As I have mentioned before, in Dec/1989, I started and headed a new subsidiary corp for a mechanical design firm. It’s initial focus was IAQ and small commercial/industrial/institutional TAB (Test/Adjust/Balancing) of HVAC systems.
I bought a duct leakage tester from England through Davis Equipment/Instruments in Philadelphia (I believe). It’s barrel resembled a small cannon with a series of finely machined nozzles of declining diameter with exterior pressure taps…they could measure down to 1-2 cfm flow!! In one situation (seniors managed care home with newly installed windows) I used the smallest nozzle to do window air leakage testing after complaints from the old folks!!
Some of the better engineers were specing main trunk air leakage rates of 3-5% of total design flows…kept the subs on their toes doing good work!!
Some companies selling blower doors now have duct air leakage testers: