The
American Petroleum Institute
The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in production, refinement, distribution, and many other aspects of the ...
gravity, or API gravity, is a measure of how heavy or light a
petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
liquid is compared to water: if its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks.
API gravity is thus an inverse measure of a petroleum liquid's
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
relative to that of water (also known as
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
). It is used to compare densities of
petroleum liquids. For example, if one petroleum liquid is less dense than another, it has a greater API gravity. Although API gravity is mathematically a
dimensionless quantity (see the formula below), it is referred to as being in 'degrees'. API gravity is graduated in degrees on a
hydrometer instrument. API gravity values of most petroleum liquids fall between 10 and 70 degrees.
In 1916, the U.S.
National Bureau of Standards
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
accepted the
Baumé scale
The Baumé scale is a pair of hydrometer scales developed by French pharmacist Antoine Baumé in 1768 to measure density of various liquids. The unit of the Baumé scale has been notated variously as ''degrees Baumé'', ''B°'', ''Bé°'' and simp ...
, which had been developed in France in 1768, as the U.S. standard for measuring the
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
of liquids less dense than
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
. 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
The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in production, refinement, distribution, and many other aspects of the ...
was to create the API gravity scale, recognizing the scale that was actually being used.
API gravity formulas
The formula to calculate API gravity from
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
(SG) is:
:
Conversely, the specific gravity of petroleum liquids can be derived from their API gravity value as
:
Thus, a heavy oil with a specific gravity of 1.0 (i.e., with the same density as pure water at 60 °F) has an API gravity of:
:
Using API gravity to calculate barrels of crude oil per metric ton
In the oil industry, quantities of crude oil are often measured in metric
ton
Ton is the name of any one of several units of measure. It has a long history and has acquired several meanings and uses.
Mainly it describes units of weight. Confusion can arise because ''ton'' can mean
* the long ton, which is 2,240 pounds
...
s. One can calculate the approximate number of
barrels
A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, u ...
per metric ton for a given crude oil based on its API gravity:
:
For example, a metric ton of
West Texas Intermediate
West Texas Intermediate (WTI) is a grade or mix of crude oil; the term is also used to refer to the spot price, the futures price, or assessed price for that oil. In colloquial usage, WTI usually refers to the WTI Crude Oil futures contract tr ...
(39.6° API) has a volume of about 7.6 barrels.
Measurement of API gravity from its specific gravity
To derive the API gravity, the specific gravity (i.e., density relative to water) is first measured using either the
hydrometer, detailed in
ASTM D1298 or with the
oscillating U-tube {{no footnotes, date=March 2014
The oscillating U-tube is a technique to determine the density of liquids and gases based on an electronic measurement of the frequency of oscillation, from which the density value is calculated. This measuring p ...
method detailed in ASTM D4052.
Density adjustments at different temperatures, corrections for soda-lime glass expansion and contraction and
meniscus
Meniscus may refer to:
*Meniscus (anatomy), crescent-shaped fibrocartilaginous structure that partly divides a joint cavity
*Meniscus (liquid)
The meniscus (plural: ''menisci'', from the Greek for "crescent") is the curve in the upper surface ...
corrections for opaque oils are detailed in the
Petroleum Measurement Tables
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
, details of usage specified in ASTM D1250. The specific gravity is defined by the formula below.
:
With the formula presented in the previous section, the API gravity can be readily calculated. When converting oil density to specific gravity using the above definition, it is important to use the correct density of water, according to the standard conditions used when the measurement was made. The official density of water at 60 °F according to the 2008 edition of ASTM D1250 is 999.016 kg/m
3. The 1980 value is 999.012 kg/m
3. In some cases the standard conditions may be 15 °C (59 °F) and not 60 °F (15.56 °C), in which case a different value for the water density would be appropriate (''see''
standard conditions for temperature and pressure).
Direct measurement of API gravity (hydrometer method)
There are advantages to field testing and on-board conversion of measured volumes to volume correction. This method is detailed in ASTM D287.
Classifications or grades
![Lower 48 U](https://upload.wikimedia.org/wikipedia/commons/a/ae/Lower_48_U.S._states_production_of_crude_oil_by_API_gravity_in_January_2015_through_July_2019_%2848880018261%29.png)
Generally speaking, oil with an API gravity between 40 and 45° commands the highest prices. Above 45°, the molecular chains become shorter and less valuable to refineries.
Crude oil is classified as light, medium, or heavy according to its measured API gravity.
*
Light crude oil Light crude oil is liquid petroleum that has a low density and flows freely at room temperature. It has a low viscosity, low specific gravity and high API gravity due to the presence of a high proportion of light hydrocarbon fractions. It generally ...
has an API gravity higher than 31.1° (i.e., less than 870 kg/m
3)
* Medium oil has an API gravity between 22.3 and 31.1° (i.e., 870 to 920 kg/m
3)
*
Heavy crude oil
Heavy crude oil (or extra heavy crude oil) is highly-viscous oil that cannot easily flow from production wells under normal reservoir conditions.
It is referred to as "heavy" because its density or specific gravity is higher than that of light cru ...
has an API gravity below 22.3° (i.e., 920 to 1000 kg/m
3)
* Extra heavy oil has an API gravity below 10.0° (i.e., greater than 1000 kg/m
3)
However, not all parties use the same grading. The
United States Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
uses slightly different ranges.
Crude oil with API gravity less than 10° is referred to as
extra heavy oil
Heavy crude oil (or extra heavy crude oil) is highly-viscous oil that cannot easily flow from production wells under normal reservoir conditions.
It is referred to as "heavy" because its density or specific gravity is higher than that of light cr ...
or
bitumen. Bitumen derived from
oil sands
Oil sands, tar sands, crude bitumen, or bituminous sands, are a type of unconventional petroleum deposit. Oil sands are either loose sands or partially consolidated sandstone containing a naturally occurring mixture of sand, clay, and wate ...
deposits in Alberta, Canada, has an API gravity of around 8°. It can be diluted with lighter hydrocarbons to produce
diluted bitumen
Dilbit (diluted bitumen) is a bitumen diluted with one or more lighter petroleum products, typically natural-gas condensates such as naphtha. Diluting bitumen makes it much easier to transport, for example in pipelines. Per the Alberta Oil Sands B ...
, which has an API gravity of less than 22.3°, or further "upgraded" to an API gravity of 31 to 33° as
synthetic crude.
[Canadian Centre for Energy Information]
"What is crude oil?"
. Retrieved on: 2012-09-10.
References
{{Reflist
External links
Comments on API gravity adjustment scaleInstructions for using a glass hydrometer measured in API gravity
Units of density
Physical quantities
Petroleum geology
Petroleum production
Gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...