Specific Gravity of Gas

Specific gravity of gas, abbreviated as \(SG_g\), a dimensionless number, is a measure of its density relative to the density of a reference gas.  It is typically defined as the ratio of the density of the gas to the density of a reference gas under specific conditions.  The reference gas is often chosen to be dry air or hydrogen.

Specific Gravity of Gas Formula
\( SG_g \;=\;  \rho_g \;/\; \rho_a \)     (Specific Gravity of Gas) \( \rho_g \;=\;  SG_g \; \rho_a  \) \( \rho_a \;=\;  \rho_g \;/\; SG_g \)
Symbol	English	Metric
\( SG_g \) = Specific Gravity of Gas (air = 1.0)	\(dimensionless\)	\(dimensionless\)
\( \rho_g \) = Gas Density	\(lbm\;/\;ft^3\)	\(kg\;/\;m^3\)
\( \rho_a \) = Air Density	\(lbm\;/\;ft^3\)	\(kg\;/\;m^3\)

The density of the reference gas (such as dry air or hydrogen) can be looked up or calculated based on its molar mass and other properties.  For example, if the specific gravity of a gas is 0.7, it means that the gas is 0.7 times as dense as the reference gas under the same conditions.

Specific gravity is commonly used in various engineering and scientific applications, especially in the oil and gas industry, where it helps in characterizing the properties of gases and determining their behavior in different environments.