Editing API gravity (section)

==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.<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" />