Synonym: Relative Density = Specific gravity
Dimensionless ratio of the actual fluid density \rho to the fluid density of a reference substance \rho_{\rm ref}, both taken at reference conditions (equal or close to STP, which should be specified individually):
(1) | SG= \frac{\rho}{\rho_{\rm ref}} |
The selection of reference substance and the corresponding value of \rho_{\rm ref} depends on the context.
For fluids which are liquid at STP this is usually a distilled water and for fluids which are gaseous at STP this is usually a dry air:
\rho_{\rm ref} = \rho_{\rm H_2O} | \rho_{\rm ref} = \rho_{\rm air} |
\displaystyle \gamma_L= \frac{\rho_L}{\rho_{H_2O}} | \displaystyle \gamma_g= \frac{\rho_g}{\rho_{\rm air}} |
See also
Natural Science / Physics / Fluid density
[ API gravity ][ Oil specific gravity (γo) ][ Gas specific gravity (γg) ][ Liquid specific gravity (γL) ]