A system (10) for measuring subject flow in a respiratory therapy device. The system may include a primary pressure generator (12) connected to a subject circuit (18), and a secondary pressure generator (30) connected to a sensor circuit (36). The sensor circuit and subject circuit may be in fluid communication such that gas form the sensor circuit is introduced into the subject circuit. A measurement of flow parameters of gas through the sensor circuit may convey information related to flow parameters of gas in the subject circuit. The system may be configured such that contaminates from subject flow in the subject circuit do not flow into the sensor circuit.
