Digital speed determination in ultrasonic flow measurements

A method of estimating an arrival time at a transducer of an acoustic wave packet transmitted at a predetermined frequency, said method comprising the steps of:

• (a) transmitting an acoustic wave packet at a predetermined frequency;
• (b) detecting at a transducer the acoustic wave packet and producing an analogue signal;
• (c) converting an analogue signal output from said transducer into digital signal data;
• (d) determining a plurality of waveform periods from said digital signal data including a waveform period determined for each of a number of different segments of said analogue signal;
• (e) averaging the plurality of waveform periods for obtaining an average waveform period, and
• (f) using the average waveform period and a corresponding number of waveform periods between a determined reference point and a point marking a start of the acoustic wave packet to estimate an arrival time of the acoustic wave packet by subtracting from an arrival time of the determined reference point the corresponding number of waveform periods multiplied by the average waveform period.

A system for measuring a speed of a moving fluid, said system comprising:

• a measurement tube through which the fluid passes;
• two ultrasonic transducers disposed in said tube and configured for transmitting ultrasonic signals between each other;
• exciting means, coupled to at least a first one of said transducers, for exciting at least said first one of said transducers to generate an acoustic wave packet that propagates at a predetermined frequency along said tube toward a second one of said transducers;
• detecting means connected to at least said second one of said transducers for detecting the acoustic wave packet,
• converting means, coupled to said detecting means, for converting at a sampling frequency an analogue signal output from at least said second one of said transducers to digital signal data,
• a first storage means for storing said digital signal data from said converting means;
• processor means connected to said first storage means for determining the speed of the fluid, said processor means comprising:
• first determining means for determining a measurement portion of said digital signal data, said measurement portion corresponding to a response of said second one of said transducers to said acoustic wave packet;
• second determining means for determining from said measurement portion a plurality of slopes Si for a corresponding plurality i of measurement segments of said response, each of said segments being about a signal level value V;
• third determining means for determining from at least selected ones of said slopes Si, an arrival time t of said acoustic wave packet.