanks. A 20 lb tanks holds 20 pounds of propane, right? Not necessarily. "Blue Rhino" and similar exchange tanks are charged with only 15 to 16.5 pounds of propane. If you notice, none of their displays say that you are getting 20 pounds of propane. You're not.
If you go to a gas dealer or hardware store, where they actually fill your personally owned, 20 lb tank, they will fill the tank completely - until OPD (overfill prevention device) clicks off - and charge you for 20 pounds. So you get - and pay for - 20 pounds of propane, right? Not necessarily. Most tanks that are refilled are not completely empty but, if they're filling a "20 lb tank" they will charge you for "20 pounds" whether they actually delivered 20 pounds or not. And they usually are not.
The only place where you get what you pay for is a dealer or hardware store that charges you for the actual amount of propane delivered - usually in gallons - as reflected on the the meter on their delivery system.
How do you know if the dealer that you just walked into charges by the pound or gallon? You ask them when you first walk in the door. You can say something like, "I have a tank that's about one quarter full but I want it filled up. Do you charge by the gallon or just by the size of the tank?" If the person has no idea what you're talking about, they charge by the size of the tank. Go elsewhere unless you're desperate