General Specifications for Bellwether ASTM Hydrometers: Manufactured in strict accordance to specifications of ASTM (American Society for Testing and Materials) for accuracy and design. All instruments are suitable for certification for use as a primary reference standard with NIST traceability.

Specific Gravity, also known as relative density, is the ratio of the mass of a solid or liquid to the mass of an equal volume of distilled water. The standard temperature of calibration in the U .S. is 60°F/ 60°F.