When flow is choked, the throat conditions follow the isentropic relations for an ideal gas. The critical density ratio relates stagnation density (≈ supply density for low approach velocity) to throat density via the ratio of specific heats \(\gamma\). For dry air, \(\gamma = 1.4\).

\begin{equation}\label{eq:orifice-method-sonic-density} \rho^* = \rho_{ca} \cdot \left(\frac{2}{\gamma + 1}\right)^{\frac{1}{\gamma - 1}} \end{equation}

Symbols

\(\rho^*\)	Air density at the isentropic sonic throat \([\unit{ \pound\per\cubicFoot}]\)
\(\rho_{ca}\)	Air density at supply conditions \([\unit{ \pound\per\cubicFoot}]\)
\(\gamma\)	Ratio of specific heats for dry air (1.4) \([\unit{ \unitless}]\)