All propane cylinders will have a TW stamped in the valve protection ring. The tare weight or TW is the weight of the cylinder empty. TW 18 would be a 18 pound empty cylinder. If you weigh a TW 18...
A quick way to check if a ship is empty or not is to look at the waterline to see how much the ship draws. Many large ships have "marks" on the hull to indicate the depth to the keel. The higher a...