A system for monitoring inspirable gas for delivery to a patient includes a branched breathing circuit having a first branch conduit and a second branch conduit. The first branch conduit is fluidly connected to a first gas source and the second branch conduit is fluidly connected to a second gas source, and the first and second branch conduits merge into a patient delivery conduit. The system includes a first flow sensor in the first branch conduit, a second flow sensor in the second branch conduit, and a third flow sensor in the patient delivery conduit. A control unit is electrically coupled to the first, second and third flow sensors. The control unit is configured to determine a blend of gas in the patient delivery circuit based on a measured flow from the third flow sensor and a measured flow from at least one of the first and second flow sensors.