Speed-of-Sound Measurements in Liquid n-Pentane and Isopentane

Abstract

Comprehensive and accurate measurements of the speed of sound in liquid n-pentane and isopentane were carried with a double-path-length pulse-echo instrument. The measurements cover the temperature ranges between 200 and 420 K for n-pentane and between 220 and 420 K for isopentane with pressures up to 100 MPa. The expanded uncertainties (at the 0.95 confidence level) are 2.1 mK for temperature, 0.005% for pressure, between 0.01% at low pressure and 0.008% at high pressure for the speed of sound in n-pentane, and between 0.014% at low pressure and 0.01% at high pressure for the speed of sound in isopentane. At high temperatures and low pressure, the uncertainty in the speed of sound in n-pentane increases up to 0.026% at 420 K, while for isopentane it reaches 0.04%. In isopentane, strong damping of the sound signals was observed at temperatures below 260 K, which indicated dispersion of the sound waves in the frequency range of our measurements near 8 MHz. Measured speeds of sound in this temperature range were corrected for dispersion effects to the thermodynamic speed of sound at zero frequency by using information on the sound attenuation in isopentane from the literature. Due to the high damping at low temperatures, the uncertainty in the speed of sound in isopentane amounts to 0.022% at some state points on the isotherm 220 K.

Speeds of Sound in n-Pentane at Temperatures from 233.50 to 473.15 K at Pressures up to 390 MPa

“…However, the equation of Thol et al 6 is at least much closer to the experimental data than the one of Span and Wagner. 5 Figures 6 and 7 also show relative deviations of literature data from the equation of state of Thol et al 6 As mentioned above, the data set of El Hawary et al 9 was also used for the development of this equation of state and agreed best with the data reported here. At temperatures between 273.21 and 353.21 K, the data of El Hawary et al agree to within ±0.015% with our data measured at RUB, while a systematically offset of 0.05% can be observed at 253.26 K. The data of El Hawary et al also differs by up to 0.06% at lower temperatures and up to 0.17% at temperatures near 420.15 K from our data measured at ICL.…”

“…However, the equation of Thol et al 6 is at least much closer to the experimental data than the one of Span and Wagner. 5 Figure 6 and 7 also show relative deviations of literature data from the equation of state of Thol et al 6 As mentioned above, the data set of El Hawary et al 9…”

“…Even though several speed of sound data sets are available in literature, some of the data sets differ significantly from each other and, thus, complicate the development of a reliable equation of state. The present study was initiated with the objective to increase the amount and scope of the available data for use in the development of the equation of state by Thol et al 6 El Hawary et al 9 completed their experimental data set at Helmut-Schmidt-University (HSU) Hamburg simultaneously but completely independent from our present measurements. The apparatuses at RUB and HSU are both designed for reliable speed of sound measurements even at low pressures and can be used to clarify the data situation by with an apparatus at ICL, optimised for high-pressure and high-temperature measurements, which extends the ranges of pressure and temperature up to 390 MPa and 473.15 K, respectively.…”

“…A fundamental equation of state for n-pentane was developed by Span and Wagner 5 and is currently being replaced by a new fundamental equation of state, currently under development at Ruhr University Bochum by Thol et al 6 as already implemented in REFPROP 10.0 7 and TREND 4.0. 8 Among other thermodynamic properties, the speed of sound data sets of El Hawary et al 9 (liquid phase), Ewing et al 10 (gas phase), and Chavez et al 11 (saturation line) were considered as primary data for the development of this equation, as well as the data investigated at RUB within the scope of the present work. The distribution of these data is presented in Figure 1, together with the state points investigated in this work at ICL as well as further data from literature.…”

“…A fundamental equation of state for n -pentane was developed by Span and Wagner and is currently being replaced by a new fundamental equation of state, currently under development at Ruhr University Bochum by Thol et al as already implemented in REFPROP 10.0 and TREND 4.0 . Among other thermodynamic properties, the speed of sound data sets of El Hawary et al (liquid phase), Ewing et al . (gas phase), and Chávez et al .…”