“We know our ultrasonic system is working properly, because it’s making plenty of noise. Why is the cleaning process ineffective?” This recurring question has vexed manufacturers for decades. Cleaning with sound is indispensible for many critical cleaning applications.¹ However, ensuring that the process is working reliably and consistently takes a bit of investigation.

THE LIGHTS ARE ON—IS ANYONE HOME?
Determining if an ultrasonic system is actually working can be challenging. Just because the unit makes noise does not mean the ultrasonic system is doing the job. The ultrasonic frequencies are...ultrasonic! That is, they are above the normal hearing range of people, so unless you are a dog, you probably can’t hear the ultrasonic concerto; and even puppies in their prime are not likely to hear ultrasonics above about 50 or 60 KHz. Also, the energy that performs the contamination removal is not from the sound waves themselves but from the shock waves and microjets associated with the collapse of cavitation bubbles.

CLASSIC APPROACH
The simplest, albeit the most unsophisticated, test for efficacy of ultrasonics is the aluminum foil erosion test. For this test, a piece of standard thickness aluminum foil is placed into the process tank. Please do not place the foil directly in the process tank, or you may be plagued by tiny “foil glitter” contaminants. Instead, place the foil into a beaker of cavitation medium (a liquid that is known to effectively transmit ultrasonics), and suspend the beaker in the process tank. Ah, wait! First, adjust the temperature and de-gas the liquids, then submerge the foil and turn on the transducers. If there is cavitation, in about 30 seconds an “orange peel” pattern (Fig. 1) will develop in the foil. Note that if the foil is left in the ultrasonics for a long time, severe erosion will occur (Fig. 2), illustrating the power of cavitation.

The aluminum foil test is simple, inexpensive, and popular. However, it has limitations. The effectiveness of this test is dependent on such parameters as the thickness (“heavyduty” foil does not work), smoothness, as well as on the ultrasonic frequency. At very high frequencies, the erosion is not apparent. The test is operator-dependent and techniquedependent.