Specific gravity (SG) is a fundamental physical property that provides a standardized way to characterize petroleum liquids. This measurement is a comparison, establishing how heavy or light a specific volume of oil is compared to an equal volume of water. It is a defining characteristic used to assess the composition and quality of everything from raw crude oil to finished products like gasoline and diesel fuel. Understanding this dimensionless number is crucial for operations, from reservoir evaluation to global commerce.
Understanding Specific Gravity
Specific gravity is defined as the ratio of a substance’s density to the density of a reference material. For liquids, the standard reference is water, typically measured at a specific temperature. The petroleum industry commonly uses the density of water at 60 degrees Fahrenheit (15.6 degrees Celsius) as the denominator for this ratio. This comparison results in a dimensionless quantity, expressed as a simple number. A specific gravity of 0.85 means the oil is 85% as dense as water at the reference temperature. Since most oils are lighter than water, their specific gravity is generally less than 1.0.
Measuring Specific Gravity of Oil
Determining the specific gravity of petroleum products relies on established measurement methods. The most common technique uses a glass hydrometer, which operates on the principle of buoyancy, floating at a level corresponding to the liquid’s relative density. For accurate measurement, the oil sample must be brought to a standard temperature, typically 60°F, because density changes significantly with temperature. The American Society for Testing and Materials (ASTM) provides standards like D1298 for performing this test, though modern methods like digital density meters (ASTM D4052) offer greater precision for opaque or viscous samples. The resulting specific gravity value is always corrected to the 60°F reference point using standardized tables.
Specific Gravity and Oil Classification
The specific gravity of oil varies widely depending on its molecular composition and is the basis for its commercial classification. The industry standard for categorizing crude oil is the American Petroleum Institute (API) Gravity, which is directly derived from specific gravity. API Gravity is inversely related to specific gravity: a lower specific gravity (lighter oil) results in a higher API number.
Crude oil is classified into three main categories based on API Gravity. Light crude oil has an API gravity greater than 30 degrees, which corresponds to a specific gravity less than approximately 0.876. Medium crude oil falls between 22 and 30 degrees API, and heavy crude is defined as having an API gravity below 22 degrees (SG > 0.922). The API scale is designed so that water, with a specific gravity of 1.0, has an API gravity of 10 degrees.
This classification indicates the oil’s value and refining complexity. Lighter crude with a higher API gravity is generally more desirable because it contains a greater proportion of shorter-chain hydrocarbons that are easily refined into high-value products like gasoline. Refined products like gasoline typically have a specific gravity around 0.72 to 0.76, corresponding to an API gravity of over 50 degrees.
Why Specific Gravity Matters
Specific gravity affects the economics, logistics, and safety of the oil industry. In commerce, oil is often bought and sold by volume, but its true value is linked to its mass, which specific gravity helps determine for accurate invoicing and taxation. In transportation, specific gravity influences the design and operation of pipelines and pumps, as a higher SG indicates a denser, more viscous liquid requiring more energy to pump. For safety and environmental planning, specific gravity is a factor in predicting oil spill behavior; since nearly all crude oil has an SG less than 1.0, it floats, which is a consideration for cleanup efforts. The measure is also integral to refinery operations, guiding separation processes and determining the yield of various petroleum products.