Watch the threads on the caps and tank, I almost knocked myself out once with the cap off a 100# LP tank. We always have 3 or 4 extra 100# tanks at the cabin, and the easiest way to check if there is gas in the tank is to grab the cap with both hands and try to lift the tank, if its empty it lifts kind of easy, if its full its pretty heavy. Well we had a tank with the cap screwed all the way on, so I tried to lift it, just about the time I was at full grunt, the cap popped off and drilled into my forehead ( I saw stars ).

CHECK YOUR THREADS!!!!!