Ultrasonic fluid flow measuring method and apparatus for inferring flow speed relative to the phase shift between signals from the transducers

A fluid flow speed detection apparatus arranged to infer fluid flow speed through a passage comprising:

• at least one input receiver arranged to:
• receive a first signal having a frequency and phase corresponding to that of a detected first ultrasonic wave, the first ultrasonic wave having been detected after passing through the fluid in a direction with a component opposed or aligned to a main direction of the fluid's flow;
• and
• receive a second signal having a frequency and phase corresponding to that of a detected second ultrasonic wave, the second ultrasonic wave having been detected after passing through the fluid in a direction with a component different to that of the first ultrasonic wave with respect to the main direction of the fluid's flow,
• at least one amplifier arranged to convert the first and second signals to respective first and second square wave signals;
• a logic gate having a first input to which the first square wave signal is applied and a second input to which the second square wave signal is applied where the logic gate used gives an output signal indicative of the relative phase shift between the first and second signals;
• an integrator arranged to receive an output signal from an output of the logic gate and integrate over the output signal, the result of the integration thereby varying with the relative phase shift between the first and second signals;
• a processor arranged to infer the fluid flow speed through the passage in accordance with the relative phase shift between the first and second signals, and
• an output arranged to output a data signal indicative of the inferred fluid flow speed,
• wherein the frequencies of the first and second ultrasonic waves are substantially the same or are substantially harmonics of one another.