Reader Questions – Monitoring Ultrasonic Transducers

Today’s blog is responding to two questions asked by readers.   Both concern monitoring of ultrasonic transducer performance.

Ralph Clendenin asks – “Is there a measurement of the frequency I need to be aware of during “good” operation I can check monthly that would show reduction in cleaning due to
change in frequency?”

Stephen Sharkey asks – “We have experienced failures of transducers coming away from the bottom of the cleaning tank. The transducer can still be operating, with less sonic power in the cleaning tank. How would you monitor this?”

These are excellent and very pertinent questions indeed!  If I were responsible for maintaining an ultrasonic cleaning process, I would be asking these questions too.  Unfortunately, the answer isn’t as simple as it sounds.

Frequency –

In answer to Mr. Clendenin’s question – The ultrasonic frequency is determined primarily by the ultrasonic generator which is factory pre-adjusted to the frequency of the transducers being used.  Some generators also have a means of automatically adjusting frequency to maximize output and performance based on feedback from the transducer of its impedance.  The optimal frequency can change, although not radically, depending on expansion and contraction of the transducer due to temperature changes and other factors including tank loading.

Mapping the impedance of a transducer can be used to indicate its optimal operating frequency.  In the days of single frequency ultrasonic units, monitoring frequency was a bit easier than it is today although it was never “easy” due to the voltages and waveform.  Today, most ultrasonic generators incorporate “frequency sweep” which “warbles” the frequency around a center frequency using a number of different algorithms and wave forms which vary from manufacturer to manufacturer.  Since the frequency is always changing, frequency measurement using relatively simple devices such as a frequency meter is somewhat difficult.  It really takes a wave analyzer and considerable knowledge of how to use it, which is beyond the scope of this blog.  Fortunately, with today’s digital technology, frequency is one variable that is not likely to change and cause poor ultrasonic performance.  Phase Locked Loop (PLL) electronics are pretty solid when it comes to frequency.  So the simple answer is, “I wouldn’t worry about frequency.”

Transducer “De-bonding” –

Mr. Sharkey’s question is directed at the same goal from a little different angle.  He’s stating that he has actually had problems with transducers becoming “de-bonded” from the bottom of a cleaning tank.  Today’s ultrasonic transducers are usually bonded to the cleaning tank using adhesive technologies.  The bonding process requires meticulous control in manufacturing and, unfortunately, things can go wrong.  Realizing this, and the fact that failure of the adhesive bond is unpredictable and gradual over time, most manufacturers offer some guarantee of the transducer bonding.  Determining if one or more transducers have become de-bonded from the tank can be difficult.

CAUTION – The following should ONLY be performed by persons knowledgeable about electrical equipment, with the power off and the unit disconnected from the power source.  ALSO, since ultrasonic transducers (like a capacitor) can retain a charge, the power side of the transducer wiring must be shorted to ground prior to your probing around in there to assure that a potentially dangerous (as in DEATH) charge does not remain.

If there is access that allows you to see the actual transducers, it is easy enough to look under the tank and give them each a wiggle to see if any of them are loose.   This method will detect a fully de-bonded transducer but will not always detect a partially de-bonded transducer.  Partial de-bonding will also result in poor ultrasonic delivery to the cleaning tank.

A bit stumped for a good answer otherwise, I took a stroll to the Blackstone-NEY Reasearch and Development Lab which is down the hall from my office and asked the question of the ultimate gurus (I’m just a sub-guru compared to them).  After a little head-scratching on their part, the following emerged.

CAUTION – The blog or Cleaning Technologies Group will not be held responsible for any damage that may result from the following procedure!  Check with the manufacturer of the unit if you have concerns.  In no case should you operate the ultrasonic unit for longer than one minute at a time with the liquid level lowered as described.

If you lower the liquid level in the tank down so that there is only about 1/2 inch of liquid over the transducers, the transducer pattern should be easily be seen as ripples on the surface of the liquid.  There should be a regular pattern as shown in the tank on the left below.

Showing transducer performance by operating with reduced liquid level.

In an ultrasonic cleaning tank operating with reduced liquid level, the ripples on the surface of the liquid on the left clearly show the pattern of the transducers on the bottom of the tank. The ripples on the surface of the liquid in the tank on the right show a non-uniform pattern due to de-bonding failure of the transducer in the position marked by an X.

Since different manufacturers use different patterns, the pattern in your tank may not appear exactly like the one above but should show an even coverage of ripples even if the spacing is irregular.  A fully or partially de-bonded transducer will result in ripples either not being present or inconsistent with the pattern in the area of that transducer.  R&D assures me that this is a pretty reliable test although not infallible.

I’m sure the above topics will become an ongoing discussion for this blog.  Also, if readers have any suggestions or experience regarding transducer monitoring, please feel free to share via the “Comments” section below.

–  FJF  –

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5 Responses to Reader Questions – Monitoring Ultrasonic Transducers

  1. peterfirefly says:

    Thank you!

  2. John Fuchs says:

    Yes, cavitation erosion will, inevitably, add particles to the liquid solution in the tank. This addition of particles, however, occurs at an extremely slow rate and, usually, is not a consideration in the overall cleaning process. I say “usually” because some users of ultrasonics in critical applications utilize a lengthy “start up” procedure with new ultrasonic tanks to allow the initial burst of particle generation to stabilize. Yes, this is a possible argument for enhanced rinsing techniques. Spray rinses, however, may also generate particles as pressurized liquid passes through orifices at high velocity. FJF

  3. peterfirefly says:

    Doesn’t the erosion of the tank cause particle problems — or are they easy to remove in the rinse phase?

    Wouldn’t they usually be paramagnetic? If I understand your post on magnetic particles right, that means a spray rinse would be a good idea, right?

  4. John Fuchs says:

    Scott –
    It sounds like your question relates to the erosion pattern on the bottom of the tank rather than to the ripple pattern seen when a tank is operated with reduced liquid level. The fact that you have had the ultrasonic unit for a relatively long period of time indicates that you will, probably, be seeing tank bottom erosion by now. This is common and unavoidable but does not tell us the same things as the liquid ripple pattern. Erosion of the stainless steel tank bottom is caused by ultrasonic cavitation at the interface between the liquid and the tank bottom. This pattern, although often well-defined really can’t be related to transducer operation or bonding. Try again using a lowered liquid level as described in the blog and look for the pattern in the ripples on top of the liquid and let me know what you see.

  5. Scott Wang says:

    This is a re-post of a question received by email –

    How do you do!
    This is Scott Wang, from CTG Asia. I hope get some edification from you for one pattern.
    Here in China we have one GMC running near two years. I saw many patterns like below picture on the bottom of tank.

    My questions are:
    1, Why the patterns look like such strange (foot print)? As I understand, it should be as some concentric circles.
    2, Some pattern is very clear, does it mean this transducer work very well? (On the other hand, if not clear, does it mean transducers’ working were not so well)

    Thanks and Best Regards

    Scott Wang

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