Direct readout gives instant gallons usable on a calibrated scale. It's easy to use, simply insert slowly until FuelHawk touches the bottom of the tank. Then place your finger firmly over the top of the opening to lock fuel in the tube. Remove and instantly see a readout in gallons of usable fuel on a calibrated scale.Specially designed for a Cessna 152 with standard 13 gallon tanks (12 gallons usable per side).