Ultrasonics – The Trouble With Watts – Part 2

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  –

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