Specific gravity is defined as the ratio of a fluid's density to the density of which reference?

• A. The ratio of the density of a fluid to the density of air
• B. The ratio of density of a fluid to the density of reference fluid
• C. The ratio of the density of a fluid to the density of water
• D. The ratio of viscosity to density

Answer: (B)

Specific gravity is a dimensionless ratio that compares a fluid’s density to the density of a reference fluid. It is defined as SG = density of the fluid / density of the reference fluid. The reference is chosen to fit the substance: water is the common reference for liquids, while air is used for gases. The key idea is not about viscosity or any fixed substance but about comparing densities to a reference density. That’s why stating that specific gravity is the ratio to the density of a reference fluid is the correct description. The other options mix in air, water as the universal reference, or introduce viscosity, which aren’t the defining elements of specific gravity.