Ultrasonic flowmeter for clean and dirty fluids

Claim 1

An ultrasonic flowmeter for measuring the flow rate of clean or dirty fluids, said flowmeter comprising: A. a pipe through which the fluid to be metered is conducted, said pipe being provided with upstream and downstream transducers; B. means including a reversing switch to alternately excite said transducers with a high-frequency transmission signal at a relatively low periodic switching rate whereby a beam emitted by the excited transducer is propagated through the fluid at angle to the flow axis of the pipe and is received by the other transducer to yield a received signal that is displaced in phase from the transmission signal to an extent depending on the transmit time of the beam through the fluid, the beam reversing direction during each operating cycle of the switch; C. means coupled to the transducers to convert the received signals yielded thereby into a square wave; D. means to compare the square wave with a reference wave derived from the transmission signal to produce a comparison wave; E. means to pass the comparison wave through a low-pass filter to exclude the high-frequency content thereof, the filter yielding a square wave in which the voltage drop between the upper level of the wave during a stable portion following an initial switching surge representing the transit time in one direction, and the lower level of the wave during a stable portion following an initial switching surge representing the transit time in the reverse direction reflects the time delay difference between these transit times; and F. means to derive a flow rate reading from said time delay difference, said means including a pair of sample and hold circuits which are alternately activated, whereby one circuit holds the voltage representing the high level of the filter square wave and the other the low level of the filter square wave, said circuits being operative only during the stable portions of said voltage levels.