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Looking for a way to calibrate an anemometer in order to measure the wind speed

gekkoscamp

Hello guys,

(My first post on this forum, so ill introduce myself post scriptum)

To get into smart home systems and arduino, I am currently building a weather station measuring temperature, humidity and all kinds of funny stuff.

I am ALSO measuring the rotations of a self built 3d printed anemometer. I know the dimensions of the "flaps/cups", i know the rotations/frequency with which the thing is spinning

but i simply cant think of how to calculate the windspeed, given rotations and the drag value of a half sphere (in German its called Cw-Wert).

The actual spinning part consists of 3 equally spaced "arms" of length d each with a half sphere with diameter a.

 

How do I calculate the Windspeed, given those parameters?

 

Any hints are super appreciated.

 

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Hi, this seems like a really cool and friendly forum so i decided to sign up.

My Name is Michael, studying engineering in Germany, eager to learn the basics of electronics and programming.

  • Hi Michael,

     

    Welcome to element14.

     

    What accuracy are you hoping to attain with your anemometer?

    A simple test is to put it on your car and drive at various speeds on a calm day.

    You can run through a series of theoretical calculations for the rotation of the device, but you still need to know the level of friction is inherent in the system.

    See if you can rent a calibrated anemometer to use beside your new unit to see how well they correlate.

     

    DAB

  • in reply to DAB

    I've seen it done from a car but better on the back of a motor bike if you have a friend to give you a ride.

    It's easier to hold the sensor out of the slipstream on a bike.

     

    If it were me I'd put it on a long pole and attach it to backof the cab on my truck.

     

    I think you will need to get the end of the pole at least 2m from the vehicle, and ideally more.

     

    MK

  • Theoretically you are looking at the rate that a small volume of air moves from the tip of your blade to the back of the blade. Each movement of the air (tip to back) creates a partial rotation of the fan. Calculation of how many tip to backs are required to produce one rotation would tell us the ratio for one rotation compared to the rate of air movement tip to back of blade. Suppose that there is one cm of depth to the fan blade and the air is moving that distance in 1/100 second. (1 Meter per second). Suppose it takes 5 of these tip to back cycles to get one rotation. From this we would know that ideally with no friction 1 Revolution would equal 5 times the the tip to back speed of the air. (5 X 0.01 sec = 1/20th second) In this example our hypothetical air speed of 1 meter per second should produce 20 rps or 1200 rpm. I did these calculations on the fly in in my head so please check them and critique is welcome. Unfortunately friction is part of the real world and in this case it isn't even linear. As the fan speed increases the friction will increase in a non-linear fashion due to the air around the fan that has to be moved by the back of the blades. In the end if you want reasonable accuracy the methods proposed by Michael and Donald are probably the best. If you can find or borrow an Anemometer then you can use a stationary fan or a hair dryer to simulate a wind and make your calibrations without trying to drive down the road with a 2 meter pole out the window. Make your calibration at a wind speed that is in the middle of the range you expect to measure for best accuracy over the entire range.

     

    John

  • in reply to jw0752

    imageMy G-d what is with all of you trining to redesign the world. I have a Fieldpiece AAV3 head for my EHDL1   >

    Reads

    Ft/mi or M/s

    MPH or KM/hr

     

    ~~Cris

  • in reply to phoenixcomm

    I can't speak for anyone else but for me it is fun to try to do it the hard way sometimes.

     

    John

  • hey guys, thanks for the great input.
    As some mentioned friction, very correctly, doesnt behave linearly which would overcomplicate a very accrate complication.

    And i lack of the tools to measure at the moment.

    But i love the idea of calibrating it by giving it a rough reference speed by car or another vehicle and do it this way.

    My uni does have a windtunnel tho, so i am thinking of continuing with the rest of the project until i have the opportunity to test it there... or a car if that doesnt work.

     

    @christina, as John pointed out. I enjoy it building it myself.

     

    Thanks again for all the hints.

     

    Stay healthy and have great holidays!

  • in reply to gekkoscamp

    Hi gekkoscamp,

     

    I was curious how you are measuring the fan rotations. Are you using a counter or is it a dynamo?

     

    John

  • Some great suggestions above for you.

     

    More so for the test from a motorbike, make sure its either very secure with a secondary safety lanyard or can completely break free i.e. a weak point in the head to Arduino wiring. Being extra cautious I'd also not post any pictures of my setup until after the tests and definitely don't add the sticker stating "If lost please return to..." image

  • You could try swinging it at a known and constant rotational speed at the end of a stick or string.

    You might get a crude calibration by using a fan and measuring the speed of a feather in the flow.