The "1-gallon" tanks for, say a Weber Q-Grill, with a hose to connect them are a worthwhile investment, over the hassle and expense of buying 1-lb "green" bottles of propane.
I'm too embarrassed to admit why I had to figure this out, but I thought I had a tank with a defective overfill protection device (OPD), until by chance I got a tech to fill the tank who was a certified fill technician instructor.
First, the density of propane is 4.4 lbs/gal, and that of water is 8.32 lbs/gal.
Second the stamping on the cylinder "TW 11 lbs" means tare weight (actually 11.1 by digital scale).
Thirdly, the stamping on the cylinder "WC 12 lbs" means water capacity (lbs), NOT weight capacity of propane
Fourth, a cylinder or tank (the latter, found in motorhomes, is not DOT regulated) may not be filled in excess of 80 percent. On a motorhome propane tank, the 100% mark on the gauge is at 80% of the capacity as stated on the manufacturer's label welded to the tank.
So, the tech checks a chart for fill volume, or you can make the calculation:
12 lb water capacity x 4.4/8.32 x 0.80 = 5.0 lbs
So the gross weight of a full (80%) cylinder is ~16.1 lbs.
Guess what: If you can't get anymore propane in a tank that weighs about 16 lbs, the OPD is not stuck; it's just full
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