Pipeline Interface Detector was initially developed for pipeline service as an outgrowth of an oceanographic in-strument which was used to measure sound velocity and other physical properties in deep-sea research. Engineers then adopted this technology and produced the first sonic interface detector for service in products pipelines in 1970. Pipeline interface detectors operate by precisely measuring the velocity at which ultrasonic pulses travel over a liq-uid path of known dimension. Sound velocity is a unique physical property of materials, as are density and viscosity.