Abstract: A system for measuring air flow rate in a duct includes an air flow sensor and an air flow conditioning device for collecting and accelerating a portion of the total air stream to the sensor, which may be means as known in the art. The conditioning device includes two opposed converging walls defining a nozzle, preferably a venturi, the sensor being disposed in the throat of the nozzle. The surfaces of the venturi walls are textured to trip the boundary layer near the wall surface into turbulence to maintain attachment of flowing air to the walls even when the angle of attack of the air is significantly non-axial. The wall texturing may be random or may be an organized pattern. A further embodiment includes a second flow conditioner/sensor in parallel with the first whereby an averaged flow measurement is taken for a more accurate reading.