Blog Purpose

The CTG Technical Blog is intended as a source of information on subjects related to industrial and precision cleaning technology. The writer of the blog, John Fuchs, has 40+ years of experience covering the entire gamut of cleaning. Mr. Fuchs has extensive knowledge of ultrasonic cleaning technology having been employed by Blackstone-Ney ultrasonics and its predecessors since 1968. The blog is also intended to serve as a forum for discussion of subjects related to cleaning technology. Questions directed to the blog will be responded to either in the blog (if the topic has general interest) or directly by email. Emails with questions about the current blog should be entered in the comments section below. Off-topic questions related to cleaning may be sent to

A Little About RMS (Root Mean Squared)

July 10th, 2015

Most of us are aware that the power we get from the power line coming into our house or factory comes as alternating current.  Basically, the voltage on the “hot” wire as referenced to ground varies, going negative then positive 60 times each second (60 Hz).

AC vs DC

The benefit and compelling argument for using alternating current to transmit electrical energy is that it is relatively easy to change the voltage of the alternating current using transformers (which can not be done with direct current).  The ability to convert voltage makes it possible to distribute large amounts of electrical power over great distances using conductors of practical size.

Note – The amount of current (amps) that a wire or other conductor can carry without a significant loss of power is determined by its material of construction, size (gauge) and length.  Long distance power transmission lines operate at very high voltage to transmit the same amount of power with smaller gauge wires than would be required at a lower voltage.

The RMS voltage of any waveform will produce the same heating effect on a resistive load as if the same direct current voltage were applied to the same resistive load.  RMS stands for Root, Mean, Squared.  To understand this better, let’s start with the portion of the sinusoidal wave where the voltage is positive.  To find the RMS voltage, we would take the voltage at every point and, first, calculate their square roots.  Having done this for every point, we would add all of these square roots together and then divide by the total number of points.  Finally, we would take the average of the square roots and square it to get the RMS voltage.  Of course, this can be done using some simple calculus but I’ll leave that for the math geeks – concept is what we’re going for here.  The result of all this effort will give us the RMS voltage.  One should not confuse RMS with average voltage which, except for a few special cases, is somewhat skewed from taking the simple average of the voltage over time by the fact that we are going for power, not really voltage, and power includes a squared factor (P=I²R).  In the diagram below, the blue area (representing the sinusoidal voltage) will produce the same heating effect in a resistive load as the yellow area which represents the RMS voltage.

RMS Positive Cycle

Now let’s look at what happens when the sinusoidal voltage goes negative – this is a bit more tricky.  The square root of a negative number is, well, difficult.  As we all know, if you multiply a negative number by a negative number, the result is always positive – - almost always.  Through the magic of mathematics, there are imaginary numbers which are represented by the addition of an i to a number so that, for example, the square root of -2 is 1.414i .  Squaring 1.414i gives you 2, not -2.  As a result, the RMS value remains positive despite the fact that the voltage is negative for 1/2 of each cycle.  This makes sense as the heating effect on a resistor is the same no matter which way the current is flowing.

RMS Both Cycles

Finding the RMS voltage of a sinusoidal wave signal is easy because some pretty simple rules apply.  The RMS voltage of a sinusoidal is always the peak voltage divided by the square root of 2 (1.414).  Yes, this means that the peak voltage of 120 volt RMS power is around 170 volts and that the peak to peak (maximum positive to maximum negative) is 340 volts.  And guess what, 340 divided by the square root of 2 is 240!  But that’s another blog.

Most of us will never encounter the need to determine the RMS voltage of a waveform other than one that is sinusoidal.  But it is important to remember that electrical energy can come in many wave shapes.  The simple rules above ONLY apply if the voltage is sinusoidal.  In order to accurately determine the RMS voltage of shapes other than those that are sinusoidal (square waves, sawtooth waves and ramp functions, for example) requires different math.  Most common meters accurately measure RMS voltage ONLY if the signal is sinusoidal!

-  FJF  -


Ultrasonics – The Trouble With Watts – Part 2

June 26th, 2015

As discussed in a previous blog, one problem with watts is that watts do not equal energy.  Let’s carry that thought a little further again using the vehicle analogy I put forth in the initial blog of this series.

