Ultrasonic concentration measurement

Measuring principle

The velocity of sound in a fluid depends on the density and compressibility of the latter, and therefore on the concentration of all substances diluted or mixed in it. The ultrasonic concentration measurement makes use of this dependency.

To determine the sound velocity, one measures the time taken by an ultrasonic signal to cover a determined distance. An ultrasonic sender receives an electric signal and transforms it in an acoustic signal. The signal propagates in the medium and reaches after a transit time "z" the ultrasonic receiver. This transit time is measured and used for the calculation of the sound velocity.

The velocity of sound in a fluid depends on the density and compressibility of the latter, and therefore on the concentration of all substances diluted or mixed in it. In order to determine this dependence, it is necessary to measure the sound velocity of the system as a function of concentration and temperature. Ultrasonic concentration measurement is only possible if the sound velocity changes by at least 0.5 m/s per mass percent.

Assuming a linear or polynomial dependency (degree 1 to 4) between concentration and sound velocity, a calibration curve is determined. The temperature dependency of the resulting concentration coefficient must be taken into account (linear or polynomial).