Density, Specific weight /volume/gravity

DENSITY (Kg/m^3  or N•sec²/m^4)

Density is defined as mass per unit volume. Mass is a property.

Density can be expressed as

ρ = m / V

SPECIFIC VOLUME (m^3/kg  or m^4/ N•sec²)

Specific volume on the other hand is the reciprocal of density.

Specific volume  = 1/ density

SPECIFIC WEIGHT (N/m³ or lb/ft³ or kg/m²•sec²)

Specific Weight is defined as weight per unit volume. Weight is a force.

γ = ρ g        

SPECIFIC GRAVITY /RELATIVE DENSITY (Dimensionless)

For liquids defined as the ratio of the density of a substance to the density of water - at a specified temperature.

SG = ρ_substance / ρ_H2O 

The reference density of water at 4^oC (39^oF) is used as the reference as these are the conditions of maximum density.

For gases

SG = Mass of gas / Mass of air = Mass of gas / 28.97