Ultrasound speed and absorption study in near-critical CO2: A sensor for high-pressure application

This paper reveals an ultrasonic sensor assembly for accurate determination of the speed of sound and sound attenuation (acoustic energy losses) in dense fluids. It was constructed by means of commercially available 200 kHz emitting/receiving piezoelectric transducers equipped with a special acoustic matching layer. The construction allows changing the distance between transducers without dismantling a high-pressure vessel. This experimental technique makes it possible to determine the sound absorption coefficient, indispensable information for the acoustic determination of gas composition and other properties related to the sound absorption coefficient. The assembly was successfully applied for measuring the speed of sound and sound amplitude in CO2 between 298 and 343 K and 6–14 MPa. The measurements of sound amplitude resulted in accurate location of the critical point of CO2 because in the vicinity of the critical point a decrease of sound signal amplitude is far more significant than a decrease of sound speed.