The measurement of the low flowrates of liquids in small tubes is difficult using conventional transit time ultrasonic techniques for two reasons. Firstly, if a diametrical beam is employed then the defining equation for the transit time difference between the upstream and downstream directions has a constant of proportionality which includes the diameter of the tube. Thus as the tube diameter becomes smaller the transit time difference for a given velocity becomes proportionally smaller. This is usually overcome by employing multiple reflections or more commonly an axial flowmeter in which the length over which the transit time difference is measured is an axial length which is de-coupled from the dimension of the diameter. These axial flowmeters can be configured as a 'U' tube. However it has been shown that the effective length of the flowmeter changes with flow velocity and this has to be compensated for in the output of the flowmeter. The second reason for the difficulty in the measurement is that as the flowrate reduces the transit time differences also become more difficult to measure. Time differences of the order of 10ns are usually measured using a digital measurement technique. This often requires multiple measurements of the time difference with a consequent reduction in the response time of the flowmeter. This paper describes a novel flowmeter configuration and time measurement technique which eases these problems. In the flowmeter configuration it will be shown that the integration length for the transit time difference is exactly the same as the geometric distance between the sensors for axially symmetric flow profiles and that there is no change in sensitivity as the flow profile changes from laminar to turbulent. The novel signal processing scheme employed in the flowmeter is one in which the transit time difference is measured by differencing the received signals from two reciprocally driven transducers. It is shown that the amplitude difference is proportional to the transit time difference for small values of the transit time difference. Referencing the difference amplitude to the amplitude of one of the receive signals enables effects of attenuation or variation between transducers to be compensated. Velocity of sound effects can be accounted for in the usual way for a transit time ultrasonic flowmeter giving rise to flowmeter whose output only depends on the velocity of the flow. The method has the advantage of being able to provide the measurement without a significant amount of averaging and thus provide a fast speed of response of less than 100ms. In a small flow tube in which the flow goes from laminar to turbulent over the measurement range it will be shown that the flowmeter has a accuracy of better than +-1percent over a turndown range of 10 to 1.
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