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bgowland
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MagPi 46 - Measuring Wind Speed question

Thu May 26, 2016 11:15 pm

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)?

Cheers,
Brian

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DougieLawson
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Re: MagPi 46 - Measuring Wind Speed question

Thu May 26, 2016 11:39 pm

The code is here: https://github.com/raspberrypi/weather-station

The interrupt daemon does the wind speed stuff
https://github.com/raspberrypi/weather- ... _daemon.py

That uses a 9.0cm radius.

The Sparkfun data sheet for it says a wind speed of 2.4Km/h will close the reed switch one tick per second.

There's some detail here: http://mile-end.co.uk/blog/?tag=anemometer and more here: http://mile-end.co.uk/blog/?p=86
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scruss
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Re: MagPi 46 - Measuring Wind Speed question

Fri May 27, 2016 12:29 am

You also have to factor in the drag of the other cup going against the wind. The official code uses a factor of 1.18 when basing the diameter on the outer edge.

There are lots of other fun corrections that you should really include in an anemometer calibrations, such as bearing stiction at startup, sensitivity to off-axis winds, and hysteresis (where you tend to read higher when the anemometer is slowing down than when it is speeding up.
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bgowland
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Re: MagPi 46 - Measuring Wind Speed question

Fri May 27, 2016 6:12 pm

DougieLawson wrote:The code is here: https://github.com/raspberrypi/weather-station

The interrupt daemon does the wind speed stuff
https://github.com/raspberrypi/weather- ... _daemon.py

That uses a 9.0cm radius.
OK, but my question was about WHY it uses 9cm radius. I was hoping to get to the root of the logic that for X number of 'clicks' per second the outside edge (at 9cm from axis) 'travels' a greater distance than the middle of a cup (at 7cm from axis), i.e., different distances in the same time.
The Sparkfun data sheet for it says a wind speed of 2.4Km/h will close the reed switch one tick per second.
Again, this implies a radius of 9cm and I still don't understand why (other than it being radius from axis to outer edge of a cup).
In that blog, the author uses 0.07m (7cm), the same as I had intended to use when I first measured radius from axis to the centre of a cup which seemed most logical to me.

I think I need to post a question on StackExchange maths site (or physics site) to get my head around this.

Cheers,
Brian

fruit-uk
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Fri May 27, 2016 6:49 pm

You could always test it using the DIY method and work backwards..
hang it out of a car window and drive a long a straight road on a calm day - both ways of course ;)

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bgowland
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Re:

Fri May 27, 2016 7:17 pm

fruit-uk wrote:You could always test it using the DIY method and work backwards..
hang it out of a car window and drive a long a straight road on a calm day - both ways of course ;)
lol - I like the idea but I live in West Yorkshire and finding a straight road and a calm day would be a feat in itself. ;)

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