Patent ID: 7299150

Claim:
An ultrasonic flow meter for measuring a flow rate of a fluid flowing through a conduit by detecting a transit time difference between a forward propagating time of an ultrasonic wave propagating within the conduit in a forward direction from an upstream side to a downstream side and a backward propagating time of an ultrasonic wave propagating within the conduit in a backward direction from a downstream side to an upstream side comprising: a first ultrasonic vibrating unit provided on an upstream side of the conduit for transmitting an ultrasonic wave propagating in the forward direction and receiving an ultrasonic wave propagating in the backward direction to generate a backward ultrasonic wave signal; a second ultrasonic vibrating unit provided on a downstream side of the conduit for transmitting an ultrasonic wave propagating in the backward direction and receiving the ultrasonic wave transmitted from said first ultrasonic vibrating unit and propagating in the forward direction to generate a forward ultrasonic wave signal; an A/D converter for sampling and converting said backward and forward ultrasonic wave signals generated from said first and second ultrasonic vibrating units, respectively into backward and forward digital data series x and y, respectively; first and second memory units for storing said backward and forward digital data series x and y, respectively; a control unit for reading said backward and forward digital data series x and y out of said first and second memory units, respectively, while data positions of these backward and forward digital data series x and y are relatively shifted; and a calculating unit for deriving total sums of absolute difference values between the backward and forward digital data series x and y read out of said first and second memory units, and for detecting a shift amount of data positions at which a total sum of absolute difference values becomes minimum, wherein an ultrasonic propagating time difference is derived in accordance with the thus detected shift amount of data positions, a flow speed of the fluid within the conduit is derived from the thus derived ultrasonic wave propagating time difference, and a flow rate of the fluid flowing through the conduit is derived in accordance with the thus derived flow speed and a known cross sectional area of the conduit.