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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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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
# '''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==
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