A breathing assistance apparatus capable of operating in alternating inhalation and expiratory phases includes a pressurized respiratory gas source, an inhalation duct to supply gas from the pressurized respiratory gas source to a patient, and an expiratory duct for expiratory gas of the patient. The apparatus further includes a set of flow rate sensors positioned to sense a first flow rate of the expiratory duct and a second flow rate of the inhalation duct, and a controller, including a processor, configured to control the gas source. The inhalation duct includes an inhalation valve, the inhalation valve configured to provide proportional operation and the inhalation valve is selectively controlled by the controller to move to a specific position to allow a desired level of flow through the inhalation valve to the patient.