Sally Goodson
Standards Associate
Standards
1220 L Street, NW
Washington, DC 20005-4070
USA
Telephone
202.682.8130
Fax
202.962.4797
Email
[email protected]
www.api.org
About API Gravity
Knowledge of density (relative density or specific gravity) has been with us since the days of
Archimedes in 250 BC, with the observation that light objects can float while heavier ones will sink in
water. In England in 1675, hydrometers were developed along scientific principles by Robert Boyle.
Later, Antoine Baumé, a French scientist, designed the Baumé Linear hydrometer scales for liquids both
lighter and heavier than water around 1798. His scales were based on salt and water mixtures.
Commercial versions of hydrometers called Jarvis Arnaboldi-C.J. Tagliabue -Baumé- type hydrometers
were widely used due to their uniformity and accuracy. It was discovered in 1881, however, that
differences in the moduli varied between 145.98 and 139.94 between the various manufacturers of these
hydrometers. All hydrometers measure relative density, but they vary in the units of measurement.
Baumé’s scale was the degree Baumé (°Bé), a unit still in common use today in certain industries.
Shortly after the formation of the American Petroleum Institute, API became involved in the study of
petroleum measurement problems. In December 1921, API, working with the Bureau of Mines and the
Bureau of Standards recommended that hydrometers should only have a scale based on the modulus of
141.5. Ever since then, such hydrometers have been known as API Hydrometers, and the official and
standard designation of the readings has been “degrees API.”
The relationship between API Gravity and Relative density is defined as follows:
141.5
API Gravity at 60°F = _______________________ - 131.5
Relative density 60/60°F