1. YOU ALWAYS OPEN THE 80% VALVE WHEN FILLING---NO EXCEPTIONS
2. If it is NOT opened when filling, you are violating the LP regulations for filling. The purpose of the 80% valve be opened while filling is NOT(NOT) to allow air to escape. There is NO air in a properly purged LP tank. ONLY LP VAPOR.
3. The REASON for the 80% valve to be opened is simple. IF you get liquid LP out of the 80% BEFORE the Auto shut off stops the fill, YOU STOP THE FILL REGARDLESS. The tank has reached its safe fill when Liquid comes out of the 80% valve. Auto Shut Offs are not that accurate at times.
4. YOU OPEN THE 80% ON PORTABLE DOT CYLINDERS ALSO FOR THE SAME REASON. AND shut off for the same reason. Doug