The screw the filler opened is an outage gauge. It is connected to a tube in the valve that goes down to the 85% level. Gas will come out at first and when liquid reaches the end of the tube a white fog will appear out of the small hole to the outside of the valve telling the filler that the tank is full. However, all 20# tanks that are legal to fill today have an auto stop fill feature that will stop the filling at 85% or less. Sort of an idiot proof system to prevent overfills which could cause the relief valve to open if the tank became liquid full and the pressure inside reached 250 psi. There is never air in a propane tank except when new. a new tank must be purged of air before filling. If air and gas were mixed it could explode although unlikely as propane's range of flammability is about 1.9 to 2.4%.
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