A patient treatment system (10) and method comprising a patient circuit (12), a pressurizing flow module (16) disposed in the patient circuit, a humidifier (18) disposed in the patient circuit, a monitor (50), sensors (22) within the patient circuit, and a processor (28). The patient circuit delivers a flow of gas from a gas source to an airway of a patient. The pressurizing flow module elevates the pressure of the gas within the patient circuit. The humidifier elevates the humidity of the gas in the patient circuit to a circuit humidity level. The monitor monitors at least one parameter of the gas from the gas source. The sensors generate corresponding signals that can be processed to estimate a rate at which vapor is added to the gas in the patient circuit.
