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 –