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Engineering LibreTexts

18.3: Specific Gravity

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  • Specific gravity is defined as the ratio of fluid density to the density of a reference substance, both defined at the same pressure and temperature. These densities are usually defined at standard conditions (14.7 psia and 60°F). For a condensate, oil or a liquid, the reference substance is water:

    \[\gamma_{o}=\frac{\left(\rho_{0}\right)_{s c}}{\left(\rho_{w}\right)_{s c}} \label{18.3}\]

    The value of water density at standard conditions is 62.4 lbm/ft3 approximately. For a natural gas, or any other gas for this matter, the reference substance is air:

    \[\gamma_{g}=\frac{\left(\rho_{g}\right)_{s c}}{\left(\rho_{a i r}\right)_{s c}} \label{18.3a}\]

    Or, equivalently, substituting Equation (18.2) evaluated at standard conditions (\(Z_{s c} \approx 1\) for most gases),

    \[\gamma_{g}=\frac{M W_{g}}{M W_{a i r}} \label{18.3b}\]

    where the value of the molecular weight for air is \(MW_{air} = 28.96\, lbm/lbmol\). Specific gravity is nondimensional because both numerator and denominator have the same units.


    • Prof. Michael Adewumi (The Pennsylvania State University). Some or all of the content of this module was taken from Penn State's College of Earth and Mineral Sciences' OER Initiative.