Measuring low airspeeds


I want to be able to measure the relatively low airspeeds (0-10 meters/second) encountered on UAVs and para-gliders. I’ve looked at three ways of doing this... 1) Differential pressure sensors – these seem to be fine for measuring airspeeds faster than 30-60 m/s, but at the speeds I’m interested in the pressure differentials are lower than any (cheap) pressure sensor can detect. 2) Hot wire anemometer – I’ve not actually tried these, but reading on the subject indicates they can be difficult to calibrate and aren’t very accurate. 3) Caged impeller.

The last option (see here for a picture) seems to be the best option (its what hanglider and paraglider pilots use)

So, my question is, how does it work? I’ve had a look at one but not been able to take it apart, the probe itself has only two wires connecting it to the display. The impeller itself is tiny (less than 12mm diameter). There’s no sign of a magnet on the impeller (which I would expect if it was using a hall sensor to detect rotation) nor is there any sign of any optical sensor to count the spinning of the blades. So there must be something inside the plastic housing that can detect how fast the blades are moving. Any ideas on how this may be working and how might be able to interface it to an arduino?


With just two wires it’s probably a reed switch. Connect to a multimeter (reading resistance) and very slowly turn the blade and see if it registers something at a certain point in the rotation

you could always use an ultrasonic transmitter/receiver pair

probably a reed switch.

That's what I thought, but the resistance is always high (I've measured it) and there's just not enough room on the impeller or axel for a magnet. I've also tried holding a compass next to the probe, there's no deviation of the needle, so there can't be any magnets involved.

you could always use an ultrasonic transmitter/receiver pair

I've not seen this done anywhere, and it sounds like a major project in it's own right. I'm looking for a lightweight and reasonably cheap and quick solution.


If there are only two wires then I suppose it's probably a passive sensor, perhaps a Magnetic Variable Reluctance pickup. If you can get hold of one for testing, you may be able to see the output signal if you connect it to a scope and blow on it. Otherwise, if you can locate a data sheet that would give a clue to how it works. Failing that, I think you may need to contact a supplier and ask.

Magnetic Variable Reluctance

Thanks, I'd not heard of this before, but a quick google suggests this is plausable option, although I would expect the impeler to be magnetic if this were the case.

contact a supplier and ask

I did try this, but the supplier just sells them, and even the company that claims to make them admitted that they actually just re-package them from a factory in the far east.


Using an ultrasonic system is not that hard!

In fact it's not hard at all!

If you've gotten past the "blinky LED" stage you should have no problem.

Schematics and built examples here

You don't HAVE to use 40kHz, a 32.768Khz watch crystal will work.

There is a good reason cars and some motorcycles use an ultrasonic system to measure - - air flow rate - temperature - humidity

Because it is EASY!!

Having A CRO to setup helps, but you can get by without one.

And befor the usual suspects start telling you the receiver bit is hard, it isn't, my initial prototype took all of an hour to get together.

With an aircraft you can not only sense airspeed but a stall condition as well.

Now I have a bit of time I'll put together some notes and a video on how to do it.

I'm interested! I fly both a Paraglider and UAV.

No progress?