Fluid Properties – Definition of Specific Gravity (Relative Density) The ratio of the specific weight of a given liquid to the specific weight of pure water at a standard temperature (commonly 4°C, where water has maximum density) is called the specific gravity of the liquid.

Introduction:
Engineers frequently compare a liquid’s heaviness to that of water without carrying units. This comparison uses specific gravity, also called relative density, which simplifies quick estimates of buoyancy, manometer readings, and pump sizing.

Given Data / Assumptions:

• Specific weight gamma is weight per unit volume.
• Reference fluid is pure water at a standard temperature (commonly 4°C).
• Gravitational acceleration is the same for test liquid and reference water.

Concept / Approach:

Specific gravity SG is defined as SG = gamma_liquid / gamma_water (at reference temperature). Because both numerators and denominators carry the same units, SG is dimensionless and independent of unit systems. For density-based definitions, SG = rho_liquid / rho_water, which is equivalent since gamma = rho * g.

Step-by-Step Solution:

Verification / Alternative check:

Example: If an oil has gamma = 7.85 kN/m^3 and water has about 9.81 kN/m^3, then SG = 7.85 / 9.81 ≈ 0.80, a dimensionless number that immediately indicates buoyancy behavior (it will float on water).

Why Other Options Are Wrong:

Density of liquid: A property with units (kg/m^3), not a ratio to water. Compressibility: Describes volume change with pressure. Surface tension: An interfacial property (N/m). Relative viscosity: Ratio of viscosities, not specific weights.

Common Pitfalls:

Mixing up specific gravity with specific weight; using different temperature references for the liquid and water; reporting SG with units (it must be unitless).

Final Answer:

specific gravity of liquid