Patent ID: 8671775

Claim:
A flow rate measuring device, comprising: a pair of transducers each of which is provided at a fluid path and transmits and receives an ultrasonic signal; a transmission means which drives the transducers; a switching means which switches transmission/reception between the transducers; an amplifying means which amplifies a received signal from the transducer; a reference comparison means which compares an output from the amplifying means with a reference voltage; a reference voltage setting means which sets the reference voltage to be compared by the reference comparison means; a determination means which determines an arrival timing of the ultrasonic signal based on an output from the reference comparison means and the output from the amplifying means; a clock means which measures a propagation time of the ultrasonic signal from the transducer on a transmission side to the transducer on a reception side; and a calculation means which calculates at least one of a flow speed and a flow rate based on the time calculated by the clock means, wherein the reference setting means changes the reference voltage in accordance with the flow speed or the flow rate calculated by the calculation means.