# Wind Sensor Testing

UPDATE2: yeah ignore the drivel in this post and check this post for correct equation for the anemometer.

UPDATE: So it turns out that I haven’t a clue about how you measure wind speed with an anemometer. I found the datasheet for the sensors, which gives 1.492mph per pulse on the anemometer over 1 second, so I have had to rework the equation and procedure below. So the screen shot below gives wrong values, but is an example of the scripts output.

Just started testing with the Anemometer N96FY. I have the anemometer connected to GPIO pin 18 and 3.3 volt supply. This wind sensor is like a switch and it’s default state is closed, meaning electricity with flow through the anemometer except for two opposing points during a full revolution where the voltage will briefly fall to zero.

I work out the equation to be

wind speed = half the circumference / time taken

Wind speed will be in metre per second. Half the circumference is 0.21994 metres.

# Anemometer N96FY

I have purchased the N96FY Anemometer from Maplin Ebay store for the tiny price of £2.49 +delivery (£3). You can purchase then through the main Maplin web site if you don’t want to go via Ebay.

It rotates very easily and seems extremely sensitive to even the slightest breeze. The anemometer has two reed switches in the unit, both pulse during 1 revolution. There is only 2 wires coming from the unit, so you can’t sense the reed switches separately. The radius is 70mm measured from the centre of the spindle to the centre point of the cup. A nice round number to work with. As a side note the mounting piece at the bottom is 18.5mm diameter (outside measurement).

## Working out the circumference

The equation is C = 2πr. I take pi(π) as 3.142 and convert radius(r) to metres, because I want to measure my wind speed in metres per second. So radius(r) becomes 0.07 metres. Therefore:-

0.43988 (circumference m) = 2 x 3.142 (pi π) x 0.07 (radius r)

So we can round this to 0.44 metres for the circumference. So 2.72 revolutions per 1 metre per second of wind speed. Which is just over 4.54 reed pulses.

## Conversion to other measurements

So it turns out the official unit of wind speed is the Knot (who knew?). I am not terribly bothered about measuring in Knots, since it’s a unit that has little meaning to me, but the Knot wikipedia entry does provide conversion to other units of wind speed.

So conversion to other units is pretty simple, just a matter of applying a multiplier, which should be easy enough in the php code.