GetDryAirDensity Function

  function GetDryAirDensity(TDryBulb, Pressure) result(DryAirDensity)
    !+ Return dry-air density given dry-bulb temperature and pressure.
    !+ Reference:
    !+ ASHRAE Handbook - Fundamentals (2017) ch. 1
    !+ Notes:
    !+ Eqn 14 for the perfect gas relationship for dry air.
    !+ Eqn 1 for the universal gas constant.
    !+ The factor 144 in IP is for the conversion of Psi = lb in⁻² to lb ft⁻².

    real, intent(in)  ::  TDryBulb
      !+ Dry-bulb temperature in °F [IP] or °C [SI]
    real, intent(in)  ::  Pressure
      !+ Atmospheric pressure in Psi [IP] or Pa [SI]
    real              ::  DryAirDensity
      !+ Dry air density in lb ft⁻³ [IP] or kg m⁻³ [SI]

    if (isIP()) then
      DryAirDensity = (144 * Pressure) / R_DA_IP / GetTRankineFromTFahrenheit(TDryBulb)
    else
      DryAirDensity = Pressure / R_DA_SI / GetTKelvinFromTCelsius(TDryBulb)
    end if
  end function GetDryAirDensity