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.
The fixed liquid level gauge is used for filling. Tanks with float gauges measure the volume of the tank as a percentage of the total capacity of the container. If the gauge reads 50% on a 250 gallon propane tank, the tank has approximately, 125 gallons of propane.