On the track of true ultrasound

Messen der tatsächlichen Kavitation in einem Ultraschallbad per Hydrophon

By measuring the cavitation noise, the effective effect of the ultrasound can be measured and adapted to the respective application (Images: Köchel)
The Cavispector in the form of a tablet PC and the evaluation electronics, including hydrophone, provide a major boost to process reliability.

Acoustic measurement of ultrasound

However, there is now a solution to close this knowledge gap, as the intensity of the cavitation process underlying the ultrasonic effect can be measured using the characteristic acoustic noise produced during the implosion of the gas bubbles.

This cavitation noise is called level cavitation noise (Lcn) and the numerical value, which is expressed in decibels (dB), is directly related to the cleaning effect of the ultrasonic bath. The more intense the cavitation, the louder the acoustic signal and the more effective the cleaning.

Standardized measurement method

To ensure comparable measurement results worldwide, the method for measuring cavitation noise in ultrasonic baths has been standardized in IEC TS 63001. Edition 1.0 was published in January 2019 and has now been updated with the publication of Edition 2.0 in February 2024.

The update of the technical standard includes the addition of a further measurement method by the National Physical Laboratory (NPL) and a more precise description of the measurement method in order to minimize room for interpretation and standardize measurement results from different providers.

 

Manual measurement with small device.

Actual effectiveness can only be measured in the bath

In addition to the method described in the standard of placing the hydrophone in the medium of the ultrasonic bath, alternative measurement methods such as measuring the cavitation noise with a microphone in air or a structure-borne sound sensor offer further possibilities.

These alternative approaches can be used depending on the application and the user's preference. However, it should be noted that numerous phenomena that take place in the medium, such as the influence of goods carriers, the loading of the bath with goods or locally varying cavitation values, can only be detected when measuring with the hydrophone in the cleaning medium, as is the case with the Cavispector.

Process defects detected

The belief that activating the ultrasound and setting a certain power level will almost certainly lead to the desired result in terms of component cleanliness is widespread and yet by no means always true. Numerous measurements carried out by Köchel Verifications in systems at users' premises have shown that in many systems either the parameters are not set correctly or the ultrasonic technology is even damaged.

Such deficiencies can either be caused by a lack of knowledge on the part of the user regarding the parameters relevant for a cleaning process, or by the fact that ultrasound and its effectiveness have not been on the test schedule of many users. In any case, cavitation measurements can be used to detect such weak points very efficiently and optimize the cleaning process in terms of efficiency and effectiveness in a way that was previously not possible through trial and error.

 

An ultrasonic cleaning process and its effectiveness can be monitored using the ultrasonic spectrum recorded by a hydrophone immersed in the medium. System defects or parameter errors can also be identified in this way.

Optimization for alternating pressure processes

There is an increasing number of closed ultrasonic cleaning devices and systems that combine ultrasound with an alternating pressure process. In these systems, pressure fluctuations also influence the cavitation - here too, the Cavispector allows the prevailing process conditions to be specified. Cavitation measurement allows the dynamics of the system to be recorded and displayed, making it possible to optimize the time constants for vacuum and aeration.

Calibration and testing are important

Another important aspect of cavitation measurement is the regular checking and calibration of the measuring equipment, especially the hydrophone. The hydrophone is the most important instrument for recording cavitation noise and should therefore be checked regularly for accuracy and reliability.

Methods such as a quick and easy impedance measurement or a comparison of the working hydrophone with a reference hydrophone enable early detection of damage or wear to the hydrophone and appropriate adjustment or replacement of the measuring device. The traceability of the hydrophone's sensitivity characteristics in the Cavispector to a natural standard ensures that the measurements can always be made on an objective and unambiguous basis.

 

The table shows the advantages of the individual measurement methods. Measurement by hydrophone in the medium provides the most information, including absolute values for cavitation. Measurements from outside the bath generally only provide relative values.

Conclusion

The continuous development and standardization of cavitation measurement helps to improve the efficiency and quality of ultrasonic cleaning processes in various industries. Accurate and reliable measurement of cavitation with a measuring device such as the Cavispector can make an important contribution to more consistent cleaning results and performance across different devices. In addition, the service life of individual components, such as ultrasonic technology, can be extended.

Köchel Verifications GmbH
www.cavispector.com