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, a 20# cylinder filled to maximum safe capacity will contain (20# / 4.24gal/#) = 4.7 gallons of propane. If you take a propane tank to a service station and fill it all the way, you will leave with 4.7 gallons in your tank, because they will not fill the tank beyond its maximum safe capacity.