HOME

TheInfoList



OR:

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 p ...
gravity, or API gravity, is a measure of how heavy or light a petroleum 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 relative to that of water (also known as specific gravity). 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 A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically calibrated and graduated with one or more scales such as specific gravity. 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 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 simpl ...
, which had been developed in France in 1768, as the U.S. standard for measuring the 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 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 p ...
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 (SG) is: :\text = \frac - 131.5 Conversely, the specific gravity of petroleum liquids can be derived from their API gravity value as :\text~60^\circ\text = \frac 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: :\frac - 131.5 = 10.0^\circ


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 per metric ton for a given crude oil based on its API gravity: :\text = \frac 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 t ...
(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 A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically calibrated and graduated with one or more scales such as specific gravity. A hydrometer ...
, detailed in
ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
D1298 or with the oscillating U-tube 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), a curve in the upper surface of liquid contained in an object *Meniscus (optics) A lens is a tr ...
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. :\mbox = \frac 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/m3. The 1980 value is 999.012 kg/m3. 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 Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union o ...
).


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

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 has an API gravity higher than 31.1° (i.e., less than 870 kg/m3) * Medium oil has an API gravity between 22.3 and 31.1° (i.e., 870 to 920 kg/m3) *
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/m3) * Extra heavy oil has an API gravity below 10.0° (i.e., greater than 1000 kg/m3) However, not all parties use the same grading. The United States Geological Survey 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 cru ...
or
bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
. Bitumen derived from oil sands 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