Fluid flow monitoring

A fluid flow monitor comprising transmitting means for transmitting acoustic signals in opposite first and second directions through a fluid with components in the direction of flow;

• means for receiving said transmitted signals;
• means for providing a first signal representing acoustic velocity;
• means for providing a second signal representing fluid flow velocity;
• integrating means for integrating said first signal with respect to time to produce a ramp signal providing a changing control function the rate of change of which represents acoustic velocity in the fluid;
• trigger means to cause said transmitting means to transmit a first acoustic signal in said first direction through said fluid and to transmit a second acoustic signal in said second opposite direction through said fluid and to initiate said integrating means;
• first comparison means for comparing said ramp signal with said second signal representing fluid flow velocity to generate a first and a second arrival signal according to the direction of said transmitted acoustic signal;
• second comparison means including storage means for comparing expected arrival times for said transmitted acoustic signals, represented by the times at which said first and second arrival signals respectively reach a first threshold, with the actual arrival times of said transmitted acoustic signals, and for storing the results of said comparisons;
• and third comparison means for comparing said stored results from two successive acoustic signal transmissions either to modify said first signal representing acoustic velocity if both said acoustic signals arrived earlier or later than their respective expected arrival times, or to modify said second signal representing fluid flow velocity if one acoustic signal arrived before and the other after their expected arrival times.