The sources are:

• Values for 0, 15, and 25°C are taken from encyclopedia.airliquide.com
• Formula 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°C', 2007
• Formula for air is taken from paper by A. Dumas and M. Trancossi: 'Design of Exchangers Based on Heat Pipes for Hot Exhaust Thermal Flux, with the Capability of Thermal Shocks Absorption and Low Level Energy Recovery', 2009
• Formula 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°C', 2007
• Formula for air is taken from paper by A. Dumas and M. Trancossi: 'Design of Exchangers Based on Heat Pipes for Hot Exhaust Thermal Flux, with the Capability of Thermal Shocks Absorption and Low Level Energy Recovery', 2009. They are calculated from polynomial curve fits to a data set for 100 K to 1600 K in the SFPE Handbook of Fire Protection Engineering, 2nd Edition Table B-2. You may find a free air property calculator from Pierre Bouteloup
• Formulae for N2 and SF6 are taken from paper by J. Vermeer: 'A simple formula for the calculation of the convective heat transfer between conductor and sheath in compressed gas insulated (CGI) cables', 1983 as published in Elektra 87
• The factor 353.0 in the formula for Air was calculated by multiplying 1.1840·(273.15+25).
• The factor 538.9 in the formula for CO2 was calculated by multiplying 1.8075·(273.15+25).

Symbol
$\rho_{\mathrm{gas}}$
Unit
kg/m³
Formulae
 $- 5.2536065 \cdot 10^{-7} \theta_{\mathrm{air}}^{3} + 3.860201577 \cdot 10^{-5} \theta_{\mathrm{air}}^{2} - 0.005538444326 \theta_{\mathrm{air}} + 1.293393662$ humid air at 1 atm (Tsilingiris2007) $\frac{360.77819}{T_{air}^{1.00336}}$ air at 1 atm (Dumas&Trancossi2009) $\frac{351.99}{T_{air}} + \frac{344.84}{T_{air}^{2}}$ air at 1 atm (UW/MHTL 8406, 1984 $\frac{353.0 p_{\mathrm{gas}}}{T_{\mathrm{gas}}}$ air (considered as ideal gas) $\frac{337.4 p_{\mathrm{gas}}}{T_{\mathrm{gas}}}$ N2 (Vermeer1983) $\frac{1666 p_{\mathrm{gas}}}{\theta_{\mathrm{gas}} + 230}$ SF6 (Vermeer1983) $\frac{538.9 p_{\mathrm{gas}}}{T_{\mathrm{gas}}}$ CO2 (considered as ideal gas) $\frac{p_{\mathrm{gas}}}{R_{\mathrm{gas}} T_{\mathrm{gas}}}$ ideal gas
Related
$p_{\mathrm{gas}}$
$R_{\mathrm{gas}}$
$T_{\mathrm{gas}}$
$\theta_{\mathrm{air}}$
$\theta_{\mathrm{gas}}$
Used in
$\alpha_{\mathrm{gas}}$
$\eta_{\mathrm{gas}}$
$F_{\mathrm{pt}}$
$\nu_{\mathrm{gas}}$
$p_{\mathrm{gas}}$
$T_{\mathrm{conv_{\mathrm{ce}}}}$
$V_{\mathrm{m}}$
Molar volume [m³/mol]
Choices
GasFormula0°C15°C25°C
Air-1.2921.2251.184
N2N21.25011.18481.145
SF6SF66.61616.25636.0383
CO2CO21.97631.87141.8075
COCO1.25021.18491.145
O2O21.42871.3541.3085
H2H20.08990.08520.0823
NH3NH30.77130.72890.7033
SO2SO22.93052.76332.6636
HeHe0.17840.16920.1635
ArAr1.78351.69031.6335
KrKr3.7483.55143.4314
XeXe5.89655.5845.3937