# 18.3: Specific Gravity

• • Vice Provost Emeritus for Global Program & Professor Emeritus (Petroleum and Natural Gas Engineering) at Pennsylvania State University
• Sourced from John A. Dutton: e-Education Institute

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.