I’d always wondered upon seeing airflow readings in cfm displayed on a room thermostat on a VAV (variable air volume) system as to where that data came from.
Well, here’s my answer. Inside a VAV box displaced during construction, I found this peeking out.
The top tube with holes in it faces the incoming airflow and is thus a Pitot tube. The dome in the middle covers and shields the end of the other hose which is the static pressure tube. A differential pressure sensor in the control unit that measures the difference in pressure can thus derive the speed of airflow, and multiplying that by the area of the duct opening gives you the volume. Simple as that.
This is actually kinda simple as vav terminals go. Some are more intricate and contain blowers. All this one has to control the airflow is a butterfly valve damper.
VAV is one of those things that looks good on paper but in practice leads to weird comfort issues. Thankfully our facility averts those all by simply never getting cool enough, so the airflow just runs full tilt all the time. Problem solved, now get me a curry chicken empanada.