If you’ve heard of ultrasonic cleaning you’ve probably heard of the aluminum foil test. The practice of putting a piece of aluminum foil into an ultrasonically activated tank of water probably began as a novelty demonstration. The holes produced in the aluminum foil by the collapse of cavitation bubbles at least showed that something interesting was happening! It was only one short step from that initial demonstration to the notion that the number, size and distribution of holes produced in aluminum foil by ultrasonic energy in water could be used as a measure of a number of ultrasonic parameters. The results have been decidedly mixed.
The “holy grail” for both manufacturers and users of ultrasonic cleaning equipment has always been a way to quantify the intensity and distribution of ultrasonic energy in a cleaning tank as a measure of the tank’s ability to clean. Numerous methods have been devised including today’s increasingly capable hydrophone technology. Most of these methods have flourished then quickly floundered when they were found to either be impractical due to their complexity or due to their lack of accuracy and repeatability. And yet, the aluminum foil test is still utilized extensively after more than 60 years. I was surprised to learn recently that a major manufacturer of the latest hydrophone technology uses the aluminum foil test as one of their standards for comparison!
Note – Those with keen interest may might want to read the article I wrote on this subject in 2002 for background. – Ultrasonic Intensity Measurement
Over the next several weeks, I intend to periodically publish blogs that explore the aluminum foil test in some detail including information on how to perform it, how it works, what it can tell (and maybe not tell) you as well as what I feel are the capabilities and limitations and why. At the outset, I should say that after peering at pieces of aluminum foil punctured by ultrasonic energy for over 50 years, I am not a fan of the foil test. This fact will be obvious to those who have read my writing over the years so I’m not going to hide it. I have repeatedly found foil tests to be inconsistent and have not been able to correlate what I’ve seen on the foil to the ability of an ultrasonic tank to clean. But I’m going to try to put all of that behind me – at least for now. As the foil test is so firmly established and is not likely to go away anytime soon, I feel that it is appropriate that we take a serious look at what might be done to make it a useful tool or, at the very least, know what it can and can’t do and why so we’re in the same chapter of the book if not on the same page.
Why Has the Aluminum Foil Test Survived?
I think there are very few people who would claim that the aluminum foil test is an absolute measure of ultrasonic activity or a reliable means to judge the ability of an ultrasonic cleaning tank to produce good cleaning results. There are two things that come to mind as the drivers for its longevity and wide spread use. The first is its simplicity. Just about everyone has access to a roll of store-bought aluminum foil, there is no other equipment needed and the procedure only takes a few minutes. The second is the fact that the result is very graphic. Differences, meaningful or not, can be seen, discussed and preserved. Foils from a previous test can be saved and used for comparison in subsequent tests.
As I embark on this adventure, I am hopeful that you will help with your input in the “comments” section below. Has the test worked for you? Do you see it as a good tool? Let me know!