With HEPA filters, the manufacturers recommended dirty filter change pressure differential (PD), is only correct if the filter is used at exactly the the same air flow as the makers specify. If there is any digression (there is often as much as 50% digression) then the makers PD must be corrected. If this is not carried out you can compromise the integrity of your room / machine pressure regimes. As you are no doubt aware this would be a very serious state to be in. There are also health and safety problems that can arise, if personnel are not getting the air flow protection that is sometimes designed into the system. This test script is not in the standard OQ, and is only available here as a download. Regulators ask to review your writen justification for these filter change PD's; make certain you have them.