Please forgive me for posting a reply with a government link:
http://hazmat.dot.gov/opd_advisory.pdf But I believe the brass cylinder shown on the OPD valve picture is a float. If you tip your OPD fitted tank upside down the float sticks straight up and pulls the valve shut. I would not buy an off the shelf refill adapter for this reason. In the old days before OPD you could put your 20 lb. tank in the sun to warm it and your empty camping bottles in the freezer. With an off the shelf adapter you flip your old 20 lb. tank over and let liquid propane flow into the camping bottle. You have to depress the schrader style valve on the side of the camping bottle. Supposedly it takes less than a minute, which would make sense with the temperature difference and flowing liquid propane.
I have been meaning to look at the numbers to see if you can get enough of a temperature difference to condense propane vapor in a camping bottle. I think it would take liquid nitrogen which kinda defeats the purpose of refilling your bottles.
You can have a >40lb. tank without an OPD valve that would allow you to refill camping style tanks. Tipping >40lbs. of propane plus the tank upside down and the cost of the tank added to the cost of the refill has kept me from pursuing this option. According to DOT, you are not allowed to transport refilled camping style tanks .
Lastly, I am not sure refilling the small tanks would work repeatily, I wish I had more data, but we definitely had problems with the internal valves in the camping bottles after hard use. Previously when I was refering to flowing liquid propane over them... the female adapter opens the camping bottle valve. When you tip the bottle upside down the liquid propane flows right out, in this case into the black pipe. The liquid propane goes right through the main valve on the camping bottle. I am not sure if it is the solvent properties of liquid propane or gross temperature changes that affect this valve. But in order to refill them you be doing essentially the same thing.
I guess we accepted the cost of buying a new camping bottle for each performance. Well really we passed it on to Fire Fabulon... I have seen a six pack of bottles that drops the price to ~$1.50 each. They last quite a long time if you use it sparingly, this design was never intented to be something tactical, in that you could use it as a weapon. But you can probably get 20 good blasts out of tank.
I'll try to post some action pictures so you can get an idea of bang for the buck.
