Petroleum property correlation: API gravity (°API) is related to specific gravity G (at 15.5 °C / 60 °F) by which standard formula?

Introduction / Context:API gravity is a widely used measure of petroleum liquids, inversely related to density. Converting between specific gravity and °API is routine in refining and trading. The formula is standardized at 60 °F (15.5 °C).

Given Data / Assumptions:

• Specific gravity G referenced to water at 60 °F (15.5 °C).
• Standard petroleum convention.
• No temperature correction needed beyond this reference.

Concept / Approach:The established relationship is °API = (141.5 / G) − 131.5, yielding higher °API for lighter crudes (lower G). This transformation linearizes comparisons and is central to custody transfer specifications.

Step-by-Step Solution:Recall standard: °API = (141.5 / G) − 131.5.Check limiting cases: G = 1 → °API = 10 (freshwater reference).Lighter oils (G < 1) give °API > 10, heavier give less.Select option (b).

Verification / Alternative check:Industry handbooks and ASTM standards list the same expression, confirming its correctness.

Why Other Options Are Wrong:Other formulas are nonstandard and produce incorrect values at the reference point.

Common Pitfalls:

• Using density in kg/m^3 directly without converting to specific gravity at 60 °F.
• Applying the formula at temperatures far from 60 °F without correction factors.

Final Answer:°API = (141.5 / G) − 131.5