In the ultrasonic world it is common to associate the effectiveness of an ultrasonic cleaning tank its rate of energy consumption (Watts)  either from the power source or in the delivery line to the transducer.  Preceding blogs illustrate (in part) why this association is basically flawed from a strictly physical standpoint.  But the problem goes even deeper.  Imagine a vehicle traveling from point A to point B.  How do you determine how much gasoline it will take to accomplish this?  The “goal” is to move the vehicle from point A to point B.  Using the laws of physics, we can easily determine how much energy this will take.  The amount of energy required to accomplish this task is a constant and never changes.  To move the vehicle from A to B requires applying a force over distance.  This is the actual amount of energy (gasoline) needed and requires adding the energy required to overcoming inertia to start the motion (acceleration), the energy required to overcome the inertia of the moving object once the destination is reached (braking) as well as static and moving friction as the vehicle is moved (surface roughness, bearings, etc.) all in addition to that constant amount of energy that is required to move from point A to point B.  All of these things are pretty straight-forward and stand to reason.

Just for fun, let’s throw in some other grantedly rather absurd but possible variables which could have a significant effect on the amount of energy required to move the vehicle from point A to point B.  Let’s say, for example, that this is a four-wheeled vehicle with four wheel drive but that one of the wheels is geared to rotate opposite to the direction of the other 3.  It is not difficult to imagine, in this case, that more energy would be required to move from point A to point B because energy delivered to the counter-rotating wheel is totally lost and, in fact, results in the other three wheels expending more energy than they would otherwise.  In another scenario, let’s imagine that the road surface is icy.  Now the vehicle may not move at all no matter how much energy is expended.  In summary, it becomes obvious that the amount of energy required to move a vehicle from point A to point B can vary widely depending on a number of other conditions.  Under ideal conditions, all of the energy delivered should contribute to the desired result but that is seldom the case.

And yet in the ultrasonic world, we seem hung up on watts as a way to express energy in an ultrasonic cleaning tank no matter how inappropriate that might be.  In fact, the only energy that really counts is that released as cavitation bubbles implode near the surface to be cleaned.  So, what happens to all that energy represented (as it is) in watts that doesn’t find its way to the site where cleaning is actually taking place?  We’ll take that up in an upcoming blog.

-  FJF  -

Ultrasonic “Shadowing”

June 22nd, 2015

We in the ultrasonics industry have long been aware of an effect which is sometimes called “ultrasonic shadowing.”  In general, this is what happens when parts being cleaned are positioned in such a way that parts cast an “ultrasonic shadow” which prevents parts in the shadow from being effectively cleaned.  This phenomenon, although we know it exists is quite difficult to characterize and predict.

Ultrasonic shadowing does not produce a distinct shadow as the case when an opaque object casts a shadow from a distinct light source like the sun or a bare light bulb.  The following illustration will help clarify this point.

An object will cast a more distinct shadow on a bright, sunny day.

An object will cast a more distinct shadow on a bright, sunny day.  A larger object will produce a darker shadow on a cloudy day as more of the diffused light is blocked.

On a bright, sunny day, an opaque object held between the sun and a surface such as a sidewalk will cast a distinctly outlined shadow of the object.  On a cloudy day, however, where the sunlight is distributed and diffused by a layer of clouds (for example), the outline of the shadow becomes less distinct as the light from a distributed source is able to illuminate what would be the shadow from a more concentrated light source such as the sun.  Even in the cloudy day scenario, however, if the object casting the shadow is large enough, there will be a dark area where even the diffused light can not reach.  The ultrasonic shadow is more like that produced on a cloudy day.  This is because the ultrasonic energy source is distributed like the light on a cloudy day and not a point source like the sun.  Even if the ultrasonic energy was delivered from a single point, there would likely be a distribution due to reflections and general diffusion of the energy produced by the transmitting liquid.

The other effect that tends to “temper” the sharpness of ultrasonic shadowing is the fact that, for the most part, items being cleaned are at least somewhat transparent to ultrasonic energy.  It can be easily demonstrated that a thin plate of stainless steel held between an ultrasonic source and an object being ultrasonically cleaned has very little effect on the transmission of ultrasonic waves.  That is not to say that this effect will go on indefinitely.  As more thin plates are added, more and more ultrasonic energy will be lost to the slight attenuation in each plate and also to reflections between the plates.  A similar effect can be seen if a light source is obscured by several sheets of glass that are separated from one another.  Eventually, all of the light is reflected away.

