P = rRT

P_h=r_hRT

(P_h)/(r_hRT_h)=(P₁)/(r₁RT₁)

(P₁)/(r₁RT₁)=(P₂)/(r₂RT₂)  

P = Pressure

r = Gas Density

R = Gas Constant 

      = 53.34 ft-lbs/lbm-°R

T = Temperature

Equation may be used if it is assumed that the gas is thermally perfect.