API gravity
From Wikipedia, the free encyclopedia
API Gravity is a specific gravity scale developed by the American Petroleum Institute (API) for measuring the relative density of various petroleum liquids. API gravity is graduated in degrees on a hydrometer instrument and was designed so that most values would fall between 10 and 70 API gravity degrees.
Contents |
[edit] History of development
The U.S. National Bureau of Standards in 1916 established the Baumé scale (see degrees Baumé) as the standard for measuring specific gravity of liquids less dense than water. Investigation by the U.S. National Academy of Sciences found major errors in salinity and temperature controls that had caused serious variations in published values. Hydrometers in the U.S. had been manufactured and distributed widely with a modulus of 141.5 instead of the Baumé scale modulus of 140. The scale was so firmly established that by 1921 the remedy implemented by the American Petroleum Institute was to create the API Gravity scale recognizing the scale that was actually being used.
[edit] API gravity formulas
The formula used to obtain the API gravity of petroleum liquids is thus:
API gravity = (141.5/SG at 60 °F) - 131.5
Conversely, the specific gravity of petroleum liquids can be derived from the API gravity value as
SG at 60 °F = 141.5/(API gravity + 131.5)
60°F (or 15 5/9 °C) is used as the normal value for measurements and further tables give adjustments for temperature.
(See ASTM D1298)
Thus, a heavy oil with a specific gravity of 1.0 (i.e., with the same density as pure water at 60°F) would have an API gravity of:
(141.5/1.0) - 131.5 = 10.0 degrees API.
[edit] Classifications or grades
Generally speaking higher API gravity degree oil values have a greater commercial value and lower degree values have lower commercial value. This general rule only holds up to 45 degrees API gravity as beyond this value the molecular chains become shorter and less valuable to a refinery.
Crude oil is classified as light, medium or heavy, according to its measured API gravity.
Light crude oil is defined as having an API gravity higher than 31.1 °API
Medium oil is defined as having an API gravity between 22.3 °API and 31.1 °API
Heavy oil is defined as having an API gravity below 22.3 °API.
Oil which will not flow at normal temperatures or without dilution is named bitumen and the API gravity is generally less than 10 °API. Bitumen derived from the oil sands deposits in the Alberta, Canada area has an API gravity of around 8 °API. It is 'upgraded' to an API gravity of 31 °API to 33 °API and the upgraded oil is known as synthetic oil.
