Accurate measurements of the sound velocity in pure water by combining a coherent phase-detection technique and a variable path-length interferometer

An automated system for measuring the sound velocity in liquids has been developed by combining a coherent phase-detection technique and a variable path-length interferometer, in which a sample liquid is filled between an ultrasonic buffer and a moving reflector, and phase changes of a given echo of the ultrasonic pulse is measured by a phase-sensitive detector (PSD) as a function of changes in the acoustic path length. For the accurate determination of the changes in the acoustic path length, the displacement of the moving reflector is measured by a Michelson interferometer that is illuminated by a frequency-stabilized He–Ne laser. The sound velocity is determined from a direct comparison of the acoustic wavelength in the sample liquid with the optical wavelength of the laser. In order to check the performance of the apparatus, measurements were performed for distilled water in the temperature range 20–75 °C under atmospheric pressure with a carrier frequency of 16.5 MHz. The precision of the phase measurements by the PSD and systematic errors in the sound velocity measurement are evaluated. The total uncertainty of the sound velocity is estimated to be 0.001%. The results agree with reliable literature values within 0.003%.