All ultrasonic cleaning systems, regardless of manufacturer or frequency, produce a scrubbing action which is distributed as a series of an equidistant bands known as Standing Waves which begin at the transducer mounting location, typically the bottom of the tank.  The frequency of the ultrasonic cleaner will determine the distance between these bands of activity.  The higher the ultrasonic frequency, the closer these bands will be to one another, but the less powerful the activity will be at any one location.  High-frequency ultrasonic systems generally produce a very evenly distributed cleaning effect, but a less powerful one, while low-frequency ultrasonic systems produce a cleaning effect with large areas of inactivity, but the cleaning action is more powerful at standing wave locations.

The photograph at the right depicts Zenith's ULTRAPROBE Ultrasonic Testing Instrument, which visually displays the ultrasonic scrubbing action distribution in a 25kHz ultrasonic cleaning system.  This patented device is composed of a quartz test tube which is filled with a colored detergent mixture, and precious metal particles of a specific size.  When the tip of the instrument is inserted into an ultrasonic cleaning system, the metal particles migrate to areas of intense ultrasonic activity.  Note that there are light colored bands spaced roughly 3/4 inch apart in this instrument.  This is the scrubbing pattern that a 25 kHz ultrasonic system will produce.  As you can see, there are a large areas of inactivity.  If a customer were cleaning parts with small threaded blind holes, and these holes happened to rest in-between the standing waves, there is a good possibility that this area will not be consistently cleaned.  This is primarily the reason why 25 kHz ultrasonic systems are no longer as common as 40 kHz or higher operational frequencies.  Even if parts are positioned perfectly in a basket, it is impossible to guarantee an evenly cleaned part in a 25 kHz ultrasonic cleaner.

Although the 25 kHz frequency produces a less evenly distributed cleaning effect, the scrubbing action that is produced is very powerful, which can be positive or negative.  The power is of benefit when cleaning heavy parts, such as a plastic injection mold weighing 2000 pounds, or cleaning those parts with highly-bonded contaminants, such as burned carbon.  The power is destructive when cleaning sensitive components, such as aluminum or soft metals, or those parts with finely-machined detail.

25kHz system may also remove thicker contaminant layers faster than higher frequencies.  Since 25kHz ultrasonic systems essentially "drill holes" on the surface of the part with a large amount of power, this frequency tends to remove thick layers of contaminants in large chunks, while higher frequencies remove the same contaminants by attempting to gently remove layer by layer.  As such, 25kHz systems may be more effective when removing paint, thick greases or coatings, or other similar contaminants.

25kHz ultrasonic systems are also very noisy during operation since the sub-harmonics are within the upper range of human hearing.  The sound is a high-powered squeal which is highly disturbing to many individuals.  Hearing protection will more-than-likely be required when operating a 25kHz ultrasonic cleaner, unless it is acoustically insulated.

25kHz ultrasonic cleaners are also highly-destructive to the cleaning tank itself.  Cavitational Tank Erosion is the slow deterioration of the cleaning tank at the transducer mounting locations.  From the very first day of operation, molecules of stainless steel directly above the transducers are eroded away.  Eventually, the tank will become porous, allowing liquid to enter the transducer area, and destroying the system permanently.  The lower the operational frequency, the faster this erosion will occur.  Acidic cleaning agents will drastically increase the rate of tank erosion. 

The photo at the right shows what cavitational tank erosion looks like on the bottom of a tank only 1 year old.  As you can see, the polish is completely removed, and the surface is being slowly destroyed.

Some manufacturers of Magnetostrictive Ultrasonic Transducers mount their transducers to tanks which are over 3/8" thick to increase the life of the tank.  However, there is a problem with this design.  The energy being emitted from the transducer must first pass through 3/8" of material before entering the cleaning fluid, which significantly reduces the effective power in the tank.  These systems are VERY loud, since the tank is being vibrated rather than the fluid in the tank, but sound is not an indication of power in an ultrasonic system.

Today, Zenith includes 25 kHz systems when higher frequencies fail to successfully clean a part, or when a potential customer must use a machine which was approved decades ago, before the advent of high frequency ultrasonic systems.  Many large, older firms developed standards which require the use of low-frequency ultrasonic systems.  Changing standards can be a very large undertaking, and rather than change standards, these firms purchase low frequency ultrasonic cleaners which have always worked in the past.  However, one must consider that it is very rare that a 25kHz system will clean an object, and a 40kHz system will not.  As such, Zenith will recommend 40kHz even for those applications that are typically the domain of 40kHz. 

When to Use 25kHz Ultrasonic Cleaners:

• When Cleaning Large and Heavy Parts with Little Detail.
• When Removing Highly-bonded Contaminants.
• If 25kHz is Required due to Standards Written Long Ago.
• When Higher Frequencies Fail to Clean a Part.
• When Cleaning Thick Contaminants Layers.

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