Measuring Method of cavispector

As described in ultrasound cleaning, bubbles are generated in the cleaning tank. When a bubble implodes, a small acoustic shock occurs, which can be measured by an underwater microphone – a hydrophone – as acoustic noise with a wide bandwidth. Normally, many bubbles implode at the same time, and the noise is summed up like clapping in a concert hall. Different surveys have shown the correlation of this noise with the cleaning behavior in the tank. The results of which were published in [1] and [2], for instance.

But measuring that noise is not trivial at all. Because of the vibration of the ultrasound transducers, which is the origin of the process, their operation frequency shows a much higher amplitude than that of the noise (figure 1, (1)). The height of the amplitude of the operating frequency is also dependent on the position of the hydrophone in the tank. This frequency and those described below must not be included in the calculation of the cavitation noise. Furthermore, the stimulation of the transducers is not a pure sine, so harmonics are also present in the spectrum (2). Additionally, further frequencies (3) are generated by the reflection of the acoustic wave from the walls of the tank and these can have amplitudes which are higher than the operating frequency itself. The spectrum becomes even more complex when the vibration of the bubbles is also considered, which occurs at half of the operating frequency in the spectrum (4). There is also a component of the rectified mains frequency (5). All these frequencies are summed up, depending on where in the tank the measurement takes place. Figure 1 shows a typical example of the frequency spectrum.

Figure 1 typical frequency spectrum

To isolate the noise of the collapsing bubbles – which represents the cleaning effect – out of this complex frequency spectrum, a sophisticated mathematical procedure has been developed. Thereby the Fourier transformation and various statistical methods are used. Figure 1(6) shows the frequency band, which is used for the further calculations. The final result of the computation is the cavitation noise level Lcn. This value is quantifiable, reproducible and documentable. For an intact device its value should be more than 171dB to 173dB (full wave rectification) or 168dB to 170dB for older devices with half wave rectification, dependent of an operating frequency from 25 kHz to 45 kHz. The measurement procedure of cavispector is described in [3].

Since a global consensus is necessary for the acceptance of a new measurement method, a multi-annual international standardization project was carried out. In the end, the standardization groups of 13 important countries unanimously approved the new technical specification. This was published as IEC TS 63001 (IEC = International Electrotechnical Commission) [4] in the beginning of the year 2019 and contains the description of the underlying mathematics in annex B. The author of this text was the leader of this project.

cavispector has been developed exactly according to this technical specification. Beyond that specification, the measurement of the bath temperature in the tank can be integrated in cavispector.

Thereby, the heating control of the cleaning device can be checked and the temperature of the bath can be measured and documented parallel to the measurement of Lcn.

[1] R. Sobotta, C. Jung: „Ein Messverfahren zur Bestimmung des Kavitationsrauschens“, Fortschritte der Akustik, DAGA 2011, pp. 921-922

[2] A. Zwahlen, M. de Wild, C. Jung: „Comparison of Methods for Testing Ultrasound in the Cleaning Bath“, Fortschritte der Akustik, DAGA 2014, pp. 716-717

[3] M. Köchel, A. Richter, R. Sobotta, „Digital signal processing for measuring cavitation noise level“, Fortschritte der Akustik, DAGA 2017, pp. 1026 – 28

[4] IEC TS 63001:2019, Measurement of cavitation noise in ultrasonic baths and ultrasonic reactors

Köchel Verifications

We will come to your site and perform the check with cavispector. You will receive a test report which documents the status of your line, regarding cleaning efficiency and cavitation. Please contact us so that we can plan the check and send you an offer.

With a software maintenance agreement you will gain access to our hotline and you will receive updates over the validity period of the agreement.

 

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