Abstract: Apparatus for detection leakage of hydrocarbon-containing liquid, such as gasoline, from a generally cylindrical tank. The apparatus includes an elongate displacer having a vertical axis and extending substantially the full height of the liquid in the tank. The displacer has an upper end and a lower end with the displacer lower end terminating closely adjacent but spaced from the bottom of the tank. The displacer is made of a material having a coefficient of linear expansion of less than 20.times.10.sup.-6 inches per inch per degree centigrade. Furthermore, the displacer is substantially symmetrical about about any plane through its vertical axis and it has an outer surface which is a surface of revolution. The displacer has a varying diameter along its length corresponding to the horizontal surface area of the tank at the same height inside of the tank. The apparatus further includes a sensor for providing an indication of the weight of the displacer disposed in the liquid.