Gary RV_Wizard said:
Pressure does decrease as the volume goes down, but we are talking a decrease from 300 psi to something less. Not what you would call "low" until it is essentially empty.
No, it doesn't. This is a common misconception that is true for compressed gasses, but propane isn't a compressed gas. It's a liquid with a very low boiling point that boils into vapor when the temperature is above that.
A propane tank is a pressure vessel and works like a pressure cooker or a pressurized cooling system in a car. As the pressure inside the tank increases, the boiling point of the liquid inside it also goes up. The liquid propane boils until the pressure in the tank raises the boiling point to match the tank temperature. Then it stops boiling until something reduces the pressure, like drawing off some vapor.
The tank pressure stays constant, as determined by the temperature, until all of the liquid is gone. That's why you can't use a pressure gauge to tell how much propane is in the tank.
Liquid propane is 270 times as dense as the vapor, so you'll have to vent 270 gallons of vapor for each gallon of liquid propane in the tank.
As far as the tank pressure, it ranges from about 250 PSI at 130 degrees F to 0 PSI at -45 degrees F. In between it's 24 PSI at 0 degrees, 92 PSI at 60 degrees and 172 PSI at 100 degrees. The regulator reduces this to about 0.5 PSI (11 inches water column) to feed the appliances.
https://www.propanecarbs.com/propane.html
If you take the tank pressure all the way to zero, you should have the tank purged to remove any air that may have infiltrated into it. Propane and air is an explosive combination so you only want pure propane in the tank.