Ultrasonics – Revisiting Watts Per Gallon

Watts per gallon as a measure of ultrasonic cleaning tank capability is under growing scrutiny.  In a series of blogs over the next several weeks, I hope to explore the origin of watts per gallon and the changes and ambiguities which have brought its continued use as a measure of ultrasonic capability into question.  Also, if watts per gallon is no longer a meaningful “yardstick” what might replace it?  Is there a more meaningful and appropriate way to measure power in an ultrasonic tank and, if so, what is it?  It seems there are lots of questions but no obvious answers regarding the use of watts per gallon as a unit of measure for ultrasonic tank capability.

The war of watts in ultrasonic cleaning has been raging since the very beginning of ultrasonic cleaning history in the 1950’s.  It was during this period of time, I suspect, that the first battle of the war began with each of a handful of ultrasonic manufacturers touting the power of their generators as an indication of the effectiveness of their ultrasonic cleaning tanks.  It was a fairly level battleground as most ultrasonic cleaning tanks were only a few gallons in size.  Watts per gallon became the accepted way to “rate” ultrasonic cleaning tanks.  In the beginning, the limiting factor was the amount of ultrasonic power that could be produced by ultrasonic generators which at that time used tube technology.  Semiconductors had not yet come on the scene.  Tubes, in general, were challenged to provide huge amounts of power at ultrasonic frequencies.  Magnetostrictive transducers of that era, however, were quite inefficient and forgiving and could accommodate much more power input than could be provided by tube type generators.  So, the battle of watts in the beginning, I believe, was all about generators.  Powerful generators meant powerful ultrasonic activity and superior cleaning results.

Todays ultrasonic cleaning systems are far advanced from those of the 1950’s.  Now, nearly all ultrasonic generators utilize semiconductor technology which is capable of delivering huge amounts of power at ultrasonic frequencies without much difficulty.  Most ultrasonic transducers are of the piezoelectric type which, although highly efficient, are, typically, rated for an input power of around 40 watts per transducer element.  This has led to the ultrasonic transducers being the first limiting link in the ultrasonic chain.  Meanwhile, many users of ultrasonic systems still view watts per gallon as the ultimate measure of ultrasonic cleaning tank effectiveness.  This results in an interesting conundrum.

In a effort to satisfy customer hunger for watts per gallon, there continues to be a seemingly irresistible temptation on the part of ultrasonic manufacturers to add more and more transducers to their cleaning tanks to accommodate more and more ultrasonic power.  However, there are certain laws of physics that, if ignored, can lead to diminishing results as transducer populations become more dense.  Maybe more power is better, but, remember, only the power that is effectively delivered in to the cleaning tank as ultrasound that creates cavitation bubbles that implode  has any bearing on cleaning.  Therein lies the root of the second and perhaps more difficult to overcome limiting link in the quest of more ultrasonic power intensity.

In short, placement of ultrasonic transducers must be done in such a way that each transducer works in harmony with neighboring transducers.  This dictates specific spacing between transducers for optimum results.  Without proper transducer spacing, the result is much akin to a four wheel drive vehicle with one wheel operating in the wrong direction.  Although more power (watts) is consumed, the resulting travel distance (ultrasonic cleaning) is diminished as the one wheel operating in opposition to the others consumes power with negative results.  Also, as excess power is delivered, an effect analogous to “spinning the wheels” occurs as the transducer looses traction with the load, so to speak.  At large transducer amplitudes, connection between the transducer (tire) and liquid in the cleaning tank (road) is lost which can result in a reduction or total loss of ultrasonic energy being delivered to the cleaning tank.

So, it may no longer be a matter of watts per gallon.  Stay tuned as we discuss the above and other issues as they relate to the evaluation of ultrasonic cleaning system effectiveness.

–  FJF  –