Flame King Overfill Protection Valve helps prevent the 20LB propane tank overfills by closing when propane liquid level reaches to an 80% fill rate. The Float Gauge measures the level of propane inside the tank – it moves up and down with the liquid propane level. The gauge dial on the outside of the tank (sold separately) then displays the approximate percentage of propane remaining. Made of high quality premium materials for your safety – UL Listed.