Abstract: A spirometer is provided having an air flow path with a housing extending between the ends of a container. A paddle wheel rotor rotates within a cavity and partially cuts across the flow path to define an air gap. The ratio of the cross sectional area of the air gap to the inlet to the flow path is approximately 1:3. A light source/detector is activated by a chopper wheel coaxially mounted with the rotor and adapted to rotate therewith. The detector generates a train of pulses proportional to the rotation of the rotor which in turn is proportional to the volume of air flowing through the air path.