Are Ultrasonic Power Ratings In Watts Meaningful?

Watts of ultrasonic generator output power is one metric often used in the comparison of ultrasonic cleaning systems.  In a previous blog, I discussed the potential foibles of using watts of consumption as a measure of comparison as there is no standard for measuring the output of an ultrasonic generator.  But there are also other potential flaws in using the ultrasonic generator output power as an indicator of ultrasonic capability.

From the time of Thomas Edison, watts of power consumption has been the standard for the specification of a light bulb.  This started, I have heard, because Mr. Edison found watts a useful means of quickly determining the size of the power source (his major interest as the supplier of these devices) needed to equip a building or other space with electric light.  Little or no concern was given to the actual luminosity (in candlepower) provided as watts and brightness pretty much went hand in hand – more watts meant brighter light and a bigger sale for Mr. Edison.  He sold watts, not candles.

Mandates to conserve energy resources in the late 1900’s led to a demand to reduce electrical consumption for lighting.    The first step in this direction was the development of the quartz light bulb.  With its increased efficiency, a quartz bulb with a power consumption of just over 80 watts of electricity would produce about the same amount of light as a standard incandescent bulb that consumed 100 watts.  The next development was the introduction of compact fluorescent bulbs which consumed just over 20 watts to produce the same amount of light as a 100 watt incandescent light bulb.  Finally, with the introduction of the LED bulb, a bulb that consumed only  7 to 9 watts of electrical energy provided the same amount of light as a 23 watt CFL, an 83 watt quartz bulb or a 100 watt conventional incandescent bulb, things started to get crazy.  Clearly the link between watts and light output had been broken.  Because of the increasing disparity between watts consumption and the amount of light that a bulb would produce, there was an attempt to begin rating light bulbs based on light output (lumens) rather than watts.  This effort flopped.  Watts were so fully ingrained in the consumer culture that manufacturers of the new LED bulbs soon took to rating them at the watts equivalent – ie. “replaces 40 watt or 60 watt or 100 watt light bulb.”  This took advantage of the old, traditional standards that had persisted from the time of Thomas Edison and were more satisfying to the consumer.

The above is an interesting narrative but we are not here to talk about light bulbs.  Let’s segue to ultrasonic cleaning systems.  It is true that ultrasonic generators have become more efficient over the past several decades and are able to produce more watts output with less energy consumption.  But power ratings of ultrasonic cleaning systems are based more on generator output.  In the final analysis, however, it is not watts of energy produced by the ultrasonic generator or the consumption of the ultrasonic transducer that counts.  The real focus should be on what those watts do.

In the 1950’s, ultrasonic transducers were magnetostrictive.  Ultrasonic vibrations were provided by a coil of wire wrapped around magnetostrictive materials which vibrated in the presence of an oscillating magnetic field.  In most cases, these devices were powered by vacuum tube generators and were rarely more than 20% efficient.  Eighty percent of the power delivered by the generator never made it into the cleaning liquid having been dissipated as heat and in energy conversion losses.  The first step forward was the first practical piezoelectric ultrasonic transducer which could boast an efficiency of nearly 50%, a far cry from the most efficient transducers today which may reach efficiencies in the high ninety percent range.  Ultrasonic transducers have undergone a metamorphosis not unlike that of the light bulb.  The trail doesn’t end with the electrical to mechanical energy conversion of the transducer!  It is becoming obvious that transducer spacing, attachment, orientation and a number of other factors also have a significant effect on overall ultrasonic performance.  The link between power consumption and performance should be starting to crumble.  Yet, much like in the case of the light bulb, watts of power consumption is still seen as the definitive measure of capability.

Do watts of power produced by the generator or consumed by the transducer matter?  Maybe, but there is a lot more to consider.  It also depends on how you use those watts and how efficiently they are applied.  Staying in the realm of sound, I am led to the example of the Bose Wave Sound System.  If you’ve never heard one, you should!  It’s like the sound of a movie theater sound system coming out of something only slightly larger than a shoe box.  In the Bose system it is not the amount of audio power the device is capable of (the specifications say the power consumption is “60 watts max”) that makes the difference.  The difference is how those watts are applied.  Bose has revolutionized speaker technology to give the most impressive sound using the least amount of power and in the smallest package possible (so far).  It’s interesting that on searching everything I could find about Bose the specifications never mention the audio output power.  There is no need to – – the sound speaks for itself.  The performance of the Bose system is based on performance, not watts.  Do watts of ultrasonic power really matter if the system does the job?

 –  JF  –

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