I'm currently working on a a few large paint booths with a 30hp make up air unit and a 40hp exhaust fan on each. In the sequence it calls for changing the exhaust fan speed based on filter loading and tracking the supply fan to keep the booth -0.05 in wg compared to the hanger its in. The previous control system\sequence would blow the doors open at start up and shut down. I've got all the bugs worked out as far as space pressure, temperature requirements, and can start and stop the system with out major pressure fluctuations blowing the doors open. I just don't know how to take in account filter loading on the exhaust fans as I've never written a program for this. Any ideas? I would like to actually track exhaust fan CFM and keep it constant but that is not really and option as any velocity probe in the duct would get clogged with over spray making it way through the filters. I'm kinda think of tracking the static on the outlet of the exhaust fan compared to the booth and trying to keep this about the same as running with clean filters and the min cfm requirement. Do you think this will work? I hope this small static tip would be more likely to give an accurate reading than a long velocity probe.