Patent ID: 7270015

Claim:
A flow meter for measuring the rate of flow of a fluid characterized by an acoustic property that varies with temperature, the flow meter comprising: a thermal input means, operable when the fluid is flowing along a flow direction, to provide a bolus of the fluid at a first selected location, the bolus having a temperature different from the rest of the fluid so that the acoustic property of the bolus is different from the acoustic property of the rest of the fluid, the thermal input means further operable to provide a thermal pulse timing signal to a logic circuit for calculating the flow rate; and at least one acoustic portion, each comprising at least one respective ultrasonic transducer and a respective receiver circuit, each acoustic portion operable, when the fluid is flowing: to define at least one respective acoustic beam extending transverse to the flow direction at a respective selected location downstream from the first selected location; and to output from the respective receiver circuit a bolus detection signal responsive to the difference in the acoustic property associated with passage of the bolus to the logic circuit.