Experiments to identify the leakage and dynamic force coefficients of a plain annular gas (air) seal are presented. The test rig consists of a rigid shaft and a seal housing supported from elastic rods. Two orthogonal impact guns excite the test seal and housing, and displacement and acceleration sensors measure the seal dynamic forced response. A frequency domain parameter identification technique allows accurate estimations of the system stiffness and damping force coefficients. Tests with no journal rotation and at 3600 rpm are conducted for pressure ratios (Ps/Pa) from 1.5 to 3.0 for centered and eccentric (50 percent of clearance) seal conditions. In all cases, measured mass flow rates and identified direct damping coefficients steadily increase with increasing pressure ratios. The direct stiffnesses, however, exhibit a peculiar behavior at and around a pressure ratio of 2.0. As the pressure ratio increases from 1.5 to 2.0, the direct stiffnesses decrease and become negative. Then, as the pressure ratio rises to 3.0, the stiffnesses steadily increase and become positive. A bulk-flow model, which includes the effect of transition flow from laminar to turbulent, demonstrates reasonable agreement with the experimental observations.Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998