The density of dry air at any pressure and temperature follows the ideal gas law. In U.S. customary units, pressure must be in lbf/ft² (multiply psia by 144 to convert from lbf/in² to lbf/ft²). The specific gas constant for dry air is \(R_{air} = 53.34\) ft·lbf/(lbm·°R). This density is used in the isentropic choking relations to find the throat (sonic) density.

\begin{equation}\label{eq:orifice-method-ca-density} \rho_{ca} = \frac{P_{abs} \cdot 144}{R_{air} \cdot T_R} \end{equation}

Symbols

\(\rho_{ca}\)	Air density at supply (compressed) conditions \([\unit{ \pound\per\cubicFoot}]\)
\(P_{abs}\)	Absolute supply pressure \([\unit{ \psia}]\)
\(144\)	Unit conversion: square inches per square foot \([\unit{ \squareInch\per\squareFoot}]\)
\(R_{air}\)	Specific gas constant for dry air (53.34 ft·lbf per lbm·°R) \([\unit{ \foot\lbf\per\lbm\per\degreeRankine}]\)
\(T_R\)	Air temperature in degrees Rankine \([\unit{ \degreeRankine}]\)