I have a slight problem with the calculation & calibration of the "Measuring Wind Speed" feature on p76-77 of Issue 46.
The anemometer sensor shown in the feature looks very much like the one I bought from Maplin and mine also has a radius from axis to outer edge of hemispherical 'cup' of 9cm. I chose, however, to measure from axis to centre of each cup (7cm) as, it seems to me, the inner portions of the cups are 'catching' the wind.
At this point my maths (or perhaps that of the feature) are falling down...
Using the calculation 2 x pi x r, the outer edge of the cups travel approx 56.5cm (based on 9cm radius) and each rotation produces 2 clicks of the reed switch. At the same time, however, the centres of the cups only travel approx 44cm (based on 7cm radius) but, again, each rotation is producing 2 clicks.
Using Speed = Distance / Time surely makes no sense (no offence meant - I'm the one who's confused here) - the reality is that different parts of the arms and cups will describe larger or smaller circumferences and as such cover more or less distance whilst at the same time, the reed switch is activated the same number of times.
If you've followed this far and have an explanation to clear my confusion I'd be grateful - should I use "outer edge" radius (bearing in mind it's unlikely to be 'catching' much wind) or "inner centre of cup" radius (which seems like the point where the wind will exert most power)?