[quote=brianalfredo14]Clevair, thank you for yor answer
so there is no method for measuring from the return air grill? even in the lowest pressure room where it gets the air from another room too? what is the idea so we have to measure it from the supply air?
where can i get the reference about it, so i can strengthening my opinion?
ISO-14644-3 sections B.4.2-3
Measurement of supply airflow velocity only is required to calculate ACH.
Assuming you’re meeting you pressure cascade and particle counts. Why are you so concerned about ACH?