The following properties are used to determine airflow rates and duct pressure losses
Density is corrected for air temperature, humidity and altitude.
Specific Heat Capacity
Specific heat capacity (Cp) is corrected for air temperature only within Fanfare. Cp reduces with reduced pressure, but within the range 0 to 2500m / 8000ft permissible within Fanfare may be considered to be independent of altitude.
The value of specific heat capacity depends on calculation/test method and the air condition used as the basis.
Within the UK (UK/Europe duct sizing options within Fanfare), a value of Cp is used which varies with temperature from approx 1.006 at 0°C to approx 1.05 at 40°C . At 20°C / 68°F, Cp is approx 1.018 KJ/kg K. (0.243 Btu/lb °F)
Within the US (USA traditional and USA current duct sizing options within Fanfare, a value of Cp is used which is almost constant value over normal range of temperatures. At 20°C / 68°F, Cp is approx 1.006 KJ/kg K (0.240 Btu/lb °F).
This leads to a 1% difference in calculated volume flowrates at 20°C / 68°F and 3 % difference at 40°C/104°F.
Dynamic viscosity is corrected for air temperature. It is usually considered to be independent of altitude, varying by no more than 2% in 5000m.