I'm the author of the article. The dual manometer suggestion below is excellent. I use external static pressure to calculate airflow using manufacturer tables q

I'm the author of the article. The dual manometer suggestion below is excellent. I use external static pressure to calculate airflow using manufacturer tables quite frequently. It's fast and only slightly less accurate than using an expensive flow grid. I also use a manometer to measure pressure drops across filters and AC coils quite a bit. I'd add a tool for measuring temperature, ideally a digital psychrometer, but even something that just measures dry bulb is useful for heating and bad AC failure. Finally a multimeter, ideally with clamp on amp readings.