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{{Short description|Measure of how heavy or light a petroleum liquid is compared to water}}
The [[American Petroleum Institute]] 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.
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==API gravity formulas==
The formula to calculate API gravity from [[
:<math>\text{API gravity} = \frac{141.5}{\text{SG}} - 131.5</math>
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Conversely, the specific gravity of petroleum liquids can be derived from their API gravity value as
:<math>\text{SG
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:
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==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 [[tonne|ton]]s. One can calculate the approximate number of [[
:<math>\text{barrels of crude oil per metric ton} = \frac{\text{API gravity}+131.5}{141.5\times 0.159}</math>
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:<math>\mbox{SG oil} = \frac{\rho_\text{crudeoil}}{\rho_{\text{H}_2\text{O}}}</math>
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<sup>3</sup>.<ref>Report on the Development, Construction, Calculation and Preparation of the ASTM – IP Petroleum Measurement Tables (ASTM D1250; IP 200), 1960, published by the Institute of Petroleum, 61 New Cavendish Street, London, W.1 and American Society for Testing Materials, 1916 Race Street, Philadelphia 3, Pa.</ref> The 1980 value is 999.012 kg/m<sup>3</sup>.<ref>API Manual of Petroleum Measurement Standards, Chapter 11.1 – 1980, Volume XI/XII, Adjunct to: ASTM D1250-80 and IP 200/80</ref> 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
There are advantages to field testing and on-board conversion of measured volumes to volume correction. This method is detailed in ASTM D287.<ref name=":0">{{Cite web |title=Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer/Method) |url=https://www.astm.org/d0287-22.html |access-date=2024-08-07 |website=www.astm.org |language=en}}</ref>
The hydrometer method is a standard technique for directly measuring API gravity of petroleum and petroleum products.<ref name=":1">{{Cite web |title=Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method |url=https://www.astm.org/d1298-12br17e01.html |access-date=2024-08-07 |website=www.astm.org |language=en}}</ref> This method is based on the principle of [[buoyancy]] and utilizes a specially calibrated [[hydrometer]] to determine the API gravity of a liquid sample.<ref name=":2">{{Cite book |last=Speight |first=James G. |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9781118986370 |title=Handbook of Petroleum Product Analysis |date=2014-12-12 |publisher=Wiley |isbn=978-1-118-36926-5 |edition=1 |language=en |doi=10.1002/9781118986370}}</ref>
The procedure typically involves the following steps:
# '''Sample preparation''': The petroleum sample is brought to a [[Standard temperature and pressure|standard temperature]], usually 60°F (15.6°C), to ensure consistency in measurements across different samples and conditions.<ref name=":0" />
# '''Hydrometer selection''': An appropriate API gravity hydrometer is chosen based on the expected range of the sample. These hydrometers are typically calibrated to read API gravity directly.<ref name=":3">{{Cite book |title=Characterization and Properties of Petroleum Fractions |url=https://www.astm.org/mnl50-eb.html |access-date=2024-08-07 |website=www.astm.org |date=2007 |language=en |doi=10.1520/mnl50-eb |isbn=978-0-8031-3361-7 |editor-last1=Riazi |editor-first1=M. R. }}</ref>
# '''Measurement''': The hydrometer is gently lowered into the sample contained in a cylindrical vessel. It is allowed to float freely until it reaches equilibrium.<ref name=":0" />
# '''Reading''': The API gravity is read at the point where the surface of the liquid intersects the hydrometer scale. For maximum accuracy, the reading is taken at the bottom of the [[Meniscus (liquid)|meniscus]] formed by the liquid on the hydrometer stem.<ref name=":1" />
# '''Temperature''' '''correction''': If the measurement is not performed at the standard temperature, a correction factor is applied to adjust the reading to the equivalent value at 60°F.<ref name=":2" />
The hydrometer method is widely used due to its simplicity and low cost. However, it requires a relatively large sample volume and may not be suitable for highly [[Viscosity|viscous]] or [[Opacity|opaque]] fluids.<ref name=":3" /> Proper cleaning and handling of the hydrometer are crucial to maintain accuracy, and for [[Volatile organic compound|volatile]] liquids, special precautions may be necessary to prevent evaporation during measurement.<ref name=":0" />
==Classifications or grades==
[[File:Lower 48 U.S. states production of crude oil by API gravity in January 2015 through July 2019 (48880018261).png|thumb|upright=1.4|API gravity of crude oil produced in the [[
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.<ref>{{cite web|title=Comments on Crude Oil Gravity Adjustments |url=http://dnr.louisiana.gov/sec/execdiv/techasmt/oil_gas/crude_oil_gravity/comments_1989.htm |archive-url=https://web.archive.org/web/20060502160519/http://dnr.louisiana.gov/sec/execdiv/techasmt/oil_gas/crude_oil_gravity/comments_1989.htm |url-status=dead |archive-date=2 May 2006 |publisher=Louisiana Department of Natural Resources |access-date=6 January 2014 }}</ref>
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