Annular gas seal clearances were simulated with closely spaced parallel plates using a Flat?Plate tester. The device is designed to measure the pressure gradient along the test specimen. The main function of the Flat?Plate tester is to provide friction factor data and measure dynamic pressure oscillations. A detailed description of the test facility is described, and a theory for determining the friction factor is reviewed. Three clearances were investigated: 0.635, 0.381, and 0.254 mm. Tests were conducted at three different inlet pressures (84, 70, and 55 bars), producing Reynolds numbers range from 50,000 to 700,000. Three surface configurations were tested including smooth?on-smooth, smooth?on?hole, and hole?on?hole. The Hole?pattern plates are identical with the exception of the hole depth. The results indicate that, for the smooth?on?smooth and smooth?on?hole configurations, the friction factor remains constant or increases slightly with increasing Reynolds numbers. Moreover, the friction factor increases as the clearance between the plates increases. However, the results from the hole?on-hole configurations are quite different. A "friction?factor jump" phenomenon was observed, and the Helmholtz frequency was detected on the frequency spectra.