GetDegreeOfSaturation Function

  function GetDegreeOfSaturation(TDryBulb, HumRatio, Pressure) result(DegreeOfSaturation)
    !+ Return the degree of saturation (i.e humidity ratio of the air / humidity ratio of the air at saturation
    !+ at the same temperature and pressure) given dry-bulb temperature, humidity ratio, and atmospheric pressure.
    !+ Reference:
    !+ ASHRAE Handbook - Fundamentals (2009) ch. 1 eqn 12
    !+ Notes:
    !+ This definition is absent from the 2017 Handbook. Using 2009 version instead.

    real, intent(in)  ::  TDryBulb
      !+ Dry-bulb temperature in °F [IP] or °C [SI]
    real, intent(in)  ::  HumRatio
      !+ Humidity ratio in lb_H₂O lb_Air⁻¹ [IP] or kg_H₂O kg_Air⁻¹ [SI]
    real, intent(in)  ::  Pressure
      !+ Atmospheric pressure in Psi [IP] or Pa [SI]
    real              ::  DegreeOfSaturation
      !+ Degree of saturation in arbitrary unit
    real              ::  BoundedHumRatio
      !+ Humidity ratio bounded to MIN_HUM_RATIO

    if (HumRatio < 0.0) then
      error stop "Error: humidity ratio is negative"
    end if
    BoundedHumRatio = max(HumRatio, MIN_HUM_RATIO)

    DegreeOfSaturation = BoundedHumRatio / GetSatHumRatio(TDryBulb, Pressure)
  end function GetDegreeOfSaturation