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Anemometer and the diameter vs circumference Answered

Various Instructables have been created for building a wind speed anemometer, and nearly all use a bicycle speedometer for the read out.

My question is, regardless of the actual diameter, the sensor can be close to the outer rim, or close to the hub, because one revolution is going to be one revolution and register as such on the speedometer.

So, setting the speedometer to whatever rim size you desire, the readout would be correct.

If you set the tire size to 24", then each time the sensor is tripped, it will show the speed for that tire size.  The same with 27" or 28" tires.

The reason I'm asking is because I'm putting together a 'gopher chaser' from Harbor Freight (on sale last weekend for $1.50) which measures 17 3/8".

I have no desire to cut new blades from sheet metal....

Thanks for any input on the issue.



7 years ago

Compare your speedometer reading to another device that reads correct. remove the bezel and glass and change the numbers with a label maker. I did this with my motorcycle riding next to my buddy in his car. I could have just used my garmin GPS to determine my speed, but it was just easier with two people. Once we were done determining increments of 5mph, i removed the dial face, scanned it into photoshop, cropped and replaced the digits, printed it out on sticker paper and put the dial back together. Analog speedos are geared like a clock....its all about wheel size.

It is a digital readout.

Next best suggestion would be to GLUE the magnet to the vane.

That way when the windspeed exceeds the magnetic attraction of the magnet to the metal vane, it won't fly off, leaving the FIRMLY attached counter balance weight to cause so much vibration that it destroys itself.......

Highest speed attained before things went badly... 49.8mph......

Either that is what happened, or a raccoon wanted the magnet for some reason?

All good ideas, answers and suggestions.


It's the magnetic sensor on a bike speedometer.

OK, so it registers a turn every time the magnet goes past?

What form is the readout?

It is a digital readout.

But, if the unit knows the circumference of the tire, then each revolution/sensor trip, would tell the unit the tire would have traveled x # of feet, yards, etc and determine the speed accordingly.

Is that a correct assumtion?

You're probably correct. With a digital readout, that is much easier to implement just with (integer) math. An analogue readout would be more complex, with a potentiometer or variable capacitor to change the output voltage/current going to the meter.

However, you mentioned putting the sensor close to the hub. If the pickup part of the circuit has a long time constant, then the frequency near the axis could be too high for it to get an accurate reading. The easiest way to find that out is just to experiment. As you take data with the sensor a smaller radii, you should see the reported speed deviate (probably toward lower values, as it misses some revolutions). A plot showing the plateau break will give you an estimate of the minimum radius of placement.


I did place it as far out on the vane as possible so half of 17 3/8".

I also used a magnet from a dead Hard Drive. Much stronger so I could place it a little further from the blade track.

So far it looks fairly close to actual breez estrength.

Is this a digital or analogue sensor we are taking about?