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FED-STD-209 Revision E, April 1992
Complete Document
Includes all amendments and changes through Cancellation Notice , November 29, 2001
Additional Comments: CNCL S/S BY ISO 14644-1 & ISO 14644-2
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This document establishes standard classes, and provides for alternative classes, of air cleanliness for clearness and clean zones based on specified concentrations of airborne particles. It prescribes methods for verifying air cleanliness and requires that a plan be established for monitoring air cleanliness. It also provides a method for determining and describing concentrations (U descriptors) of ultrafine particles.

The requirements of this document do not apply to equipment or supplies for use within clearness or clean zones. Except for size classification and population, this document is not intended to characterize the physical, chemical, radiological, or viable nature of airborne particles. No universal relationship has been established between the concentration of airborne particles and the concentration of viable airborne particles. In addition to the need for a clean air supply that is monitored for total particulate contamination and that meets established limits, special requirements are necessary for monitoring and controlling other forms of contamination.

Verify that the microscope has eyepiece-objective combinations capable of 100- to 250-fold magnification. Adjust the lamp and focus the microscope to illuminate evenly the entire field of view. Place the stage micrometer on the mechanical stage. Adjust and focus each eyepiece independently to give a sharp image of the gradations on the stage micrometer.

If an image analyzer or projection microscope is used, perform a similar calibration.

The following steps are used to calibrate a specific ocular reticle paired with a specific stage micrometer for the measurement of particles at any selected level of magnification.

(a) Determine and record the number of stage micrometer divisions, 8, of size M (micrometers), corresponding to the number of divisions, R, in the full scale of the ocular reticle for each magnification of interest.

(b) Calibrate the scale of the ocular reticle for a given magnification using the formula: (Equation A60-1)


For a given ocular reticle and stage micrometer at 100-fold magnification, let 150 divisions of the reticle correspond to 100 divisions, each 5.0 µm in length, of the stage micrometer. Using equation A60-1,

(c) Calculate the number of divisions of the ocular reticle corresponding to each specific particle size of interest.


Using the same data as in (b), calculate the number of divisions of the ocular reticle required to size particles in the range of 10 to 20 µm.

Since, at 100-fold magnification, each division of the ocular scale equals 3.33 µm, counting particles whose longest dimensions span 3 to 6 divisions will size particles in the range of 10 to 20 µm.

If the microscope has a zoom mechanism, appropriate intermediate magnifications may be selected in order to calibrate the ocular scale to integral values only. A change in interpupillary distance between operators changes focal length and, therefore, calibration.