Mechanical seals generally fail as the result of the collapse of the lubricating film between the faces. In the present research, a system that detects the collapse of the lubricating film in a gas seal has been developed. The system uses an active ultrasonic transmission technique to monitor the fluid film. Sensors are placed behind each stator of a double mechanical seal. A series of ultrasonic sinusoidal wave packets is generated by one transducer while the second transducer monitors the signal transmitted through the seal interface. A weak transmitted signal indicates a continuous fluid film, while a strong transmitted signal indicates contact between the seal faces. The transmitted signal is relatively independent of the seal characteristics and operating conditions, and can be easily distinguished from machinery noise since its frequency and amplitude can be controlled.Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999