Christian Missions in Persia. Christianity was introduced in Persia in the Parthian period, and several bishoprics were established there (Latourette, II, pp. 263ff.).
You will likely pay more for the exchange, even if your tank is completely empty. An empty 20 lb tank will hold about 4.7 gallons of propane. That is $18.47 at $3.93 per gallon, which is the low quantity price (I just paid $3.44 per gallon when filling several tanks). The exchange is about the same price. Keep in mind this comparison is as if the exchange gives you a FULL tank.
Therefore, to prevent too much pressure from building in the tank, they are only filled to a maximum of 80-85% of their water capacity to allow for the expansion and contraction of the propane. So 85% of a 500 gallon tank would be 425 gallons. This is the maximum amount of propane that can be put in that tank.