You are correct but that is not the issue here. The propane "full" capacity of a 20# BBQ tank is 80% of the tanks water capacity, but the 4.7 gallons you cited previously is for propane, meaning it has already taken the 80% into account. The exchange tanks are a further reduction of that, generally 75-80% of the original 80%.My understanding is either your tank or an exchange tank is only filled to 80 percent per regulations.
ant21b said:Cost to fill a 20lb or 4.7 gallon propane tank. Average across U.S.A.