## RPM/MPH formula

I'm designing an anemometer with Arduino and I need a formula to figure out the speed at which the rotor turns.

I have a rotor with three magnets, spaced 71mm apart, triggering a Hall Effect sensor which the Arduino reads and counts. Now I can't seem to figure out a formula to calculate wind speed from this information. Anybody got an Idea (or the answer?)

I have a rotor with three magnets, spaced 71mm apart, triggering a Hall Effect sensor which the Arduino reads and counts. Now I can't seem to figure out a formula to calculate wind speed from this information. Anybody got an Idea (or the answer?)

active| newest | oldestif it takes 50ms (0.05s) between any two magnets (71mm) we can figure that to be about 3mph.

if I were to use this method, how would I tell the arduino to start counting in between pulses.

in plain english:

start counting when magnet is sensed then stop counting when another magnet is sensed. Store that time into a variable and do the calculation to figure out MPH.

on HallEffectSensor high

push timerCount onto myStack

reset timer

do calculations

end

You don't have to use a stack, but it might be handy if you want to do some smoothing, averaging, &c.

is one method.

. RPM * EmpiricallyObtainedFactor = MPH

. You may be able to compute the factor, but I have no idea where you would find the formula(s).

Rat which his anemometer cups are placed from the axis rotation. Then 2pi * R * RPM gives the linear velocity of the cups in /minute. Converting to miles and multiplying by 60 will give miles per hour.However, that assumes that the cup is 100% efficient at being driven by the wind. If not, there is an additional emprical factor which needs to be applied.