Prandtl number for gas

The Prandtl number is a dimensionless number, defined as the ratio of kinematic viscosity (also called momentum diffusivity) to thermal diffusivity.

Equation for humid air is taken from paper by P.T. Tsilingiris: 'Thermophysical and transport properties of humid air at temperature range between 0 and 100$^{\circ}$C', 2007

$4.969218948 \cdot 10^{-9} \theta_{air}^{4} - 4.162785412 \cdot 10^{-7} \theta_{air}^{3} + 2.240599044 \cdot 10^{-5} \theta_{air}^{2} - 0.0003703124976 \theta_{air} + 0.7215798365$humid air @ 1 atm (Tsilingiris2007)
$\frac{c_{p_{gas}} \eta_{gas}}{k_{gas}}$general formula for gases