Flow Velocity Measurement by Cross-correlation with Tailored Modulation

For measuring a flow velocity, a cross-correlation method may be used to determine the transit time between two points in a flow system. Cross-correlation is a powerful mathematical tool used in many flow meters. Since it is principally based on the time delay measurement, it imposes very few restrictions on the transducers. Various sensing techniques have been employed in flow meters involving in cross-correlation. The bandwidth of the transducers to acquire fluctuation signals is a key factor, which affects the measurement accuracy. However, most of the transducers suitable for industrial use have a limited bandwidth, performing like a low-pass spatial filter. To extend their bandwidth, the axial length of the sensing electrodes in the transducers should be as short as possible. On the other hand, the transducers need a moderately large area or length to attain an acceptable signal level. Therefore, there is a compromise in designing the electrodes for cross-correlation flow meters. In this paper, a differential capacitance transducer is proposed for the use in flow measurement by cross-correlation. In contrast to the conventional capacitance transducers, the frequency response of the differential capacitance transducer performs like a band-pass filter. As a result, it could achieve a bandwidth of 1.8 times of a conventional capacitance transducer with same axial length of electrodes. Also it offers a valuable characteristic of no standing capacitance, which enables it to take full advantage of the measuring circuit.