If you do it, use the filter and weigh it compared to and new 1lb. If the 20lb tank is full, it takes about 20 to 30 seconds to fill the 1 lb. As the level in the 20lb goes down, it takes longer. Use soap and water to test the seal after on the 1 lb. After a few uses (5-6) they will leak.There's lots of crap in most 20lb tanks. This said, buy yourself the hose for the Buddy heater and use a 20lb tank with it. Saves a lot of pissing around.