An apparatus for the measurement of the speed of sound based on the pulse-echo technique is presented. It operates up to a temperature of 480 K and a pressure of 125 MPa. After referencing and validating the apparatus with water, it is applied to liquid ammonia between 230 and 410 K up to a pressure of 124 MPa. Speed of sound data are presented with an uncertainty between 0.02% and 0.1%.

Equations of state in terms of the Helmholtz energy are presented for octamethyltrisiloxane and decamethyltetrasiloxane. The restricted databases in the literature are augmented by speed of sound measurements, which are carried out by a pulse-echo method. The equations of state are valid in the fluid region up to approximately 600 K and 130 MPa and can be used to calculate all thermodynamic properties by combining the Helmholtz energy and its derivatives with respect to the natural variables. The accuracy of the equation is validated by comparison to experimental data and correct extrapolation behavior is ensured.

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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.

Vinyl chloride is one of the world's most important industrially synthesized substances, but due to its physico-chemical nature comparably little is known about its thermodynamic behavior. Accurate density and thermodynamic speed of sound data of vinyl chloride in the liquid state are measured along nine isotherms, covering the temperature range from 283 K to 362 K up to a pressure of 91 MPa. Data are presented with a maximum expanded uncertainty (k = 2) of 0.15% for the density and 0.16% for the speed of sound. They are compared with all available literature sources and a preliminary equation of state. Present density data are in good agreement with the literature data and have a maximum deviation of 1.5% from the equation of state. However, no experimental speed of sound data are available in the literature for comparison and the equation of state diverges up to -12.4% from the present data. keywords: Density, speed of sound, densimeter, pulse-echo technique, vinyl chloride.