The Correct Answer and Explanation is:
The correct answer is B. API gravity.
API gravity is a specialized scale used primarily in the petroleum industry to measure the relative density of crude oil and other liquid petroleum products. The term directly references the American Petroleum Institute (API), which, in collaboration with the U.S. National Bureau of Standards (now the National Institute of Standards and Technology, or NIST), established this standard in 1921. This historical partnership directly aligns with the details provided in the question.
The scale is considered “arbitrary” because it is not a direct measurement of density but is instead derived from the specific gravity of the liquid. The formula used is: API gravity = (141.5 / Specific Gravity at 60°F) – 131.5. This formula creates an inverse relationship between API gravity and density. Consequently, a higher API gravity value indicates a lighter, less dense petroleum liquid, while a lower API gravity value signifies a heavier, denser liquid. For instance, water has an API gravity of 10, while crude oils with an API gravity greater than 10 are lighter than water and will float.
This measurement is critically important for both technical and commercial purposes. It helps classify crude oils, with “light” crudes (high API gravity) generally being more valuable than “heavy” crudes (low API gravity) because they typically yield a greater percentage of high-value products like gasoline and diesel fuel during the refining process.
The other options are incorrect. A grade number usually relates to a product’s quality or viscosity, like motor oil grades. Hydrometer units refer to the readings from a general instrument used to measure liquid density, not the specific petroleum scale. The cetane number is a measure of the ignition quality of diesel fuel, which is unrelated to its gravity or density.
