[TowerTalk] Wired Anemometer for Tower?
k1ttt at verizon.net
Wed Apr 30 08:11:08 EDT 2014
and accuracy compared to what, and over what period?? wind speed is HIGHLY variable over small distances and elevation change. There are specifications for measuring wind that require so much open area, a specific height above terrain(much lower than you would think), distance from buildings, etc... basically measuring wind speed on a tower is only interesting for you and your antennas, no one else would have any use for it. take a look at my station's quality control charts... http://weather.gladstonefamily.net/qchart/AR841?date=20140428 you will see that normally things like temperature and barometric pressure track the expected values pretty well... but my wind speed, when I just looked at yesterday's values, were off by a factor of about 10 (my reading is higher!) from the expected values. this is because my anemometer is 50' up and not far enough from the house, and my house is almost 1000' above the nearest airports or nws offices. and yet when measuring gusts using skywarn tree motion estimates or a handheld pith ball pressure gauge it usually reads low by a factor of 2 (a gust estimated at 50-60mph only reads 25-30mph on the anemometer). part of that is the response time, but most is probably the averaging over 3 to 5 seconds that I think the davis uses. I tried at one time to look up the specs for reading gusts, it gets very confusing even on the noaa web site, there are at least 3 different specs (that I found) for different instruments that use 3, 5, or 8 second gust averaging. If you really want an eye opener about wind and height above ground come up my 180' tower some day, it can be dead calm on the ground, and even at the 50' elevation of the anemometer, but still have a nice 5-10mph breeze at the top... nice for me, but tough for the ground crew fighting black flies, mosquitoes, and deer flies.
Apr 30, 2014 07:40:14 AM, K8RI-on-TowerTalk at tm.net wrote:
On 4/29/2014 9:59 PM, Jim Lux wrote:
> 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.
Just count the pulses, and divide by time. Plastic anometers have very
little rotational mass.
With out getting fancy you can easily get better than 1 mph accuracy.
To me, more than every three seconds is wasted energy/effort/money/time
as is all the effort expended to measure errors that are meaningless
unless you want lab accuracy for a study. Then calibration becomes a
major portion of the effort. Then you need standards traceable to the
NBS and someone certified to do it. Been there and done that, but not
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