The “take away” here is that although ultrasonic shadowing is real and does have an effect on the strength of the ultrasonic field, it is not a sharply defined phenomenon.  Racking of multiple parts for ultrasonic cleaning is possible even if one part is partially “hidden” from the ultrasonic source.  At some point, however, there will be a reduction of the ultrasonic field and subsequent cleaning effect as part density becomes more than the ultrasonic energy can overcome.  The best way to determine if cleaning problems are related to ultrasonic shadowing either by the number of parts or as an effect of the fixture is to try cleaning a single part (hung on a wire, for example) in the tank.  If it is possible to clean a single part but not a full load of racked parts then there is a possibility that ultrasonic shadowing may be the cause.

- FJF  -

The Trouble With Watts

June 8th, 2015

Some time ago I wrote a paper entitled “What is a Watt.”  Although this paper seems lost in history (I can not find a copy of it), I can remember the point that I was trying to make when I wrote it.  In essence, it made the case that a watt is only an instantaneous measure of the rate of energy development or consumption (power), not a measurement of overall energy.  Part of the problem is the fact that “Watt” is a somewhat unique unit of measure.  A watt equals a rate of energy consumption or generation of 1 joule of energy per second.  If you substitute joules per second for watts, it is easy to see that watts does not equal energy (joules) but, rather, a rate of generation or consumption of energy (joules).  You need to integrate the rate of energy consumption or generation over the amount of time the energy is applied to achieve a measure of accumulated energy.  I say that a watt is unique because in most cases the rate of achieving a total quantity is defined as the amount of that quantity per unit of time  eg. miles per hour.  An example? – There is no unit that I know of that is the equivalent of miles per hour.  The closest is the knot which is one nautical mile per hour.  Using a reverse analogy, it might be simpler if we adopted the term Andretti as the equivalent of one mile per hour. Our speedometers would be marked in Andrettis, not miles per hour. In simple terms, Andretti’s would be to miles as watts are to joules.

Watts can be related to miles per hour as the rate of travel but not the total distance covered.  Miles per hour, for example, is not an appropriate response to the question, “How far is it from Detroit to Cleveland.”  Similarly, watts, by itself, is not an appropriate answer to, “How much ultrasonic energy is there in this ultrasonic tank.”  The dimension of time is missing.  Think of it this way – - If you were on your way from Detroit to Cleveland you could drive at a constant speed of 65 miles per hour or, you could drive at a speed of 80 miles per hour from Detroit to Toledo (60 miles) and then at a speed of 45 miles per hour from Toledo to Cleveland (118 miles) on the snow-covered Ohio Turnpike.  You could boast that you had traveled at 80 miles per hour in the course of your trip but, in truth, you had really only travelled at a speed of 52.8 miles per hour overall for an accumulated distance of 178 miles in 3.37 hours.  If you had travelled at a continuous 80 miles per hour for the entire trip, you would have been at your destination in 2.23 hours.  In summary, miles per hour does not describe the distance travelled any more than watts describes the amount of energy in an ultrasonic cleaning tank.

Sometimes speed is important while at others, it is distance that counts.  Although both move by moving their legs, a sprinter wants speed while a hiker wants distance.  In the ultrasonic world, any power in a liquid above that required to produce cavitation at a given frequency will provide cleaning.  A higher rate of delivering power (watts) to the liquid will result in larger numbers of more energetic cavitation bubble collapses and could result in faster and, perhaps better, cleaning but not always as we will see in upcoming blogs. 

-  FJF  -

Ultrasonic Performance – Ceramic Ring Test – Post Script

June 5th, 2015

I usually try to make sure I’ve done my research prior to posting a blog.  As I was thinking about the post about the ceramic ring test, however, it occurred to me that there are a couple of variables that I had not previously considered – - the pencil and the means of applying the markings to the rings.  So I decided to do a little research.

On researching pencils, I found that there is no unified specification for pencils.  There are some guidelines the most common of which are the European Scale and the American scale.  The following illustration shows how wide the range of pencil hardness can be.

Pencils vary widely in hardness and blackness.  The American scale goes from a #1 to a #4.  The rest of the world uses a much expanded scale.  A pencil of a particular hardness made by one company can not be reliably compared to a pencil of the same hardness made by another company.

Pencils vary widely in hardness and blackness. The American scale goes from a #1 to a #4. The rest of the world uses a much expanded scale. A pencil of a particular hardness made by one company can not be reliably compared to a pencil of the same hardness made by another company.

In most cases, pencils are rated in “hardness (H)” and “blackness (B).”  Individual manufacturers set their own standards and there is no reliable “conversion chart” – - note that the numbers have been “penciled in” in the above.

