I’m seeing 22–28% spread below 150 CFM when I cross-check a Testo 420 against a freshly calibrated TSI Alnor EBT731, and a pitot grid + TEC DG-1000 in a straight section trends closer to the EBT. Has anyone found a hood that holds +/-5% at low velocities without adding a baffled cone, or is the better workflow to measure duct flow and back-calculate register k-factors to tighten balance time on 12-register jobs?
On Testo 420 “below 150 CFM”, gasket leaks dominated — foam collar cut spread to about 6%; still traverse once.
I’ve noticed similar discrepancies with lower airflow settings, especially with capture hoods. One thing that helped me was ensuring that all connections were airtight; that alone reduced variability significantly without needing to add baffled cones. How often do you check for leaks during your setup?
I’ve found that using a good quality duct tester can sometimes help refine those discrepancies at low CFM… It’s like trying to tune a guitar — you think it sounds right until you get it next to a perfect one. Have you ever tried comparing the readings with a different style hood, @mike_r75?