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What is an API scale? What is the difference between API gravity and density?
Density and derived quantities such as specific gravity and API gravity are used worldwide to characterize crude oils and to compare their quality. API gravity was defined as an adapted version of the Baumé scale in 1921 when a manufacturing error in the hydrometers caused many wrong measures, and finally the American Petroleum Institute (API) decided to adopt the incorrect scale as an official standard. The formula is the following:
And it differs to the specific gravity (SG) as it the ratio of the density relative to the water density:
How is crude oil classified based on the API gravity?
The oil industry generally classifies crude oil according to its API gravity. In the USA, API gravity is used to classify petrochemical products in light, medium and heavy oils and to compare their quality.
How do you calculate the API gravity of crude oil?
Nowadays, the majority of the oil industry uses hydrometers to measure the density although the method is very inaccurate and poorly reproducible. Alternatively, digital density meters offer accurate density determination of highly viscous samples or even samples that are solid at room temperature, following four basic standards:
- ASTM D 4052-96: Shows how to determine the density of petroleum oil distillates and viscous oils that are liquid at measurement temperatures of 15 to 35 °C and that have a vapor pressure of less than 80 kPa and a viscosity of less than 15,000 mm2/s (cSt).
- ASTM D 5002-99: Describes the determination of the density of crude oils that are liquid at 15 to 35 °C, and for crude oils that exhibit a high vapor pressure.
- DIN 51757 (1994-04): This standard method shows how to determine the density of crude oil, petroleum oil products and similar materials.
- DIN ISO EN 12185: This standard is suitable for liquids with any vapor pressure because is assumed that the loss of volatile components can be excluded prior to measurement.
In summary, this application note teaches you about different types of regulation requirements, as well as tips and tricks on how the measurement of oily substances with a METTLER TOLEDO digital density meter can be optimal. Learn how you can obtain good and repeatable results.
