How fast is wind ?

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rigidigital

Senior Member
yes, a silly title but i wanted to get attention :)

I need advice on a couple of issues. First I can't remember which electronic stores i used in the past(its been ages) except for Little bird Electronics, Wiltronics (Australia) There was one I used somewhere in Asia that was cheap[, terrific range and website but i don't remember ? probably will at some moment when I'm not thinking!!

So any tips on online stores you use be great.

Secondly, what sort of encoder do I need so that I can measure the speed of rotation of a small free spinning prop I'm going to try and make out of a propeller i have that is about an inch and a half diameter (off a helicopter tail rotor) ?

Ok, thanks for reading!
 
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Dippy

Moderator
I'm confused.
In post#1 you imply you want to make a DIY windyometer and in post #8 you link to a ready-made cheapy....


"hard to beat"
- price maybe, but it's only hard to beat value-wise if it works at an acceptable level for your application.
A digital display doesn't guarantee it's fit for purpose.


I've never used that model but I have used small cheapies for air-flow tests and they were rubbish at low speeds because the blades couldn't generate enough torque to drive the mechanism.
Whereas a much more expensive item did the job well.

And that will probably be the same problem if you use any kind of motor/generator to produce a voltage with a small fan blade.
Your mechanical section should produce as little drag as possible other than a bit to damp the system.

A friend turned up the other day with a £15 current clampmeter from cheapBay.
"Brilliant value" he said as he waved his Ting Tong Tool at me.
It had an error of 10% - 15% compared to a Fluke.
So, if +/-15% is good enough then it is brilliant value, otherwise it's merely a paper-weight.
It often feels like people are having a buy-cheap competition rather than a buy-good-value competition.
 
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rigidigital

Senior Member
It's highly classified :) I'm full off rubbish !!

I guess your very correct. The cheap windyometer I found after I first posted, just gave me hope that I was somehow going to be on the right track soon.
But the problem as you've seen is there is not going to be enough wind. If I held that toy and jogged around the block and could work out my speed..maybe that was taking things in the right direction. But no....I think it'd be lucky to give any reading. and since the amount im will to spend is not much more than that, well its foolish!!! thx 4 the wakeup !!!!
 
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AllyCat

Senior Member
Hi,

As usual, the crucial question is "what are you trying to do?".

Do you want to measure windpeed (which may come from any direction), or the speed of an "airflow" in a specific direction? A classic "cup anemometer" measures the windspeed from any direction and can be made to work below 1 mph, a propeller needs to point into the wind. Both methods need calibration of revolutions against airspeed (the cup anemometer is probably easier) but at least they have a fairly "linear" (revs/airspeed) characteristic (unlike hot wires).

The choice of a rotation sensor depends on whether the "prop" is metallic (it presumably isn't), how much resistance to motion (friction) can be tolerated and what degree of "unbalance" is acceptable (or can be corrected). A common method is to attach a small (neodymium) magnet and detect its movement with a reed relay, a Hall sensor or a "search coil". To minimise unbalance and "friction" an optical method would be best.

Cheers, Alan.
 
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Dippy

Moderator
Yes, a description of the requirement could get some good suggestions from the Collective.
Maybe he has a prop kicking around and was bursting to give it a job? ;)
 
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