Based on my experience, I would have to agree with Don's basic ideas. When I was responsible for such operations, I used a conductivity meter with the probe in the same cell as where the water was introduced in the counterflow series of cells. I set the meter so that it turned on at twice the conductivity of the incoming water. This was an arbitrary starting point; I am sure that with further testing, we could have lowered the setpoint. And this was with softened city water. DI and RO water have their uses, but in an electroless line, for example, they sound like overkill. If possible, a hang time of 10 seconds over a process bath maximizes the drainage in a reasonable amount of time; minimizing water usage means minimizing the need to rinse. As Don implied, making theoretical calculations is a lost cause since variable interactions are too costly to figure out. An empirical approach is most cost effective: what works for your specific situation will be determined by trial and error. Rinsing may also be most effective with warmer water vs. cold water and bubbling vs. no bubbling or other agitation to maximize mixing. And don't forget simpler things like: no uncontrolled hoses, automatic shut-offs, spray rinses, restrictors, etc. I once heard someplace: What is Clean? Better is the evil of good enough. Robert Lundquist Assistant Director -- MnTAP Suite 207 1313 Fifth St. SE Minneapolis, MN 55414 612-627-4557 612-627-4769 FAX [log in to unmask]