Don't overestimate clean rooms - they contain 100 particles per cubic
meter as opposed to an "average" room containing 600 particles. A
"clean" "average" room will contain far less than the 600 particles.
Well, having managed a real clean room , you are way off base. First of all,
the particle count is per cubic foot. Clean rooms are classified by
the sustained particle count averaged around the entire room.
The average count in a typical home/office/light industrial room is about
500,000 particles per cubic foot, and the particlas are quite large (several
microns or tens of microns).
The first level of clean room we define is a class 100,000. This isn't a real
hard level to achieve and can sometimes be done without real expensive HEPA
filters if the working conditions are clean enough. The Space Shuttle high bay
room is maintained at class 100,000 (my experience is with NASA).
The next level is a class 10,000, which certainly requires a high level of
filtering and monitering with special clothing for the occupants. Next comes a
class 1000 which is getting serious. You are into laminar flow air systems and
special training for the people.
Computer chips are assembled in class 100 or even class 10 (!) rooms, since a
single particle can ruin a product. At this level, even the way you move can
disturb the room's particle count. Everyone is trained to move slowly and be
aware of where the downwash from the airflow over your body goes.
By this level, the particle size is usually measured at a much smaller,
sub-micron level also. A single small tear in a HEPA filter can take the room
out of spec for quite some time, requiring a long, slow damp swabbing of all
surfaces.
Dennis
