A patient treatment system and method comprising a patient circuit, a pressurizing flow module disposed in the patient circuit, a humidifier disposed in the patient circuit, a monitor, sensors within the patient circuit, and a processor. 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. The processor determines the humidity level of the gas in the patient circuit downstream from the humidifier based on the signals generated by the sensors and the at least one parameter of the gas in the gas source.