Ultrasonic Interferometer PSAW It is a simple device to determine the ultrasonic velocity in liquids ,the principal is based on accurate measurement of wave lenght in the medium. Ultrasonic waves of known ferquency are produced by a quartz plate fixed at the bottom of the cell .The waves are reflected by a movable metallic plate. If the separation between these plate is exactly whole multiple of sound wavelenght , standing waves are formed. The acoustic resonance gives rise to eletrical reaction on the generator and anode current becomes maximum. From the knowledge of wavelenght , velocity can be obtained.