More Weather - there's a lot of it about!

Build a machine to measure wind speed said the teacher... OK. In that case I'm going to make my machine using this anemometer I just happen to have in one of the many boxes of "interesting components" that are lying around Sniff labs. Of all places it came from Maplin (kind of a UK Radio Shack), who typically charge twice as much as anyone else for stuff, but in this case they had this as a spare part for their weather stations, currently retailing for £2.49. That's not a typo - stupid cheap. They also have  a rain guage for £4.99.

It has a simple switch so that once per revolution it closes a switch. Connecting one side of the switch to ground, we wire the other side to an Arduino, with a pull up to 5v.  Of course I did this using the preferred 3pin dupont header, so it plugs straight into a sensor shield!

From there it seemed pretty obvious to use an i2c lcd display (my current favourite - use it in every project device). I powered it from a USB mains charger, and put the wires through the window, out on to the balcony where the anemometer was installed.

make anenometer digital input 2

make count number

when start
.forever
..wait until anenometer
..wait 0.01 secs
..change count by 1
..wait until not anenometer
..wait 0.01 secs

The code to actually process the input is nice and simple - wait for the switch to go high, increment the count, wait for it to go low again. There's a couple of delays in there incase the switch bounces. I've not fine tuned them but it works pretty solidly.

make i2c device

make display lcdi2c device

make message string

make fastest number

when start

.set fastest to 0

.forever

..

..set count to 0

..wait 4 secs

..set count to count / 4

..

..if count > fastest

...set fastest to count

..tell display to "clear"

..set message to join "now:"[count]

..tell display to "show"

..set message to join "max:" [fastest]

..tell display to "show"

As we're only measuring whole revolutions, we average the speed over 4 seconds. That seemed a good compromise - to short a measuring period, and we wouldn't be as accurate, but any longer and we might be averaging the speed over several gusts. It would be interesting to know if there's an official way to do this - our local "real" weather station records both average speed and maximum gust speed, so there's clearly more we could do here.

We record the maximum speed as well as displaying the current speed. This is revolutions per second. I happen to know (because I looked it up, but you could measure the circumference) that one revolution per second is about 1.5mpg. Over the few days we've been running it the max recorded each days has been about 6.5 or 7 revolutions, which is about 10.5mph. Converting to metric thats about 16.5kph.

Checking with the official records for the last few days:

Wind speeds have been peaking at round 17kph each day!! There are a couple of spikes we missed, but our anemometer's location is on the balcony of the Sniff Mansion, which was chosen for convince and safety of installation, rather than accuracy! I'm calling that a pretty good win.

Just for fun I added a ds18 to record min and max temperature, and of course we could easily log all this to an SD card, but maybe thats for next time!