The problem with flow measurements is that the flow velocity varies in magnitude across a channel. The good news is that a velocity across a round duct for fully developed, turbulent flow is fairly uniform. One just has to have a straight round duct about 10 pipe diameters long to have fully developed flow. Most people do not have a suitable, accessible length of duct that they are willing to drill holes in in an existing system that can serve as a test section. So what can be measured?

Velocity varies a huge amount in both magnitude and direction across an open duct outlet which seems to be the way YouTubers measure velocity. The pitot tube sensor and the hot wire anemometer measure at a particular point and direction. That point and direction can be very different from the average flow in the duct. Spatial averaging, ie taking a series of flow measurements across a duct can compensate for point versus average problem if the averaging is done correctly.

Also the sensor response, ie the sensor reading versus the true flow velocity at that point, can be different. The reality is that the point response of hot wire and pitot tube is pretty good. This analysis changes for the vane anemometer. It averages over an area in a complicated way. Plus, its sensor response under uniform velocity field is not so linear. So it has to be used within its calibrated range of velocity. Ideally the velocity profile across vanes should be flat to improve the accuracy of its response. The limitation of the hot wire anemometer is range. My Testo 405i maxes out at 5900 fpm. For duct velocity in 6” duct, 5900 fpm is not enough for a true high performance dust collection.

The question for the shop owner is mostly how good is my own system and could I make better at a reasonable price. The good news is that all the measurement methods generally track velocity proportionately if not accurately in magnitude. Since we don’t know exactly what velocity is needed, this knowledge can be enough. I take readings on velocity readings around the air inlets and gaps of cabinet tools. I find 200-300 fpm enough to entrain fine dust particles. Your mileage and collection efficiency may vary.