Aluminum Foil Test for Ultrasonic Performance – Potential Problems

Task At Hand –

In a previous blog I declared that I am not a fan of the aluminum foil test for evaluating ultrasonic performance.  I question its validity and relevance on a number of fronts.  If my purpose here is to question my own skepticism, then I guess I should probably define the factors that are the basis for that skepticism.

The object of cleaning is to remove dirt from parts.  My first hangup is that I’m not sure that putting holes in aluminum foil has anything to do with removing dirt from parts.  They may correlate but they may not.  There are only a handful of cleaning tasks that can be accomplished using water and ultrasonics alone and none (that I know of) involve putting holes in aluminum foil.

My second “hangup” is that it seems to me that putting holes in aluminum foil would only prove the ability of ultrasonic energy to damage delicate parts.  If ultrasonic equipment is judged based on its ability to produce holes in aluminum foil then how would you propose to clean a part similar to aluminum foil without damaging it?.  This, by the way, is done on a daily basis!

Thirdly, after dunking literally thousands of pieces of aluminum foil in water over the past 50 years, I have seldom (and then only randomly) been able to achieve consistent results despite meticulous efforts to standardize test conditions.  Sample foils dipped one after the other in the same tank with the same ultrasonic parameters, at the same temperature, with the same liquid, and positioned as closely the same as possible (even using a fixture) more often than not net vastly different results evident even to the inexperienced eye.

 

Background –

The aluminum foil test has been around forever!  It was well-entrenched when I started in this industry in 1968.  To the best of my knowledge, there is no formally published universal standard for aluminum foil testing of ultrasonic equipment to establish performance.  The basic idea is to immerse a piece of aluminum foil vertically in an operating ultrasonic tank filled with water and then visually examine the foil for holes or “pock marks” after a pre-determined exposure time.  Explanation beyond that is either non-existent or incredibly vague.  Although one will occasionally find an internal procedure specified, In most cases, instructions are passed from “generation to generation” with important parameters (if they ever existed) lost or distorted in the transmission.

Request – If anyone has a procedure for using aluminum foil to quantify ultrasonic cleaning performance I would REALLY appreciate it if you would share.  As I said, I haven’t seen one that addresses the issues below and which I feel have significant consequences in the validity of such a test.  I can be reached through the “comments” section below or directly at jfuchs@ctgclean.com.

 

Plan –

As first step in looking at the aluminum foil test as a potentially viable measure of cavitation intensity or cleaning ability one must first look at the obvious variables which must be controlled as part of a meaningful test..

Foil Consistency–

The test is usually performed using “household aluminum foil.”  How consistent is the foil?  What is the possible effect of known or potential variations in foil on foil perforation?

Liquid Media –

The test is usually performed using water.  Options include tap water, RO water, de-ionized water, distilled water, etc.  In some cases a random or defined concentration of a chemistry is added to the liquid.  What effect does the liquid media have on foil perforation?

Liquid Depth –

Some ultrasonic cleaners have a precisely defined operating depth which is often indicated by a scribe mark  or dimple in the side of the tank.  Many times, however, there is no liquid level specified. Liquid level has been demonstrated to have a significant effect on the performance and distribution of the ultrasonic activity in a cleaning tank.  What effect does liquid depth have on foil perforation?

Liquid Temperature –

Liquid temperature may range from tap temperature (which may vary from place to place and time to time) up to near boiling depending on the available water source.  Temperature has been demonstrated to have a significant effect on ultrasonic performance. Is there a temperature effect on foil perforation?

Gas Content –

The gas content of a liquid is important to performance.  What effect does degassing have on foil perforation?

Foil Placement –

Consistent placement and support of the foil during testing is difficult. How critical is it?

Test Procedure and Sequence –

The sequence of test events can have an effect on outcome.  Things like whether the foil is immersed before or after the ultrasonic energy is turned on can make a difference.  What effect does this have?

Evaluation –

Evaluation methods are usually visual and “subjective” to say the least.  A foil produced using one set of the above variables in a particular ultrasonic cleaning tank and a second produced using in the same tank using another set of variables can look vastly different yet both represent the same well (or badly) performing tank.  Is it possible to develop a repeatable means of evaluation?  To be acceptable to me, a test should produce a number.

Finally, I am hoping that the community will help me out on this effort.  I’m pretty sure this list will change and evolve into something more meaningful with your help.  Because it is so simple and available, I want to find a way that I can be confident that the foil test is meaningful beyond simply indicating the presence of ultrasonic cavitation and implosion. The ultimate goal would to establish a reliable connection to cleaning performance if that is possible.