Has anyone heard of DC200 antifoam (silicone) causing significant particle count level increases as defined by NAS 1638 and using a Hiac automatic particle counter?

Water dropplets and other dispersables will show up as "contaminants" in the fluid. Dust and dirt will also show positive for Si in the metals analysis. DC200 anti-foam will show possitive for Si as well. Parallel testing needs to be done to fully understand the contamination profile of a critical system. Historic trending analysis is your best bet.

Yes we have had problems with a silicone product in hydraulic oil. I do not remember what brand it was, but there where significant problems in automatic counts vs. microscopic. We got NAS10-12 in the automatic count, and NAS 4-5 in microscopic. Because of the difference we run an element analysis and found 50-70ppm of Si. The automatic particle counter (An advanced model) also took pictures of the particles over 20µm in size. They looked like half-moon shaped (not like cutting particles). Anyhow the excess of particles was found under 20µm in size. : -Therefor I recommend using a microscopic count (ARP598b or ISO4407).