Disclosed is a moving vane type air flow sensor designed particularly for use in measuring inhalation and exhalation flow rates, respiratory rates and capacity of human lungs. Included in the combination is a tubular conduit or air passage and two sets of swirling stator vanes for imparting a helical flow to breath passing therethrough. A flow sensitive rotor is journaled for rotation responsive to air flow through the sensor. A pair of light sources and a pair of photo sensitive elements are arranged to detect movement in the rotor. Disclosed also is circuitry for counting the number of revolutions of the rotor as an indication of flow through the sensor and logic circuitry for determining the direction of flow.