Find out what the initial pressure drop of the HEPA filter is (this can be found on the side of the filter housing) and what the final recommended pressure drop is for filter replacement from the filter manufacture you are using.
Gage: Dwyer # 2001 ( 0.0-1.0” Wg.)
Tubing size: 3/16”-1/4” ID (to fit the Dwyer hose fitting that comes with the gauge)
Drill a ¼” hole in the “Tin Can” portion (between the air inlet and above the HEPA filter media). of the HEPA filter housing and install the high side of gauge tubing.
Seal this penetration (tight) with a non-silicone approved sealant.
Run the tubing across the ceiling within the plenum to a corner of the cleanroom and drop this tubing down into the cleanroom to about 6 feet above floor level for ease of access to hook up a portable differential monitoring device (or the in-place Dwyer # 2001 gauge mentioned above and leave the low port open).
Make sure the ceiling penetration is also sealed with the approved sealant.
Repeat this sequence for each room you want to monitor.(using one HEPA filter in each area as a typical)
The monitoring and recording of each designated HEPA filter should be part of the certification visit and be part of a in-house monitoring program on a monthly basis.