What an awesome example of fantastic innovation!
Check out the TrueFlow Capture Box CEE made!
So that's where you use those True Flow plates.....
It was designed for air handlers to determine the total system air flow. I It would be installed in the area that the return air flow filter would have been and then match the systems pressure to calculate the air flow. Using the True Flow plates with a box or capture hood is the same as using the duct return line as the hood.