Synonym: Relative Density = Specific gravity

Dimensionless ratio of the actual fluid density $\begin{array}{l}\rho\end{array}$ to the fluid density of a reference substance $\begin{array}{l}\rho_{\rm ref}\end{array}$ , both taken at reference conditions (equal or close to STP, which should be specified individually):

(1)	$\begin{array}{l}\displaystyle SG= \frac{\rho}{\rho_{\rm ref}}\end{array}$

The selection of reference substance and the corresponding value of $\begin{array}{l}\rho_{\rm ref}\end{array}$ 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:

Liquid specific gravity $\begin{array}{l}\gamma_L\end{array}$	Gas specific gravity $\begin{array}{l}\gamma_g\end{array}$
$\begin{array}{l}\rho_{\rm ref} = \rho_{\rm H_2O}\end{array}$	$\begin{array}{l}\rho_{\rm ref} = \rho_{\rm air}\end{array}$
$\begin{array}{l}\displaystyle \gamma_L= \frac{\rho_L}{\rho_{H_2O}}\end{array}$	$\begin{array}{l}\displaystyle \gamma_g= \frac{\rho_g}{\rho_{\rm air}}\end{array}$

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See also