Burst design and signal processing for the speed of sound measurement of fluids with the pulse-echo technique

Abstract

The pulse-echo technique determines the propagation time of acoustic wave bursts in a fluid over a known propagation distance. It is limited by the signal quality of the received echoes of the acoustic wave bursts, which degrades with decreasing density of the fluid due to acoustic impedance and attenuation effects. Signal sampling is significantly improved in this work by burst design and signal processing such that a wider range of thermodynamic states can be investigated. Applying a Fourier transformation based digital filter on acoustic wave signals increases their signal-to-noise ratio and enhances their time and amplitude resolutions, improving the overall measurement accuracy. In addition, burst design leads to technical advantages for determining the propagation time due to the associated conditioning of the echo. It is shown that the according operation procedure enlarges the measuring range of the pulse-echo technique for supercritical argon and nitrogen at 300 K down to 5 MPa, where it was limited to around 20 MPa before.

Speeds of Sound in Methanol at Temperatures from 233.33 to 353.21 K at Pressures up to 20 MPa

“…The waveform of the two resulting echoes is averaged from 16 consecutive pulses and a bandpass filter based on a Fast Fourier Transform with a bandwidth of 1.6 MHz is applied, to enhance the signal-to-noise ratio. Based on these processed data, the time difference ∆t echo is computed according to the algorithm described by Dubberke et al [65].…”

Speeds of Sound in n-Hexane and n-Heptane at Temperatures from (233.33 to 353.21) K and Pressures up to 20 MPa

“…The averaged waveform is treated with a bandpass filter, being based on a Fast-Fourier-Transform with a bandwidth of 1.6 MHz, to enhance the signal-to-noise ratio. Ultimately, the speed of sound c is determined by the following equation where the time difference ∆t echo between the two echoes is computed from the averaged waveform according to an algorithm by Dubberke et al [169]. Equation 1includes the time difference between the ideal and the real case of the wave propagation.…”

Thermodynamic speed of sound of xenon

“…where a = 219711. clean signal with a high resolution was prepared for analysis. [35] The delay in time of flight ∆t was derived from a peak-to-peak measurement approach, [35] which leads to the time difference between the maximum amplitudes of both echoes. Moreover, a correlation method and Hilbert transform function [35] were used too, to select or verify the correct maximum peaks.…”

“…For xenon, these effects were significant at low pressures, the second echo became wider than the first one and the maximum amplitude within the echo was reached later in the second echo than in the first. This phenomenon was suppressed by applying a brake function as described by Dubberke et al [35]. Xenon was supplied in a gaseous state at the ambient temperature with a pressure of almost 5.3 MPa, which is near its critical region [2].…”