Abstract: A peak expiratory flow rate measuring apparatus (PFM) having at least one measurement channel and at least one bypass flow channel including at least one entrance, at least one exit, at least an inner wall and an outer wall, and wherein at least a portion of said at least one bypass flow channel has a variable cross-sectional area, is provided herein. Further disclosed is a PFM having at least one measurement channel comprising at least one first flow entrance, at least one bypass flow channel comprising at least one second flow entrance, and at least one guide wall, having an axially forward portion adjacent to the second flow entrance and an axially aft portion adjacent to the first flow entrance is provided.