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==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, 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 web |title=Characterization and Properties of Petroleum Fractions |url=https://www.astm.org/mnl50-eb.html |access-date=2024-08-07 |website=www.astm.org |language=en |doi=10.1520/mnl50-eb}}</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 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 viscous or opaque fluids.<ref name=":3" /> Proper cleaning and handling of the hydrometer are crucial to maintain accuracy, and for volatile liquids, special precautions may be necessary to prevent evaporation during measurement.
==Classifications or grades==
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