Apparatus for measuring velocity of sound in fluids

A universal apparatus was developed for simple and accurate measurement of the velocity of sound in almost any fluid, including a chemically active one. Measurements are made by the fixed-distance pulse passage method. An ultrasonic signal travels through the test fluid betwen two optically treated plane-parallel windows in a hermetically closed quartz cell. The latter is dropped into a glass vessel with circulating thermo-static liquid for low-temperature measurments or into an electric furnance for high-temperature measurements. The temperature is measured with a resistance thermometer and read on a digital voltmeter, constancy of the temperature is also monitored on an oscillograph. The velocity of sound is determined on the basis of absolute measurement of the pulse travel time or on the basis of relative measurement. Absolute velocity measurement also requires correction for diffraction, especially in the Fresnel near-field region, which must be subtracted from the velocity reading to yield the true thermodynamic velocity of sound. A significant systematic error of both absolute and relative measurements, in addition to the errors of distance and time measurements, is the error in referring the readings to specific state parameters.