The “lead” in a pencil is not really lead (of course) but, in most common wood encased pencils, is a mixture of graphite and clay and in some cases “grease” in varying amounts.  The mixture is then molded and fired to harden.  In an internet search, I discovered that there are also other materials which may be added to the mixture to provide certain properties.  Any additive would, it seems, alter the properties of the pencil marking as it is applied to ceramic making it easier or more difficult to remove.  One that was specifically mentioned was wax.  I’m pretty sure that wax would make the pencil marking more difficult to remove.  Also, polymers are often used in the lead for mechanical pencils to add hardness and durability to limit breaking of the thin lead.  I don’t know if a polymer would make the pencil marking easier or more difficult to remove.

The other variable(s) would be speed and pressure.  I’m sure the ceramic ring test was first applied by simply scribbling on the ring by hand.  I have, however, encountered a number of home-brew contraptions that been used to speed up the tedious process of scribbling.  One I remember had a little platform on a spinning motor to hold the ceramic ring while the pencil was drawn over its face (a little like the track on a record.)  In this case, the high speed may have caused heat which would also change the properties of the deposit on the ring.  Varying pressure might also change the properties of the deposit left on the ceramic ring making it easier or more difficult to remove

In summary, despite the efforts to keep the surface of the ceramic rings consistent, keeping the process of depositing graphite on the ring may not be as controllable as one might think.  In fact, there is little or no assurance that a #2 pencil supplied by the same manufacturer in two different (or even the same) lots will be the same.  I don’t mean to debunk the entire test, but the above variations have not, as far as I know, been investigated.  They may make a difference or they may not.  The best advice at this point would be to try to keep things as consistent as possible.  I know I have violated that rule many times as I pulled out my mechanical pencil with an HB lead to scribble on rings.  Knowing what I know (or don’t know) now, I’ll try to be more careful in the future.

-  FJF  -

Ultrasonic Performance – Ceramic Ring Test

June 4th, 2015

Other than to say that there is no perfect, infallible way, the blog has not addressed assessing ultrasonic performance.  In an earlier blog, I did suggest that any of many methods are appropriate to compare ultrasonic performance of an ultrasonic system from day to day but that no further inference should be made.

One test that’s been around for a long time is the “ceramic ring test.”  In this test, ceramic rings with a particular surface finish are “contaminated” with pencil lead using a standard #2 pencil.  The contaminated rings are immersed in the ultrasonic cleaner under test under specified conditions for a specified time.  Performance is judged by the amount of pencil lead remaining on the surface of the ceramic after ultrasonic exposure.  The cleaner the rings, the better the performance.  The good thing about this test is that it does actually reflect ultrasonic cleaning performance.  Unless ultrasonic cavitation and implosion are present, the pencil lead is not removed.  As proof of this, a contaminated ring can be processed through several cycles using a commercial dish washer without seeing an appreciable removal of the pencil lead.  Also, most standard cleaning chemicals alone will not remove the lead.  The problem is that small changes in procedure can result in significant changes in results.  Although the instructions for the procedure are quite specific, there are still some unmentioned variables that may contribute to inaccurate results.

  • The test temperature should be not only achieved but stabilized.  An ultrasonic cleaning tank that is increasing in temperature immediately prior to or during the performance of the test will not perform as well as one that has been stabilized at the test temperature for at least 15 minutes prior to the test.  The reason for this variance is that as long as the temperature of a liquid is increasing, degassing continues to progress.  A liquid that is degassed at one temperature will no longer be degassed if the temperature is increased.  The best way to assure total degassing is to heat the liquid to a temperature somewhat above the test temperature and then turn off the heaters and let it cool down to the test temperature before proceeding.
  • Turbulation in the tank may also cause poorer results.  Re-circulating pumps, filters, spargers, agitators and any other feature that may disrupt the ultrasonic field should be turned off during testing.  A slight up and down movement of the basket as suggested in the instructions is beneficial to achieving consistent results.
  • The ceramic ring test has been shown to be frequency sensitive.  In general, results at frequencies of 20 to 60 kilohertz are meaningful.  Above these frequencies, less pencil lead is removed from the rings.  This is not to say that the ultrasonic cleaner at higher frequencies is not performing well but, rather, that the task of removing pencil lead from a ceramic ring is best achieved at a relatively low ultrasonic frequency.  Higher frequency ultrasonic and megasonic systems should not be judged using the ceramic ring test.

