When filling my 20# bottles, I have been told that filling above 80% is now prohibited by law.

I've done a little looking around on the web, and it appears that the DOT and NFPA folks have conspired somewhat to require all new bottles in the consumer range to be fitted with overflow protection devices, or OPDs. These are used to prevent "overfilling".

However, I can not find any code requirement or description of what "overfilling" means.

Can anybody point to a document on the web that defines what overfilling might be, or is this just a nebulous concept? Who defines what overfilling is?