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Title: A Novel Ultrasonic Flowmeter For Low Flowrates In Small Tubes
Author: m. Sanderson, R. Al-Rabeh
Source: Flomeko 2005
Year Published: 2005
Abstract: 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.

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