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Quantitative determination of the cleaning effect of ultrasound baths

03.11.2005

The recording of spectral sound field parameters directly in the cleaning bath allows the efficiency of ultrasound-based cleaning processes to be determined and optimized.

Ultrasound in the frequency range 20 - 200 kHz is used for a wide range of applications in research, medicine and technology, especially for the cleaning of all kinds of components. Despite its widespread use, no general cleaning and equipment principles are available so far, however. The actual cleaning effect can only be estimated and for optimizing the processes, one relies to a large extent on empirical approaches.

The metrological approach which characterizes the sound field as the cause and driving force of the processes taking place in the cleaning bath opens up the possibility of understanding the complicated processes and their description and control more precisely. Within the scope of a project supported by AiF, PTB's"Medical Acoustics" working group was for the first time able to determine direct spatial correlations between spectral sound field parameters and the cleaning effect achieved by using, on the one hand, suitable sensors for the spatial determination of the power sound fields and by refining, on the other hand, classical physical methods for effect determination.

The three-dimensional detection of the cavitating ultrasound fields is achieved by using either fibre-optical or piezo-electric sensors. The cavitation effect is determined two-dimensionally using aluminium foils. Thereby, the perforations and indentations having occurred during the application of ultrasound are quantified with special image processing software. In this way, they can be correlated with the spectral parameters obtained by the sound field measurement (e.g. the frequency-band-related noise powers and the amplitudes at the fundamental frequency, the first harmonic, subharmonic and ultraharmonic) in the corresponding plane. By linking the sound field data directly with the effect data it shows to what extent sound field measurements can be used for the optimization of application processes.

Fig. 1 shows by means of a typical example the results having been obtained so far. A good correlation between the amplitude distributions of the sub- and ultraharmonic components could be found (k = 0.77) and a high correlation of these components with the amplitude at the fundamental frequency (k > 0.90). Also, the noise powers in the high (1.0 - 1.2 MHZ) and low (100-200 kHz) frequency band showed good agreement with the parameters mentioned (k = 0.50 .. 0.85). The coefficients for the correlation of the cavitation effect with most of the spectral parameters were at k = 0.40. Here, by refinement of the method, an improvement in the selectivity shall be achieved. Not suited for judging the cleaning effect is the amplitude of the first harmonic whose correlation coefficients were always below k = 0.20.

Two-dimensional distribution of spectral parameters from the sound field measurement

Figure 1: Two-dimensional distribution of spectral parameters from the sound field measurement.
(avg f: amplitude at the basic frequency,    harm: first harmonic,    subharm: subharmonic,
ultraharm: ultraharmonic,    BIE low: noise power in the low frequency band) and the
cavitation effect (erosion) in an ultrasound cleaning bath.

Contact person:

Klaus-Vitold Jenderka, FB 1.6, AG 1.63, E-mail: klaus-vitold.jenderka@ptb.de