ASTM D1298 Density, Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method – Please see our description for ASTM D4052 for a discussion on density, specific gravity and API gravity.
A hydrometer is a calibrated, weighted glass bulb with a cylindrical stem. When the hydrometer is placed in a liquid sample, the bulb and part of the stem become submerged. The stem is marked with a graduated scale that is read at the submersion line. Hydrometers vary in weight and markings – different hydrometers are used for API gravity than for density (although the value from one can be used to calculate the other). The sample is placed into the test cylinder and brought to the test temperature. The appropriate hydrometer is gently placed in the sample, and allowed to equilibrate. The scale on the hydrometer is read and the density (in g/ml), relative density (unitless number) or API gravity (OAPI) is determined and reported.