The most accurate method to measure gas in a LP tank is by weight. This is because gas expands. When the gas expands it can give a false reading to the magnetic strips you place on the side of the tank. To effectively measure your gas level by weight you can use a standard bathroom scale or a LP tank scale. 1. Search the tank for the letters "TW." This stands for tare weight, or the weight of the tank when there is no LP Gas or propane within it. The letters will be followed by a number, "TW 18." This means the tank weighs 18 pounds when the tank is empty.
Every pound over the TW number represents a pound of propane remaining in the tank.
Remember a good tool, a 20lb propane tank lasts for about 1 lb per hr on HIGH.