In the blog “Reader Questions – Monitoring Ultrasonic Transducers,” I suggested a couple of ways one might test individual ultrasonic transducers to assure they have not become ineffective due to de-bonding from the cleaning tank. In an extension of the spirit of that blog, quality control measures for ultrasonic cleaning performance, I decided to check out a paper I wrote for presentation at the Precision Cleaning Forum in 2002 entitled “Ultrasonic Intensity Measurement Techniques.” On reading it for the first time in several years, I was, first, disappointed that with all the technological advances we have made in the field of ultrasonics since then, there has been very little progress in the development of ways to quantitatively measure Ultrasonic Intensity in a cleaning tank. And then I started to think – which is always dangerous – especially for an engineer (and an old-fashioned one at that).
It is assumed, perhaps wrongly, that a measurement of the effects the implosions of cavitation bubbles relates directly to the intensity or amplitude of the ultrasonic pressure waves that produces them. In fact, it is really the energy and density of the implosions of cavitation events that, probably, relates most directly to ultrasonic cleaning performance. Most of the techniques described in the paper are intended and purport to measure and quantify the effects of (rather than the cause of) the cavitation implosions. Which is first, the cart or the horse? If a way was found to maximize the effects of cavitation bubble implosions and, therefore, the intensity of the ultrasonic waves required to produce a desired effect could be reduced, ultrasonic intensity no longer be directly related to effective cavitation bubble implosion.
It is no deep secret that we already know that there are variables that directly affect the relationship of ultrasonic field intensity to ultrasonic cleaning performance. These variables include temperature, chemistry, the quantity of gas dissolved in the liquid and the particle load in the cleaning bath along with a myriad of others which, according to some, include the phase of the moon! The same ultrasonic tank can, indeed, perform well one day and poorly the next without a measurable change in the magnitude of the ultrasonic field (as measured by electrical energy accepted by the ultrasonic transducers).
Looking back on the Ultrasonic Intensity Paper, I now realize that, in some ways, there was an effort to maximize the cavitation implosion effect in the interest of consistency of result but without a direct connection to logic. Yes, consistency is important (and illusive) but we may be better served to understand the why and not the wherefor. The goal should be cleaning performance, and should encompass all variables with the intensity of the ultrasonic field being only one of many. In fact, the goal is CLEAN PARTS, not the most powerful ultrasonic source.
With that off my chest, I think there are some very good points made in the above paper which is now nearly 10 years old. It is worth the effort for anyone involved with ultrasonic cleaning to re-read it with the above perspective in mind.
Happily, there IS some new work that has been reported in an article Measuring Cavitation in Ultrasonic Cleaners and Processors
published by Mark Hodnitt of the National Physics Laboratory in England. This article references many of the above variables. I think it would be worthwhile to study and establish relationships between the variables and the cleaning results. It appears that the NPL device is intended to measure cavitation implosion, not cavitation. I feel this is where we should be headed – with an acknowledgement that all variables affecting cavitation implosion intensity and its relationship to cleaning performance should be optimized, not just the ultrasonic intensity field.
– FJF –