[TowerTalk] Wired Anemometer for Tower?

[Jim Lux]

On 4/28/14, 6:43 AM, Al Kozakiewicz wrote:
> Regardless of the specific instrument, ANY digital measurement system will require some amount of time to sample the transducer output.  The time required depends on the resolution of the transducer, stability of the reference timebase and the measurement accuracy required.
>
> You're calculating windspeed from rotational velocity.  The latter is probably measured using either an optical sensor or some electromagnetic device.  In either case, you're counting pulses over a specific period of time to get a large enough sample to outweigh the measurement uncertainty, which increases as the sample time drops as well as with decreased encoder resolution.
>

Actually, you can measure the time between pulses, so you can get a 
pretty rapidly updated instantaneous measurement.

For a rotating cup or a propeller/windmill style anemometer, there's the 
rotational inertia to consider too.

Interestingly, the rotation rate of the rotating cup anemometer is 
fairly linear as a function of wind speed.


> I have no first hand knowledge of converting anemometer RPM's to windspeed, but if the relationship isn't linear, you have the additional step of a table lookup or curve fit to convert the rotational speed to wind speed for display.


It is linear, but in any case, the calibration lookup even if a 10th 
order polynomial, is something that can be done in milliseconds, if not 
microseconds.


There's all kinds of clever wind speed measuring designs out there. Some 
measure direction at the same time (e.g. by making one of the cups on a 
standard anemometer a bit different, the rotation speed varies 
cyclically, with the phase of the cycle depending on the wind direction.

There's hot wire anemometers which can have very fast response times 
(milliseconds or less), but which require a bit more calibration, and 
are sensitive to ambient temperature.

There's various pressure based schemes (pitot tubes, vanes with strain 
gauges, etc.) which have a "square of the wind speed" characteristic.

The rotating cup and windmill styles are popular because they tend to 
read out linearly, and can be made with entirely mechanical components 
(e.g. a turns counter)



There's also a whole lot of instrumentation and measurement things that 
might be considered, especially when looking at things like gust speed. 
  For instance, if you're looking for "fastest mile" wind speed, you 
might post process the raw sensor data differently than if you're 
looking for fastest 10 seconds.

There's some changes in how gusts are defined over the years too.

NWS defines it currently as max instantaneous wind speed in last 10 
minutes, and the "wind speed" is defined as the 2 minute average.

There's also a "maximum sustained speed" about which those who care can 
read more at this interesting WMO report

http://www.wmo.int/pages/prog/www/tcp/documents/Doc2.3_WindAveraging.pdf