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 = ρsubstance / ρH2O
The reference density of water at 4oC (39oF) 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