Inexpensive Ultrasonic Air Flow Measurement
Sound travels through the air at 767 miles per hour under ideal conditions, but if the air is already moving in one direction this relative speed is increased proportionally. Thus, how long an ultrasonic pulse takes to get from emitter to receiver can be used to measure the airspeed between the two.

After researching the subject, including viewing this writeup, YouTuber “ItMightBeWorse” considered the possibility that this type of measurement could be done in HVAC (heating, ventilation, and air conditioning) ducts using inexpensive components, namely the SR-04 ultrasonic sensors that you often see used on robots. These sensors are not only available for a couple dollars, but have well-established Arduino libraries.
In order to test this theory, he disassembled two of these emitter/receiver pairs, and mounted each receiver and emitter at a distance to each other on the original PCBs. This enabled him to measure the time from the emitter to each receiver directly, not based on reflection against an object as they are normally used.
He found that with no air movement, transmit/receive time in each direction was nearly the same, but after adding air flow via a hairdryer, this changed significantly, allowing him to calculate the air flowrate based on the formula found here. It’s a very useful concept, however, ‘MightBeWorse still needs to test out the setup’s accuracy, especially in the context of the air speeds experienced in HVAC systems. He plans to test this in the next video, so consider subscribing, or check back on his channel if you want to see more.
[h/t: Reddit]


