What is specific gravity and how is it used in fluid mechanics?

Specific gravity (SG) is the ratio of a fluid's density to the density of a reference substance (water at 4°C for liquids, air at standard conditions for gases). Being dimensionless, SG = ρ_fluid/ρ_reference. For water at 4°C, SG = 1.0. Applications include: calculating fluid density (ρ = SG × 1000 kg/m³), determining buoyancy forces, selecting appropriate materials for tanks and pipes, sizing pumps for different fluids, and converting between mass and volume flow rates. Common values: gasoline (0.72-0.78), crude oil (0.8-0.95), glycerin (1.26).