This international Standard specifies requirements for five basic series of glass hydrometers of constant mass which indicate relative density 60/60 F° with reference to water and compiy with ISO 387. NOTE- The use of a scale other than one based on density ( mass per unit volume ) is not in general recommended, butin view of its importance in trade between various countries, the scale based on relative density with reference to water is accepted. The dimensions of the hydrometers have been chosen in such a way as to lead to convenience in use and economy in manufacture.