Abstract: An improved apparatus and method for measuring fluid flow in a conduit enables the transmission of ultrasonic signals through the fluid with low signal attenuation and unique phase determination. Ultrasonic burst signals are simultaneously transmitted to axially spaced receivers adjacent a conduit. The transmitted signal is made up of a signal burst having two different frequency signals. The signal frequencies are selected to prevent unwanted signal attenuation by entrapped gases in the fluid and to allow an unambiguous phase determination for the received signal burst.

Abstract: Facing each other diagonally across a pipe for flowing fluid is a pair of like, transmitter-receiver units operating with signals of supersonic frequency. The units are in a joint circuit and periodically are simultaneously energized to transmit identical signals to each other in opposite directions in high frequency bursts, each burst being of a chosen number of cycles and being shorter than the transmission time between the units. The bursts, received between successive transmissions, are phase shifted with respect to each other, resulting from one burst being speeded up and the other burst being slowed down in accordance with the velocity of the liquid traversed. The received bursts, in sine wave form, are heterodyned with a cosine wave form modulating signal to produce a sine wave resulting signal dependent upon phase difference between the bursts and at a low frequency.