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A sensor to determine the effectivity of ultrasonic cleaning devices

07.06.2013

Ultrasonic baths have long been widely used in industry and trade. Despite this, no uniform procedure has existed so far to compare the cleaning results obtained by different devices or the cleaning effectivity of a device in various operating modes. PTB has developed a procedure which allows the abrasion of particles from a surface – hence a cleaning process – to be measured directly.

There is no standardized procedure to assess the effectivity of ultrasonic cleaning devices, manufacturers in particular all use different methods. In addition, the procedures used by each of them often have no proven direct reference to the cleaning effectivity. PTB has now developed a sensor which allows the abrasion of thin test layers exposed to ultrasound to be determined quantitatively with a quartz micro-balance. Hereby, the change in the mass of the test layer indicates the progress of the abrasion – and thus the progress of the cleaning.

The change in mass of the test layer deposited onto the quartz surface is in linear connection with the change in the resonance frequency of the quartz which is used for a weighing. Contrary to the widely used procedure where the quartz is operated as a frequency-determining element in an oscillator circuit, it is here excited externally by a signal whereby the frequency is swept in the range of the resonance frequency of the quartz of 10 MHz. The HF voltage at the quartz is rectified and low-pass filtered  in order to measure the amplitude of the resonance curve and to derive the resonance frequency from it. Hence, it becomes possible to measure this frequency even at a high attenuation as occurs in a liquid environment. Figure 1 shows a diagrammatic sketch of the set-up. An AT-cut quartz crystal is used in order not to emit an acoustic wave and to avoid influences due to waves reflected in the environment.

Figure 1: Sketch of the oscillating quartz crystal and its actuation.

The test layer is made from a suspension of silicate micro-particles in a diluted varnish. This mixture is deposited onto the quartz surface by spin-coating. Due to its mass, the layer shifts the resonance frequency of the quartz down to lower frequencies. During the cleaning process, the frequency increases again due to the abrasion of the layer. The layer can be adapted to the conditions in the cleaning bath via the dilution of the varnish and the size of the particles in the suspension; for example, ultrasound with a higher frequency is used to remove smaller particles. The prototype of this sensor was developed and tested for cleaning baths with a working frequency of 45 kHz.

At constant cavitation, abrasion is fastest at the beginning of a measurement with a newly deposited layer. Later, the remaining frequency shift – i.e. also the remaining test layer – approaches the value of the uncoated surface. Figure 2 shows the quartz surface immediately after coating as well as after one and after five minutes of ultrasound irradiation.

Figure 2: From left to right: (a) Coated quartz surface, (b) surface after one minute, and (c) surface after five minutes spent in the active cleaning bath. The light-coloured surfaces are coated with particles.

The next step will be to enable this measurement for as many practical applications as possible. A quartz crystal having been coated once can be used for approx. 5 to 10 minutes in an active cleaning bath, which corresponds to approx. 30 tests of the cleaning effectivity where the test layer is gradually removed. The quartz can then be coated again. For this purpose, it is first cleaned with a solvent before a new test layer is deposited.

Contact person:

Matthias Jüschke, Dept. 1.6, WG 1.62, e-mail: matthias.jueschke@ptb.de