Development of sweep frequency ultrasonic interferometer for high precision velocity measurement in liquids

An ultrasonic interferometer with variable separation between the transducer and reflector is widely used for the measurement of ultrasonic propagation velocity in liquids. The inherent limitation of such an interferometer is due to the mechanical movement of its reflector for ultrasonic wavelength measurement in a liquid medium. It is observed that the ultrasonic velocity measurement precision is adversely affected at higher frequencies compared to lower ones. For instance, in our experimentation, a standard deviation of +/- 21.5 m/s (+/- 1.43%) was obtained for velocity measurement at 1.84 MHz with the consideration of two consecutive maxima, which increases drastically to +/- 76.8 m/s (+/- 5.12%) at 9.4 MHz. These measurements can significantly be improved by considering many maxima and averaging for wavelength estimation. However, it still requires design attention and improvement, particularly for higher frequencies. In this article, a sweep-frequency based ultrasonic interferometer design with a fixed separation for liquid characterization is proposed and described. This technique overcomes the limitations of mechanical movement systems and also provides a better and uniform precision for lower as well as higher frequencies. The functionality of the developed sweep frequency method was tested in water, carbon tetrachloride, ethylene glycol, and glycerol, which shows good agreement with literature values. The velocity measurement in double distilled water by the developed technique at 1 Hz sweep resolution has shown an improved standard deviation of +/- 0.74 m/s (+/- 0.05%) at 9.4 MHz.