When to Use a High Frequency Ultrasonic Cleaner

One of the purchasing decisions to be made when considering an ultrasonic cleaner is the frequency at which it operates. Ultrasonic frequency is the term applied to the ultrasonic energy provided by ultrasonic generators and delivered to the transducers affixed to the cleaning tank. It is expressed in kilohertz (kHz).

Ultrasonic cleaners such as those manufactured by Elma and available from Tovatech are available in several frequencies, ranging 25 kHz to 130 kHz. Several units offer a choice between two different frequencies, examples being the Flex and TI-H lines that operate at either 25/45 kHz or 35/130 kHz, and the P line at 37/80 kHz.

While one might think the higher the frequency the more vigorous the cleaning action, such is not the case. The cavitation action that does the cleaning is much more vigorous at the lower frequencies, which is why low frequency ultrasonic cleaners are generally selected to remove grease, rust and other contaminants from relatively tough materials such as iron and steel stampings and castings.

Complex and Delicate Cleaning at High Frequencies

But this same vigorous cleaning action can damage delicate parts such as printed circuit boards, disc drives, those fabricated of materials such as aluminum, and parts with polished or mirror finishes. It can cause breaks in PCB circuits and cavitation “streaks” on soft and polished surfaces.

Otherwise explained, higher ultrasonic frequencies generate larger numbers of smaller sized cavitation bubbles per unit volume in the ultrasonic cleaning tank. The smaller the bubbles the less cavitation energy released during implosion. While a higher frequency produces less intense cleaning, the upside is that more bubbles are produced for cleaning delicate parts or those with very tiny features such as crevices and blind holes.

Even high-frequency ultrasonic cleaning can damage parts due to the “standing wave” produced by ultrasonic energy. These waves are relatively constant, which means that the ultrasonic energy at that point is more intense than between the waves, and can result in possible damage such as streaks. Ultrasonic cleaners equipped with a Sweep function overcome this by distributing the cleaning action more evenly in the bath. It is also worth noting that higher frequency ultrasonic cleaners operate more quietly than low frequency units.

Add cleaning solution decisions to ultrasonic frequency and Sweep when considering a ultrasonic cleaner. This too can be influenced by what is being cleaned.

Calculating Cavitation Bubble Size

High-frequency ultrasonic cavitation bubbles are better able to penetrate small openings and blind holes. The cavitation bubble must be smaller than the smallest hole or cleaning will not take place. To calculate the bubble radius divide 3.28 by the frequency of the ultrasonic cleaner in Hertz then multiply by 2 to get the approximate diameter. For example, an 80 kHz frequency = 80,000 Hertz or cycles per second. 3.28 divided by 80,000 = 0.000041 m or a cavitation bubble radius of 41µm. For the higher frequency 130 kHz cleaners the cavitation bubble radius is 0.0000252 m or 25.2 µm

In summary, decisions on ultrasonic cleaner tank size, operating frequency, Sweep and cleaning solutions should not be made lightly. Discuss all details with component manufacturers and with equipment providers such as Tovatech to be sure your selections are correct.