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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 STP:

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

The value of $\begin{array}{l}\rho_{\rm ref}\end{array}$ depends on context:

Liquids	Gases
$\begin{array}{l}\rho_{\rm ref} = \rho_{\rm H_2O}\end{array}$	$\begin{array}{l}\rho_{\rm ref} = \rho_{\rm air}\end{array}$

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Specific gravity

See also