Skip to main content
Engineering LibreTexts

18.4: Specific Gravity

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:

Contact your instructor if you are unable to see or interpret this graphic.(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:

Contact your instructor if you are unable to see or interpret this graphic.(18.3a)

Or, equivalently, substituting Equation (18.2) evaluated at standard conditions (Contact your instructor if you are unable to see or interpret this graphic. for most gases),

Contact your instructor if you are unable to see or interpret this graphic.(18.3b)

where the value of the molecular weight for air is MWair = 28.96 lbm/lbmol. Specific gravity is nondimensional because both numerator and denominator have the same units.

Contributors

  • 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.