I really have a problem with the big fans, several hundred locally, that
blow almost constantly around EN50nj. (tongue firmly in cheek)
73 & DX,
Gary - AB9M
-----Original Message-----
From: David Robbins
Sent: Wednesday, April 30, 2014 7:11 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Wired Anemometer for Tower?
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 usi
ng 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@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
with anemometers.
73
Roger (K8RI)
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