It would make sense that variations in the aluminum foil used for testing in the foil test for ultrasonic performance could have a bearing on the outcome of the test. Most people use standard “grocery store” aluminum foil for testing because it is inexpensive and widely available. So let’s talk about that first. The most popular brand is probably Reynolds but there are several store brands and generics as well. Could they differ in properties? To find out, I did a little research. What I learned was this – –
How Aluminum Foil is Made –
Aluminum foil is made by rolling aluminum between heavy rollers to progressively reduce its thickness. The aluminum may start as ingots or slabs or as a continuously cast sheet. During the rolling process, the aluminum may be sprayed with lubricants to reduce friction and/or heated to increase ductility and relieve stress hardening created by deformation as the material is rolled thinner and thinner. As the foil approaches its final thickness (thinness) two sheets are rolled together face to face. An oil coating is applied to prevent them from sticking together. This, by the way, is why aluminum foil has a shiny side and a dull side. The shiny side is where the double thickness of foil is in contact with the rollers while the dull side is where the two parallel sheets of foil are touching. Once the rolling process is complete the two pieces of foil are separated, slit to the desired width, collected on large rolls and annealed. Later it is re-rolled onto cardboard tubes for final sale to the customer. It appears that the process may vary considerably from manufacturer to manufacturer.
To start off with, aluminum foil as purchased at the grocery store varies considerably in thickness. In general, thicknesses are .016mm for “standard” and .024mm for “heavy duty” foil. One advertised tolerance is plus or minus 3%. At first that sounded reasonable but then I learned that “thickness” is determined by weighing a sheet of a specific size and extrapolating that weight to thickness. This, in my mind, measures the overall average thickness of the sheet but says nothing for the uniformity of the foil thickness. Variations in thickness in different areas of the sheet will not be detected by this method. And, to my surprise, there is no claim that grocery store foil is impermeable! Pin holes and stress cracks are allowed within certain limits. Foil may also be quilted, coated with a microscopically thin layer of polymer or other materials (non-stick), bonded to paper or undergo any one of many other treatments depending on its intended use.
The foil sold in the grocery store may vary considerably in alloy. One source says it may range from 92 to 99% aluminum but it is seldom pure aluminum. Additives are used to improve things like wettability, tear strength, puncture resistance, pliability (for forming around the rims of dishes), and so on. These modifications have an effect on the ductility or the “temper” of the foil making it more or less susceptible to fatigue failure. An earlier blog explained that it is fatigue failure that produces holes in the aluminum foil test for ultrasonic activity. The outer parts of the roll as it is annealed after rolling will see a higher temperature than the inner portions of the roll. The central part of the roll may see less temperature and shorter holding times which, again, may affect ductility and temper. Heating for annealing, also intended to remove oil used in the rolling process, may be more or less effective depending on the position of the foil in the roll.
Based on the above, aluminum foil, at least that you can buy at the grocery store, doesn’t look like a very reliable standard. My best advice if you are committed to an aluminum foil test is to stay with one brand and weight and type of foil. Avoid heavy duty, non-stick, quilted, textured, colored and other specialty foils! I would also suggest that you consider buying foil by the case or at least several rolls at the same time from the same source in the hope that there will be some lot consistency. When you move from one roll or lot to another, do a side by side comparison testing the same ultrasonic machine using the last test of the old roll or lot and the first test of the new roll or lot and comparing the results. If they are vastly different, it may be due to differences in the foil and should be noted immediately.
There are sources for “laboratory grade” aluminum foil. Thirty or so years ago, many companies tried using laboratory grade foil but found that even it was inconsistent – – maybe even more inconsistent than grocery store foil. There may be a better standard now but I have not tested that.