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'Success? Failure?' Rating: ★★★★☆
Last weekend I built a wind speed meter based around a cheap (NZ$15) cycle computer that I picked up in a camping store sale.
The cycle computer sensor is mounted on the circumference of a freely spinning plate (in my case a peanut butter jar lid screwed to a sliding door castor) which is surrounded by three rounded cups (baby food cups, NZ$4 the set) and mounted on a pole (an aluminium towel rail). All parts are either naturally weather-resistant (e.g. plastic or aluminium) or made so (e.g. varnished the wood, used silicone sealant to prevent water getting in). The unit was assembled and then calibrated out the window of a moving car.
To calibrate the wind speed meter I drove up and down a stretch of road at exactly 30km/h and adjusted the 'wheel circumference' of the cycle computer until the meter was reading 30km/h (increase the 'wheel circumference' to increase the speed reading). Ideally this is done on a windless day, but you can allow for a head-/tail-wind by averaging an up-/down-wind run (choose another road if you've got any kind of cross-wind blowing). My set up was reading true with a 'wheel circumference' of 2400cm.
Once assembled and calibrated, the unit was erected on the roof of my garage/shed and the display was set up in a nearby window so I can see what the wind is doing whilst I'm safely inside.
One final note concerns your choice of cycle computer. The one I chose turned out to have a sensor set-up that was a challenge to mount and balance - take a good look before you buy.
Pros: Cheap and effective.
Cons: Long-term weather resistance was not great (bearings got clogged with salt and sand after less than a year).
Enhancement Suggestions: Use a more robust (and better sealed) set of bearings and mount the read-out inside for easier viewing.
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