Two safe methods to fill cylinders, according to the US "bible" for propane, NFPA 58 (link below):
1.
Fill by volume. Open the bleeder valve; fill until it spews; immediatly close the bleeder valve.
2.
Fill by weight. Set scale to reflect (a) capacity of cylinder in pounds + (b) tare weight of cylinder + (c) weight of filling apparatus.
Note that under NO circumstances are fillers permitted under these codes to fill based on the behavior of the OPD valve.
Of course, some fillers do both method (1) and method (2) at the same time. Kind of overkill, given that they amount to the same thing. Nonetheless, sometimes a filler will run into a curious situation regarding some cylinder or another; better safe than sorry.
NFPA standards are not national; they are offered by the codes association, and states are free to accept fully, reject, modify, whatever. Some states may, for instance, prefer or even require one filling method over the other. A common reason is that filling by volume, number (1) above, inevitably spews some propane into the air.
Lynn
http://www.nfpa.org/codes-and-standa...detail?code=58