In-situ particle monitoring (ISPM) provides a tool to observe particle removal behavior in the ultrasonic cleaning bath process at work and in real time, thus providing the ability to quickly select, adjust, and optimize the cleaning process. ISPM measures particle counts directly during the cleaning process in both ultrasonic rinse and wash baths.

An Example of How ISPM Works
A known amount/type of particle contamination was deposited on glass slide test coupons. Several cleaning agents were tested for their effectiveness in removing the contamination by monitoring the rate of particle removal in the bath.

Figure 1 shows the addition/mixing of the agent C at three different concentrations. Each concentration change appears as a rise in particle counts, until the mixing is complete. The test coupon is then inserted into the bath and the resultant peak particle count is noted.

The sudden and dramatic jump in particle counts after the 5 percent concentration was added to the bath — but before the test coupon was inserted — was caused by bubbles in the bath and was characterized by high counts for all particle sizes, unlike that of typical contamination. Particle counts larger than 5 microns are typically close to zero after mixing and before insertion of the test coupon, whereas during bubble interference they approach 100 counts per ml.

With appropriate precautions, bubble interference can be minimized and even eliminated. Bubbles are often a sign of over concentration of the cleaning agent and frequently do not appear at optimal cleaning agent concentration.

Figure 2 shows particle count response using cleaning agent E at various concentrations.

To determine the actual total particles removed, the area under the particle count curve over time should be integrated. However, for the sake of convenience, only the particle peak was initially used as a measure of comparing cleaning agent effectiveness. The flaw in choosing this method was revealed when agent E was tested.

Agent E’s composition was decidedly different from the other cleaning agents tested. Its cleaning behavior was a surprise as well. While the other agents appeared to achieve their best cleaning results immediately, agent E seemed to keep on attacking successive layers of contamination. This was confirmed by close visual examination after rinsing and drying of the glass slide coupons.

Figure 3 shows the cleaning bath peak particle counts for several cleaning agents versus concentration. While it at first appears that agent C yields the best results, closer examination confirms that agent E did indeed remove the most particles.

D. John Hunt is an Applications Research Engineer at Pacific Scientific Instruments (Grants Pass, OR).