specific gravity (relative density) - for petroleum products, the ratio of the mass of a given volume of product and the mass of an equal volume of water, at the same temperature. The standard reference temperature is 15.6°C (60°F). Specific gravity is determined by test method ASTM D 1298: the higher the specific gravity, the heavier the product. Specific gravity of a liquid can be determined by means of a hydrometer, a graduated float weighted at one end, which provides a direct reading of specific gravity, depending on the depth to which it sinks in the liquid. A related measurement is density, an absolute unit defined as mass per unit volume - usually expressed as kilograms per cubic meter (kg/m3). Petroleum products may also be de fined in terms of API gravity (also determinable by ASTM D 1298), the API gravity value, the lighter the material, or the lower its specific gravity.