No ... Al is not correct in his assessment, at least not at a practical
level. The 12.5% "inaccuracy" he claims is purely arbitrary ... it's a
12.5% inaccuracy for one revolution, which is meaningless when you're
trying to measure speed and not rotational position. And at speeds of
interest it's only an inaccuracy for the very briefest interval of time
... 12.5 milliseconds at 25 mph. After that you inherently get an
averaging effect that greatly improves the accuracy.
You guys need to step back and actually think about what you're talking
about. It does NOT take "big $" to measure wind speed reasonably
accurately for our purposes here. The most relevant uncertainty I can
think of in the case of the Vortex anemometer is the effect its
rotational mass has on it's ability to respond to CHANGES in wind speed.
Dave AB7E
On 4/28/2014 10:58 AM, Kim Elmore wrote:
Al is exactly correct in his assessment. Meteorologists (like me) worry about
this stuff all the time. If we want to measure the turbulence spectrum down to
the eddy dissipation rate, we need extremely high resolution and accuracy. That
means big $ instrumentation.
Kim N5OP
"People that make music together cannot be enemies, at least as long as the music
lasts." -- Paul Hindemith
On Apr 28, 2014, at 12:39, Al Kozakiewicz <akozak@hourglass.com> wrote:
I don't think it's misleading, but you'd have to look at my prior post.
If you're going for a "fast" measurement technique, you can measure the time
between two pulses. If you do that, the absolute position uncertainty is +/- 12.5% of a
revolution - based on the very nature of having only 8 pulses per revolution.
On the other hand, if you count pulses over a fixed period of time, the
measurement uncertainty drops (logarithmically, I suspect) with the number of
revolutions in the sample. It is not constant, but +/-12.5% of a revolution
added to the entire sample, which (without doing any math) gets down to 4% in
just a few revolutions.
All I'm pointing out is that with a digitized anemometer you cannot have both
accuracy AND high measurement speed at the same time. For accuracy you either
add more resolution to your transducer or sample over a longer period of time.
No free lunch here!
Al
AB2ZY
________________________________________
From: TowerTalk [towertalk-bounces@contesting.com] On Behalf Of David Gilbert
[xdavid@cis-broadband.com]
Sent: Monday, April 28, 2014 12:35 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Wired Anemometer for Tower?
Sorry, the calibration factor is 2.5 mph per revolution per second. I
got sloppy when I typed that, although I would have thought that was
almost intuitively obvious.
But your +/- 12.5% assertion is greatly misleading. If indeed the 2.5
mph per Hz factor is (as InSpeed claims) accurate to +/- 4%, then the
absolute accuracy is degraded further only by the accuracy of whatever
is measuring the length of time between pulses (which in most cases is
negligible by comparison). The 12.5% figure would apply to the
accuracy of measuring ACCELERATION of the wind speed, but not the wind
speed itself ... at least not unless somebody was silly enough to care
about wind speed accuracy within, say, a 6 msec window (160 pulses per
second at 50 mph). I'd bet that the mass of the cups dominates within
intervals of that magnitude.
73,
Dave AB7E
On 4/28/2014 7:12 AM, Al Kozakiewicz wrote:
but at 2.5 mph per revolution and 8 pulses per revolution
You're missing a time constant there. 2.5 mph per revoution per ????
A single revolution or a single pulse has a measurement uncertainty of +/- 12.5
% with 8 ppr. So if you're going to measure velocity by measuring the time
between two consecutive pulses (the smallest possible sample), your accuracy is
never going to be better than that. But response time will be fast.
Al
AB2ZY
________________________________________
From: TowerTalk [towertalk-bounces@contesting.com] On Behalf Of David Gilbert
[xdavid@cis-broadband.com]
Sent: Monday, April 28, 2014 1:26 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Wired Anemometer for Tower?
Not to pound the InSpeed drum, especially since I have no connection to
them at all other than as a long ago customer, but InSpeed also sells a
high resolution anemometer for $89 that puts out 8 pulses per
revolution. The response of either Vortex version is of course limited
by the time between pulses, but at 2.5 mph per revolution and 8 pulses
per revolution for the hi-res version the ability to almost instantly
track wind gusts would be limited primarily by the mass of the rotating
cups (no spec that I am aware of) ... and they are very light weight.
73,
Dave AB7E
On 4/27/2014 9:23 PM, Kim Elmore wrote:
I can't speak to the Pro2 model, but any anemometer must average over a period.
I was incorrect about the Vue: it yields 4 s averages in each packet. Sonic
anemometers average over something like 10 pulses. Depending on the brand,
that's anywhere from 1 s to 0.1 s. So, if you really need wind speed resolution
better than 2 or 4 s, you need to obtain a sonic anemometer. Because of the
mass of our antennas and towers, I'd be surprised if they have a significant
response to frequencies much higher than 0.25 to 0.5 Hz, but to know that
requires an engineer's analysis.
