There are various types of flowmeter used in gas flow measurements, including differential pressure meters [1], turbine meters [2-3], and positive displacement meters [4]. These common flowmeters are all well characterized and readily available. However, they may cause an unsatisfactory obstruction to the gas flow and an associated pressure drop in the system. Ultrasonic flowmeters [5] are another type of flowmeter that use the interaction of acoustic waves with the moving fluid to measure the fluid flowrate. Ultrasonic flowmeters have many advantages over traditional techniques: they offer little or no obstruction to fluid flow, have a fast response, and may produce a negligible pressure drop in the system. There are three basic ultrasonic techniques. The first operates using Doppler shift [5] and hence relies on the frequency variations between the transmitted and received signals. The second uses cross-correlation to provide an estimate of the time for a particular disturbance in the flow to travel between two ultrasonic beams a known distance apart [6-7]. The third technique makes use of the ultrasonic time-of-flight between transducers using paths upstream and downstream in the fluid flow and such devices are known as transit time ultrasonic flowmeters [5,8]. Ultrasonic flowmeters in general can be used either as clamp-on or wetted [6,9], where sensors are fitted inside the pipe in contact with the fluid, but do not intrude significantly into the flow path. This usually provides increased acoustic signal strength, because no signal attenuation occurs through the pipe wall. Piezoelectric transducers are available for use in gas flowmeters under different conditions [10,12] but there is usually a large mismatch between the acoustic impedance of piezoelectric materials and that of gases. This means that they are not very efficient and the use of impedance matched layers is necessary, which has been successfully implemented and showed good results [13]. However the bandwidth of such devices is usually limited and the matching layers may be difficult to manufacture. An alternative is the capacitive ultrasonic transducer (CUT) [14-15] which is more suitable for generating and receiving ultrasonic waves in gases, where the vibrating transducer surface is a very thin membrane with an impedance more closely matched to that of air and other gases. By exploiting the advantages of these devices in fluid flow measurement, capacitive ultrasonic transducers have been successfully applied to gas flow measurement [16][21]. The objectives of this work are to investigate the application of high frequency capacitive transducers in transit time ultrasonic flowmeters for measuring gas flowrates in different meter configurations using the same set of ultrasonic transducers for time-of-flight and frequency analysis.
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