Check if a Gas Grill Tank Is Full

Hungry dinner guests will not forget that you forgot to get gas to cook their meat. Sushi, anyone?

Steps

Weigh empty tank. Write number on tank with permanent marker. [The empty or "tare weight" (TW) should also be stamped on the tank collar.]
Fill tank and weigh it again. Write on tank with permanent marker.
When you want to know how much gas is left in tank, weigh the tank and compare the numbers.
Propane weighs 4.24 pounds per gallon. Each pound contains about 21,500 BTUs which translates to 91,500 BTUs per gallon. A standard '20 pound' tank will be filled to about 17 pounds or a little more. Thus, a 'full' tank contains roughly 365,000 BTUs of energy.

Pour a cup of hot water over the side of the tank.
Then slowly feel down the side of the tank with your hand.
Near the top it will feel hot, as you go down the tank it will get cold.
At the point the tank changes temperature, that is the level of the gas in the tank.

Under certain conditions, condensation will appear on the outside of the tank up to the level of gas inside the tank. If you keep tank in a cool garage, and bring it into humid weather, the empty space in the tank will warm up, but the cooler gas in the bottom of the tank will allow condensation on the outside.