# computing mph for a weatherstation with opto (anemeter)

Hi I'm looking on some information on how compute miles per hour from a round disc that has 32 shaded area or slots using a photo transistor and led. I'm planning on using external interrupt on the Arduino to detect a shaded area on the disc. Say I count how many pulses that have been count in 1 second. Don

If the diameter of the anemometer is D and it rotates at R revs per minute, then the wind speed is: S = R*D*pi/60 If D is specified in inches then S will be in inches per second which you will have to convert to miles per hour.

Pete

el_supremo:
If the diameter of the anemometer is D and it rotates at R revs per minute, then the wind speed is:
S = RDpi/60

The above formula will calculate the tip speed of the rotating “disc”, but the wind-speed to tip-speed ratio is not 1:1.

You will need to calibrate your anemometer by plotting RPM against a known wind-speed. You could attach it to the roof top of a car, drive at varying speeds and then collect and plot RPM measurements against car speed.

Hi, If you look at the documentation part of this: http://www.nrgsystems.com/sitecore/content/Products/1901.aspx you will at least have a first-order approximation of the transfer function of MPH to rotation speed.

I have a few of these (Fortunately my Kid is the main Engineer there and gets me some returns to use with schools etc..)