In the standard FED-209-E it is stated how you should plan and perform measurements in a cleanroom. 
Additional you can measure a cleanroom when it is in work and when it is in rest.
In work the rating for a eg. 10 K room has to be less than the rating for 100K room in work.
For a 100K room in work there is no limits.

When determing your cleanroom rating you should always state the highest counts.

Jan Thuesen

-----Oprindelig meddelelse-----
Fra:    Ed Cosper [SMTP:[log in to unmask]]
Sendt:   9. oktober 1997 17.04
Til:    [log in to unmask]
Emne:   Re: [TECHNET] fab, cleanrooms rating

I'm not positive, but when you give a room a class, i.e. 10K., You are stating that you do not have more than that classification. Therefore, it makes sense to me that you would  state the worst case. 

Ed Cosper

----------
From:  John Nelson[SMTP:[log in to unmask]]
Sent:  Thursday, October 09, 1997 10:54 AM
To:  [log in to unmask]
Subject:  Re: [TECHNET] fab, cleanrooms rating

If you were to monitor your clean room with a calibrated instrument (Met One) and take multiple measurements each day of the number of 0.5 micron particles per cubic fot of air.  And then you
constructed an X-bar  control chart with a calculated mean and with control limits at +/- three standard deviations.  

What number would you use to state the class of your room?  Would you use the
mean number of particles per cubic foot or would you use the upper control limit 
as the worst case particle count?

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