This column is about understanding how and why sonic transducers perform as they do in cleaning operations. That understanding is based on an analysis of how the power applied from transducers is distributed throughout the tank. The involved science is sophisticated but the understanding may come mainly from a simple and old method.

“BLACK ART”
Use of ultrasonic transducers can be as much “black art” as science — both despite and because of the best efforts of suppliers. There are two reasons: the technical one is that the continued reflection of pressure waves off tank walls, parts and part baskets, and the fluid-air interface produce a somewhat variable and irreproducible environment; the commercial reason is that it is a way of differentiating what has become a commodity product — sonic transducers.

ENTER SCIENCE
As does nature abhor a vacuum, so does those valuing science abhor “black art.” This has led over at least the past decade and one-half to many suppliers and users attempting to measure the local effects of pressure waves produced by sonic (ultra and mega) transducers.

HIGH-TECH SCIENCE
Scientific developments with various levels of success include: measurement of temperature profile throughout a cleaning tank using a polyethylene calorimeter,¹ measurement of radiation pressure using a velocity sensor,² measurement of local velocity gradients via laser interferometry, ³ and analyzing the optical diffraction patterns of short laser pulses due to pressure waves.⁴ All of these approaches are useless for cleaning operations.⁵

MEDICAL CALIBRATION INSTRUMENTS
The mechanical effect of the ultrasonic transducer is quantified through a radiation force balance (RFB). The low-wattage transducer is aimed at a hollow triangular target. The net applied force causes the target to deflect, and the miro-force required to sustain its position is measured by a strain gage bridge. The gram value of force is directly proportional to the absorbed power level. Cavitation is not involved as the frequencies are megasonic.

These devices also operate in undisturbed tanks, are useless for cleaning operations, but do provide good value to sites doing medical work who must calibrate the output of therapeutic ultrasound devices.^6,7