High Integrity air filters have recently figured in a several very targeted warning letters. It is very important to remember they are not Fit and Forget items. The filter makers give as part of a new filter specification a PD (Pressure Drop) at a certain flow. Since you will rarely have this exact flow rate; your clean filter PD will differ from this specified PD. It is imperative (for product protection) that the filter dirty PD or (change filter) indication is correct for the actual airflow in your system.

To reiterate; the filter manufacturer's clean PD value is always given at a specific air flow rate. When the actual flow in use is lower than this flow; your filter clean pressure drop will also be low and when it is higher your filter clean pressure drop will also be higher. You must therefore measure the flow and calculate; what the corrected "Change Dirty Filter" PD is.

Regulators expect to be able to review your documented methodology for this important task. We have a very simply easy to use test script used for this purpose. Why not purchase it and paste it into all you SOP's and protocols that deal with HEPA filter testing.