A very interesting and informative article, but I’d like to add a couple things. Some 500 gallon propane tanks are fitted with what’s known as a “wet leg”. It is another valve situated on the top of the tank, in addition to the main valve. It’s plumbed to a pipe running to the bottom of the tank, with its purpose being refilling smaller tanks, like 20 lb portables. It requires a specially fitted hose, the shorter and larger diameter the better; 10 feet works well in 3/4” diameter. I mention a short length as disconnecting the hose from the bottle causes you to lose whatever propane was in the hose, and larger diameter to reduce the time required for filling. I purchased three used tanks from a propane supplier, with the specific requisite that at least one have a “wet leg”. It cost the same as the other used tanks. The hose, including already attached fittings cost $38. They are available; you just have to find them. Also, the “tare weight” of the bottle itself will be stamped on the valve guard/handle. Using a bathroom scale on a board, adding the tare weight to the desired amount of propane weight, 17 lbs, will give you an accurate stop point. – O. T.