I feel like I've seen this discussed before on here but I'm not finding it at the moment.  We're wondering if a VAV system serving multiple spaces can use AFMS at fan inlet bell for supply and return fans and utilize the difference as the "OA" measurement?  Discussions have been around ongoing maintenance and reliability of stations in the raw OA path in the long term and that fan tracking of the SA and RA airstreams may make for a more reliable setup.  Has anyone looked at or attempted this?