Glass hydrometer method for the laboratory determination, of the density or relative density of crude petroleum, petroleum products of mixtures of petroleum and non-petroleum products normally handled as liquids, and having a Reid vapour pressure of 1,8 bar (180 kPa) or less, determined according to ISO 3007. Particularly for mobile transparent liquids, viscous oils and opaque oils. Includes determination of API gravity and essential requirements of short hydrometers.