Probably the biggest problem with the ceramic ring test is that evaluation of the rings is very subjective.  Different people will score rings differently.  To help minimize this problem, it is suggested that one person (or a coordinated team) should be assigned to evaluate all rings.

The ceramic ring test is only one of many tools that can be used to judge ultrasonic performance of the same ultrasonic tank from day to day.  It should not, however, be used to replace periodic evaluation of actual part cleanliness using other means.

-  FJF  -

The Trouble With Watts – Efficiency

June 2nd, 2015

One of the problems with using the number of watts consumed to produce a particular output of another form of energy (light, motion, heat, etc) is that there are always losses when one form of energy is converted to another.  Although the law of conservation of energy always applies, energy lost in conversion to other forms of energy is not always obvious.  Let’s consider devices that convert electrical energy into light as an example.

There is no better example of the trouble with watts than we see in the lighting industry today.  When Thomas Edison invented the first practical incandescent light bulb, he rated his bulbs in candlepower.  In his wisdom, he knew that the “deliverable” was the amount of light produced.  Also in his wisdom, however, he knew that the customer would use a certain amount of energy to produce that light – - and guess who provided the energy.  He quoted new lighting installations based on so many light fixtures each providing the equivalent light of so many candles as it was the amount of light produced that was important to the potential buyer.  As time passed, the candlepower rating gave way to watts as a way to rate light bulbs.  Although I have not been able to find a clear explanation of why this happened, I suspect that the cost of electricity had something to do with it.  Knowing the rate of energy consumption of each bulb provided a way to determine how much energy would be used overall.  This determination, of course, would allow for the time that each bulb would be illuminated and could be converted directly into cost.

Despite his best efforts, Edison’s first light bulbs were not very efficient.  It takes a lot of energy to heat a piece of carbonized bamboo to incandescence with a lot of the energy ending up as heat.  Over the years, there were continued efforts to produce more light using less energy.  Over time, the efficiency of the light bulb did improve but, having become a convention, watts remained the way of expressing the light output capacity of a light bulb.  In simple terms, a 60 watt light bulb, for example, became a bit brighter with each improvement.  The changes in efficiency were not huge so people were happy to continue to use watts as a way to, indirectly, define the brightness of a light bulb.

In more recent years, starting with the invention of the fluorescent bulb, improvements in efficiency became significant enough to begin the downfall of the watt as a means to express the brightness of a light source.  At first, the unique design of fluorescent lamps (tubes instead of bulbs) provided enough distinction that the inequality of watts vs. brightness wasn’t an issue.  However, as technology advanced even further to light emitting diode light sources and fluorescent lamps took on a form emulating that of the incandescent bulb, the inequity in the relationship between watts and brightness became so significant that a new way had to be found to express the brightness of a light source that was not linked to its energy consumption.  As a result, light bulbs today are rated in lumens with watts being used as an indication of efficiency.  One lumen is defined the amount of light that falls on a one square foot area at a distance of one foot from a burning candle (right back to Edison).  Now, instead of shopping for a 60 watt light bulb (for example) we shop for a light source that produces a certain number (or range) of lumens.  The following chart shows some approximate ranges for comparison.

Light Bulb Watt Equivalents Revised

Watts have given way to lumens as a way of specifying light sources.  But this is just one problem with watts.  Others will be discussed in upcoming blogs.

-  FJF  -

Ultrasonic Machining – A New Use for Ultrasonics?

May 28th, 2015

Over the years, there have been several anecdotal references to otherwise unexplained changes in the properties of surfaces exposed to ultrasonic energy in a liquid.  In some cases, it would make sense that the change was due to increased cleanliness.  In others, however, the benefit of cleanliness alone would seem questionable.  One incident in particular sticks in my mind.

Many years ago, a printing company reported that ink rollers (not analox rolls but just smooth rolls) carried ink much more efficiently and uniformly after they were ultrasonically “cleaned.”  In a series of (not so) controlled experiments, it was shown that the use of a chemical cleaner played virtually no measurable role in whatever was happening.  Rolls exposed to ultrasonics using tap water showed the same change in ink carrying properties as those cleaned ultrasonically using a variety of chemicals.  It was finally deduced that ultrasonic exposure had “roughened” the surface of the steel rolls just enough to allow the ink to adhere more effectively.  At that time (30 or so years ago) we did not have the means to measure what exactly had changed when the rolls were exposed to ultrasonics.  No difference could be seen using a microscope - the result was strictly empirical but significant.

