Typically, the easiest way to measure in-duct airflow in residential and light-commercial HVAC is by doing a timed average traverse. In this case, we made five equidistant holes in the ductwork to do a traverse. We go into the Smart Probes app and set up the timed average traverse with the proper length and width dimensions of the duct.

Make sure that the probe tip is open. Then, place the probe fully into the duct and press the button on the handle to start measuring in-duct airflow. Hold the probe flat and pull it out slowly. Repeat those steps for the remaining ports/holes. Complete the average by pressing the button at the bottom, and you will see the in-duct airflow results. We measured 755.3 CFM and 388 feet per minute (air velocity).

Testo: https://www.testo.com/en/