If you work around ultrasonic cleaning tanks you have probably heard a tank “squeal.” If you’ve never heard this, (1) consider yourself lucky and (2) be aware that a “squeal” is not the normal hissing sound associated with an operating ultrasonic tank but, rather, an extremely high amplitude sound at an audible frequency that can be heard several rooms or even floors away from the source. In short, if an ultrasonic tank squeals, you KNOW it! The phenomenon is well-known and, to my knowledge, no ultrasonic tank is immune although it seems more prevalent in larger tanks and at lower ultrasonic frequencies – below 100kHz. Despite the best efforts of the most knowledgeable people in the business, the reason for and source of the annoying sound eludes explanation or a sure means of prevention. What we do know is that tank squealing seems related to certain conditions, the avoidance of which may or may not prevent squealing. Be aware that the following is anecdotal based on collected reported observations, not science!
The likelihood of tank squealing seems high in a freshly prepared tank once cold water and chemistry have been added and the tank is in the process of being heated to the desired temperature for use. Applying ultrasonic energy during this period often initiates squealing. Squealing has also been seen when a large, cold part is introduced into a heated cleaning tank. Both of these point to some connection between temperature stratifications and squealing.
Stirring a tank during initial heat-up or following a period of non-use may trigger squealing. Stirring might be caused by a re-circulating filter loop, the introduction of a cleaning load or any other disturbance that causes turbulence or a shearing action in the liquid.
Some cleaning chemistries seem to be more prone to squealing than others. My personal observation is that chemistry containing c0-solvents or ones that are formulated as emulsions of a solvent with emulsifiers in water are more prone to squealing. Also, squealing seems more likely when a powdered chemistry is not thoroughly dissolved.
There have been alleged associations between tank geometry and the likelihood of squealing. Certain depth to width and length ratios have been theorized to trigger squealing, notably in tanks that are near-perfect cubes although no combination of dimensions seems immune. One suggested solution has been to slightly angle tank walls and bottoms to avoid parallel and/or perpendicular surfaces. To my knowledge this has never been definitively explored.
It’s hard to argue with the notion that squealing is caused by something resonating. It could be the tank structure, constructive interference of sound waves due to displacement of the sound waves by liquid motion or, as some have theorized, the excitation of resonant cavitation bubbles in a state of stable cavitation. These theories seem plausible but neither has been identified as the definitive “smoking gun.”
The good thing is that squealing is normally temporary and not as likely to happen in a tank in constant use. Should an ultrasonic tank squeal, the obvious first step is to turn off the ultrasonic source – it’s likely you won’t need an explicit invitation to do this! Try to identify if any of the possible causative actions listed above were co-incident with the onset of squealing. That may allow prevention of a recurrence next time. Heat the tank to a temperature slightly above the desired operating temperature and stir it to eliminate temperature stratifications – temperatures may vary considerably in a stagnant tank. Turn on the ultrasonics and wait for normal degassing to occur without disturbing the tank. Degassing may be immediate or may take considerable time depending on the tank size and temperature. Once the tank has stabilized and degassed, begin normal production.
Finally, I would like to ask your assistance in further understanding the cause and solutions for tank squealing. If you have experienced this condition and especially if you have managed to find a way to prevent it in a particular situation, I would appreciate your input either as a comment here or directly to my email which is firstname.lastname@example.org. Maybe we can work together to get to the bottom of this. I will post updates in future blogs. Thanks!