The particle counter test method employs discrete particle sizing and quantitative analysis. This test is performed by introducing an artificial aerosol challenge upstream of the filters and verifying the installations integrity by scanning the downstream side of the filters with the particle counter.
The challenge aerosol particle concentration must be greater than 3.53 x 10⁷ (35,300,000) particles per cubic meter, (3 x 10⁶ [3,000,000] particles per cubic foot of air) based on using a probe that is 0.4 inches in the direction of scan and at a scan rate of 2 inches per second. This concentration will allow a reasonable scan rate that can be manually performed. Particle concentrations of 2.12 x 10⁸
(212,000,000) particles per cubic meter, (6 x 10⁶ [6,000,000] particles per cubic foot of air) are desirable and will increase the accuracy with which leaks are detected.
This test be limited to cleanrooms where the cleanliness classification is ISO Class 6 or cleaner.

INSTRUMENTATION AND EQUIPMENT
A particle counter.
A hand held isokinetic sampling probe configured to provide adequate residence time while scanning.
An aerosol particle generator.
A diluter or a reduced flow rate particle counter.

TEST PROCEDURES

• Introduce the aerosol into the air supplied to the filters in a manner which will produce a uniform challenge concentration at each of the filters being exposed at the same time.
• Measure the upstream particle concentration as the challenge is introduced. Verify that the upstream particle challenge shall be greater than 3.53 x 10⁷ (35,300,000) particles per cubic meter, (1 x 10⁶ [1,000,000] particles per cubic foot of air). While this concentration will allow a reasonable scan rate, a higher concentration will improve the accuracy and may allow for increased scan rate.
• The resulting upstream challenge concentration, along with other information specified below is used in Equation to calculate the Acceptable Scan Rate when utilizing a Discrete Particle Counter Test.

Where:
Sr = Acceptable Scan Rate—cm/s (in/s)
Cc = Upstream challenge concentration—particles/L (particles/ft3)
Ls = Significant leak—percentage of upstream concentration (typically 0.01%)
Fs = Sample flow rate of instrument used—L/min (cfm)
Dp = Probe dimension parallel to scan direction—cm (inches)
Np = Number of particle counts that indicate the leak (1 through 10)
60 = Conversion—60 sec/min

• Using the resulting scan rate (Sr), scan the filter face and the perimeter of the filter assembly by passing the probe in slightly overlapping strokes so that the entire area of the filter and installation is tested. The probe should be held approximately 25 mm (1 inch) from the area to be tested during scanning. Separate passes should be made around the entire periphery of the filter, along the bond between the filter pack and the frame, and around the seal between the filter and the device.

 
NOTE: If a lower value of Np is used (1), the allowable scan rate will be increased but the probability of finding a leak will be decreased and the probability of false leaks is increased. If a larger Np is used (3 or greater) the allowable scan rate is decreased but the probability of detecting a leak is increased.

• When scanning a filter and the supporting assembly installation a particle count detection exceeding Np will indicate a potential leak. If particles are registered that exceeds Np, then the particle concentration penetrating the defect shall be determined if a significant leak exists. 

ACCEPTANCE:- An unacceptable leak is defined as a sustained reading greater than 0.010% of the measured upstream concentration for the particle size of interest.
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Reference links
http://standards.nsf.org/apps/group_public/download.php/29691/DPC%20Filter%20Test%20Draft%20-%2011-16-2015.doc
http://electroiq.com/blog/2006/03/iest-rp-cc0342-hepa-and-ulpa-filter-leak-tests/