Experimental Measurements

At present there are few ultrasonic instruments which can be purchased off-the-shelf which are specifically designed for food processing applications. This is the main reason why ultrasound has not been used more frequently in the food industry. This situation is already changing, and a number of instrument manufacturers have recently developed ultrasonic sensors for application to food materials (e.g., Cygnus Instruments, Dorchester, Dorset, UK;

Nusonics Inc., Tulsa, OK, USA). These sensors are suitable for simple applications which rely on measurements of the ultrasonic velocity at a single frequency, e.g. concentration determinations, flow rates, thickness measurement, detection of foreign bodies. More sophisticated instruments are likely to be developed in the near future which will open up a wider range of applications, e.g., particle sizing. The ultrasonic properties of materials can be determined in one of two ways: either the wavelength and amplitude of ultrasound is measured at a known frequency, or the time-of-flight and amplitude of a pulse of ultrasound which has traveled a known distance are measured [9, 10]. The latter technique is by far the most popular and useful technique for food processing applications, because the experimental configuration is simple to design and operate, measurements are rapid, non-invasive and non-intrusive, there are no moving parts and the technique can easily be automated.

The simplest and most widely used technique is called the pulse-echo technique. More sophisticated pulsed methods have been developed to improve the accuracy of measurements [9, 10], however, the operating principles are basically the same as those of the pulse-echo technique. For this reason, only the pulse-echo technique is described and some of the modifications are mentioned in passing.


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