Ultrasound Cleaning

An ultrasound cleaning device contains a tank made of stainless steel (figure 1 (1)), in which an aqueous liquid can be filled. For a better cleaning efficiency detergents can be added to the bath (2).

Figure 1 Intersection ultrasound cleaning device

Ultrasound transducers (3) are mounted at the bottom and sometimes also at the sides of the tank. These transducers convert an electrical alternating voltage in a mechanical vibration, like a loud speaker. An ultrasound generator (4) produces this alternating voltage, which has an operating frequency from about 25kHz to 50kHz normally used for cleaning purposes. Higher frequencies are used to support the subsequent rinsing.

When the transducers are stimulated by the alternating voltage, the vibration spreads through the material of the tank into and in the fluid. Bubbles (5) which are in the fluid are brought to oscillation, so that their volume increases and decreases. When the intensity of the oscillation reaches a sufficient level these bubbles can implode. Then a so-called jetstream occurs, which removes by its stimulus the dust from a polluted surface like a micro brush.

However, the bubbles in the ultrasound bath are much smaller than shown in the video.

To increase cleaning efficiency, most of the ultra sound devices are equipped with a heating (6) to warm up the liquid.

Many ultra sound devices also offer the option to select different frequency modes. “Sweep” is normally the standard mode. Here the operation frequency sweeps a range of about +/- 1 kHz around a middle frequency. This causes a more common distribution of the bubbles in the tank. Another mode is “pulse”. Here the operating frequency switches very fast between two frequencies, which have a separation of about 1 kHz. The advantage is a higher cleaning efficiency. Older or simpler devices only have a fixed frequency. A bath with a fresh liquid should also be degassed. Therefore some devices offer a special “degas” function.

The following figures 2 – 4 show examples of the amplitudes of the acoustic pressure at the operating frequency of different modes in logarithmic presentation.

Figure 2 frequency spectrum ”sweep“

Figure 3 frequency spectrum “pulse“

Figure 4 frequency spectrum ”fix“

The light blue dots show the middle frequency and the frequency with the highest amplitude of the pressure, respectively. These are identical at fixed frequency.

/1/ O. Supponen, Ph. Kobel, D. Obreschkow, M. Farhat, “The inner world of Collapsing bubble”, Physics of Fluids 27, 2015.

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