Liquid flow meter

A sonic flow meter for measuring the rate of flow of a fluid within an enclosed axially elongated hollow conduit, comprising:

• first and second transducer means secured to the exterior surface of said conduit;
• said first and second transducer means being spaced from each other along the axis of said conduit such that said first transducer means is located downstream from said second transducer means with respect to the direction of flow of said fluid;
• each of said transducer means being adapted to transmit to each other, and to receive from each other, sinusoidal sonic signals which pass through said fluid in said conduit;
• sonic signal transmission means for causing said first transducer means to transmit N sonic signals in an upstream direction from said first to said second transducer means and for causing said second transducer means to transmit N sonic signals in a downstream direction from said second to said first transducer means during each of a plurality of successive transmission cycles;
• and
• flow measurement circuit means for measuring the rate of flow of said fluid as a function of the difference in time it takes to transmit said sonic signals in said upstream and said downstream directions, said flow measurement circuit means determining said time difference by measuring said upstream and downstream transmission times with reference to a single zero cross-over point of each of said received sonic signals, the particular zero cross-over point referenced during any given transmission cycle being the same for each sonic signal received in that transmission cycle, the particular zero cross-over point referenced during each successive transmission cycle being different than the zero cross-over point referenced during a prior transmission cycle.