# 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.

## Contributors and Attributions

• 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.

This page titled 18.3: Specific Gravity is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Michael Adewumi (John A. Dutton: e-Education Institute) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.