Ultrasonic thickness measurement with two-phase flows requires a liquid/gas interface in order to capture useful information. Thus, when the pipe is filled with either pure liquid or pure vapor, the ultrasonic sensing means is no longer capable of capturing the flow dynamics. To obtain useful measurements in both single- and two-phase flow conditions of cryogenic propellants (for NASA), Foster-Miller developed an instrument based on dynamic pressure measurements. Despite the fact that this required pipe penetrations, this proved to be a highly advantageous approach for flow velocities in the range of 5 cm/sec up to 17 m/sec with current equipment. In addition to providing accurate measurements with liquid-gas flows, it provided unexpectedly good accuracy with single-phase liquid flows:  the flow totalizing error was only 0.66 percent of full scale.

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