[TowerTalk] Wired Anemometer for Tower?

Jim Lux jimlux at earthlink.net
Tue Apr 29 21:59:05 EDT 2014

On 4/29/14, 5:08 PM, Al Kozakiewicz wrote:
> I'm not going to belabor the point.  Wind speed is derived by measuring the change in rotational position divided by time.   The shorter the sampling interval (time), the lower the measurement accuracy. There is no reason for this to controversial.

not true. Consider a frequency counter in your shack.  You can have a 
particular gate time and count positive zero crossings during that gate. 
   For this simple scheme, yes, short measurement gate means low precision.

But instead, if you do a ratiometric counter, and count the length of 
time it takes for N cycles, then compute the frequency by N/(time 
duration), you can potentially more accuracy.  Then your accuracy 
depends on how good your time stamps are.

For low frequency signals (e.g. 1 pps from a GPS) doing "time 
measurements" is almost always better than doing "frequency measurements"

Same with the anemometer.  If I get 8 pulses/rev and it's spinning at 1 
rev per second, and I count for 1 second, then sure, my uncertainty is 
0.125 count.

But if I hook up my 10 MHz clock and count the time between pulses, my 
accuracy is +1/-0 count out of 1,250,000 counts.  That's a whole lot 
more accurate.  (actually, i'd worry more about the mechanical accuracy 
of the thing making the pulses at that point).

In any case, getting 1% accuracy on rotation rate is easy.

Other effects will be much larger.

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