[TowerTalk] Wired Anemometer for Tower?
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
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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