Abstract: An air flow measurement apparatus for verifying one or more apertures in a test piece 126 by mounting the test piece in an air flow such that air flows through the aperture(s) therein. The apparatus comprises a servo pilot valve 114 for adjusting the air flow from the source 104, a pressure sensor P3A for measuring the flow and a computer for determining the effective area of the test piece 126 at a range of different pressures across the test piece, the range including a desired pressure differential. The computer continuously tracks the readings and, when sufficient data has been obtained, computes a curve fit to calculate the effective area of the test piece 126 at the exact specified pressure differential. The present invention avoids any problems associated with differences in atmospheric pressure as well as any problems that are associated with obtaining the exact desired pressure differential across the test piece 126.