Contemplating the above over the years, I have come to the conclusion that there are two possible explanations for the change seen in the ultrasonically cleaned ink rolls – -

  • The implosions of cavitation bubbles may have created small “craters” in the surface which improved the “tooth” of the surface.
  • The surface had been “machined” sufficiently to remove a thin skin of oxide or other barrier which otherwise prevented adhesion of ink.

Either of the above is logically possible based on our knowledge of ultrasonic cavitation and implosion.  But, the important thing at that time was not what happened but the fact that something happened.

Since that time there have been instances in which, although not quite as dramatic, I have seen evidence of changes in surface characteristics that are not consistent with cleaning alone.  But the purpose here is not to teach but to learn.

Ultrasonics is still a growing technology.  The more I know (after nearly 50 years) the more I find that I don’t know.  I see this potential use of ultrasonics under controlled conditions and with known parameters a rich opportunity to enhance coating technology and to improve bonding technology.  On a somewhat larger scale, we already commonly use sand blasting, as an example, as a way to increase bond strength when two surfaces are to be held together using an adhesive.  Maybe ultrasonic “machining” would provide a means to improve bond strength on a much smaller scale – maybe in optical coatings, semiconductor and other applications.  In fact, this effect may already be benefiting in instances where the operative term should not be cleaning but, rather, “machining.”

With the capability we now have to utilize a variety of ultrasonic (and megasonic) frequencies, power levels and waveforms, I believe it would be worthwhile to research this, as yet, poorly understood potential use for ultrasonics.

-  FJF  -



Ultrasonics – Revisiting Watts Per Gallon

May 21st, 2015

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  -

Capillary Flow

May 18th, 2015

In the process of parts cleaning, capillary flow can be our friend or our enemy – sometimes both at the same time.  So let’s take a minute or two to understand this interesting phenomenon.

Capillary flow is a property that is exhibited when liquids spontaneously penetrate narrow passageways.  The classic example of a capillary (narrow passageway) is a small internal diameter tube.  The following illustration demonstrates how three different liquids might respond in a capillary space.

Liquids climb to different heights in the same diameter capillary space depending on their properties.  The size and contact angle of the meniscus where the liquid meets the capillary wall also changes based on the properties of the liquid.

Liquids climb to different heights in the same diameter tube depending on their properties. The size and contact angle of the meniscus where the liquid meets the capillary wall will also change based on the properties of the liquid.

The flow of liquid into a capillary space is spontaneous and occurs without pressure, vacuum or any other outside influence.  The height (or depth) of penetration can, of course, be affected by pressure or vacuum but without them, the degree of penetration in similar capillary spaces is related most directly to the surface tension and specific gravity of the liquid.  The meniscus formed at the point where the liquid contacts the surface of the capillary space is analogous to the contact angle measurement for surface tension.   Contact angle was discussed in previous blogs as both an indicator of cleanliness (if the liquid is held constant) or as a measure of surface tension of a liquid (if the contact surface is maintained constant).

Although we often think of capillaries as tubes, the same phenomenon will occur wherever there are closely spaced surfaces of any geometry including flat plates or more complex shapes like screw threads in contact with a threaded fastener or tapped hole.

Effect of varying capillary spacing

Varying the capillary spacing using the same liquid and surface conditions will produce different results in capillary flow. Closer spacing results in deeper penetration.

The degree of capillary penetration is also directly related to the dimension of the capillary space or gap.  A larger capillary gap results in less penetration.  Once a specific spacing is exceeded (depending on the surface activity of the capillary walls and the surface tension of the liquid), there will be no noticeable spontaneous penetration into a capillary space although the meniscus effect will still be evident where the liquid and surfaces forming the capillary space meet.

Note – Capillary flow is also commonly referred to as “wicking” as it is this effect which causes liquids to rise in the wick of an oil lamp or candle.  Liquid may also “wick” in porous solids.

In cleaning, capillary flow results in difficulties in both rinsing and drying.  Once liquid has entered a capillary space (by overcoming gravity) the only way to remove it is evaporation or use of mechanical means such as high pressure air or centrifugal force.  The liquid will not just “drain” out.

The purpose of this blog is not to suggest ways to overcome the effects of capillary flow but, rather, to make the reader aware that the effect exists any time small passageways are encountered.  The result may be inconsequential in some instances but in others is a means for real concern.

-  FJF  -