For a while the 227b dust monitor in this clean room has been reporting high counts. (see attached plot back to May 15th)

One theory was that this may be due to a missing prefilter on one of the HEPA units. (alog 6375)

This cleanroom is split in two, with a garbing area on the HAM2 side and cleaned parts on the other. Each half has two HEPA units. The HEPA unit with the missing prefilter is over the cleaned parts. The 227b dust monitor is also in this half.

This morning I took further measurements with one of the HHPC-6 particle counters. In the cleanrooms over the two test stands, and the larger one closer to HAM3, the counts were flat zero.
The counts I measured in the small cleanroom with the HHPC-6 appeared to confirm earlier counts with the 227b. The counts were elevated in both halves. I did not see a noticeable difference directly under the HEPA unit with the missing prefilter. However, inside the cleanroom near the floor of the entrance to the gowning side, the counts were repeatably and exceptionally high, I believe up to from 480 counts at 0.3 um to 160 counts at 5.0 um.

I plan to do more detailed measurements tomorrow.