Low-end sonic anemometers can be had for a few thousand dollars. They can,
however, require periodic maintenance due to bird pecks (birds peck at the
transducers) and bird droppings that contaminate the transducer surfaces.
If you must have higher resolution than this you need very deep pockets indeed
and a lidar anemometer.
As for radar: the NEXRAD (WSR-88D) requires 5 min to complete a volume scan,
but the latency for each radial of data is on the order of a second or so. So,
it's available in real time. Thus, there's no significant latency once a
volume is complete. It does take a few seconds to move the antenna back to the
0.5 deg elevation and start the next volume, but it moves pretty fast in
elevation -- several degrees per second.
Kim N5OP
"People that make music together cannot be enemies, at least as long as the music
lasts." -- Paul Hindemith
On Apr 27, 2014, at 22:29, "tomkd8deg" <kd8deg@centurytel.net> wrote:
The unit I have is the Davis Vantage Pro2 wired, and the 4 second delay is what
Davis tech support told me when I questioned the accuracy of the wind readings
that were being displayed.
Tom KD8DEG
From: Kim Elmore [mailto:cw_de_n5op@sbcglobal.net]
Sent: Sunday, April 27, 2014 8:08 PM
To: tomkd8deg
Cc: EZ Rhino; <towertalk@contesting.com>
Subject: Re: [TowerTalk] Wired Anemometer for Tower?
I'm not sure that's true about the wind. I have a Davis Vantage Vue. The remote
unit does some processing to generate a 2 s mean speed and a peak gust value
within the 2 s window. Data are then sent in packets every 2 s to the base unit.
Mine is up 10 m (a WMO standard height). During calm winters nights it will
measure an anomalously high minimum temperature. The anemometer is shielded a
bit by the tower structure when it's downwind of the tower and so the wind
speed is biased low. Direction appears unaffected.
The rain gauge is a tipping spoon with a 3" diameter catchment. It under
samples only at high rain rates. The temperature sensor and dew cell are not
aspirated in this unit and so temperature is biased a bit high due to radiation
errors on calm, clear Summer days.
I highly recommend any of the Davis units, but the Peet Bros. Ultimeter 2100 is
also a very fine unit. Both are comparable in price (~$400).
Kim N5OP
"People that make music together cannot be enemies, at least as long as the music
lasts." -- Paul Hindemith
On Apr 27, 2014, at 18:29, "tomkd8deg" <kd8deg@centurytel.net> wrote:
The Davis wired version, which I am using, has a 4 second delay for wind,
and direction display. That can mean the loss of 10 to 20 mph in a gust
situation.
Tom KD8DEG
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of EZ
Rhino
Sent: Sunday, April 27, 2014 4:10 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Wired Anemometer for Tower?
I should mention one other thing about the "cheap" weather station
anemometers that drive me crazy. The first WX station I had only updated
the wind speed about every minute. The LaCrosse updates about every 12
seconds. This is almost worthless...I want to know the wind speed NOW! The
stuff from Inspeed is real time, very nice.
Chris
KF7P
On Apr 27, 2014, at 14:03 , Roger (K8RI) on TT wrote:
On 4/27/2014 3:15 PM, EZ Rhino wrote:
I have tried a few different weather stations, so I can unequivocally tell
you what NOT to get.
DON'T buy anything under $300. *sigh*
Mine cost 40 something, works fine, is made of cheap plastic. Uses plain old
Duracell non rechargeable. Last 2 years or more outside. The ones indoors
only last 6 mo to a year. Probably cost twice that much now...if available
Mounted on the tower where I can just reach it with an 8' stepladder.
Does nothing except report wind speed, direction,temp, humidity, and
relative temp.
73
Roger(K8RI)
Unfortunately the cheap ones are total garbage. The recent one I got is a
LaCrosse station, purchased from Costco. Junk! Junky cheap plastic, the
rechargeable batteries in the solar powered wireless anenmometer/wind vane
unit went bad in the first year, and the rain bucket stopped working in the
first few months after purchase.
The Davis units are good, have a buddy with one and it is quality stuff.
And about $300. You pay for what you get. I agree, I think the wired units
are more reliable and last longer.
Ok now take a look here: www.inspeed.com These guys make anemometers
that are decent quality and not expensive. They also have a cool computer
hookup version that allows your PC to track all the data and put it in a
spreadsheet. AND it has the wind switch option, where you can program the
system to turn an appliance on or off (read: lower your tower!) if the wind
goes above a preprogrammed speed. Pretty neat. I don't have this option
yet. I think AB7E has one of the inspeed units too, maybe he can provide
some feedback.
Chris
KF7P
On Apr 27, 2014, at 11:01 , Steve Jones wrote:
Can any of you recommend a wired anemometer or weather station unit?
I want to install an anemometer to monitor the wind speed at the top
of my tower. All I can find online are wireless units. On this
reflector I have seen a number of bad reviews of wireless weather
sensors, because their little wifi transceivers get damaged by the RF
from a ham antenna. I have a spare CAT5 cable at the base of the tower
available.
73,
Steve
N6SJ
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