[TowerTalk] Wired Anemometer for Tower?

Grant Saviers grants2 at pacbell.net
Tue Apr 29 21:37:17 EDT 2014 

I also think Al's math is incorrect.  My frequency counters measure 
intervals to nanoseconds, better ones can to picosecond resolution, 
these are standard features on better frequency counters.  So one can 
measure rotational speeds to many digits, with a one pulse per 
revolution or 1000 pulses per revolution of the cups.  Since the 
physical measurement is the time period between pulses, converting to 
speed, which takes simple arithmetic, will be the "average wind speed 
between pulses" and  this can be averaged over any number of pulses.  
The practical limits are more likely the jitter of the pulses being 
measured and that can come from a lot of sources - electrical noise, 
mechanical vibration of the transducer, inaccuracies in the angular 
division scheme, etc.  I also wonder if the rotational speed of the cups 
is precisely linear with wind velocity and suspect a calibration factor 
is probably needed.  Also, as others have mentioned, the rotational 
inertia of the rotor is a limiting factor re measuring very short 
gusts.  However, since tower structural calculations are based on "3 
second gusts"  that is some guidance re what makes sense to measure.  I 
agree with the post that suggests about 2 to 4 Hz total system bandwidth 
is probably enough (ie 0.25 to 0.125 second sample period to keep 
Nyquist happy and a rotor inertia to match).

Arduinos can be programmed to measure to about 0.5 microsecond 
resolution, so there are big bucks NOT solutions.

Grant KZ1W


On 4/29/2014 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.
>
> I'm not the one who insisted that measurements had to be instantaneous, only the one who pointed out the inherent problem in attempting such with a rotating instrument that is a slave to the time domain.
>
> If gusts lasting less than the refr3esh rate of the instrument are really that much of a concern, then maybe the tower shouldn't be extended.  If a guyed tower, it's not information that you can do anything useful with anyway....except maybe run away!
>
> Al
> AB2ZY
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of David Gilbert
> Sent: Tuesday, April 29, 2014 1:36 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Wired Anemometer for Tower?
>
>
> 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 at 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 at contesting.com] On Behalf Of David
>>> Gilbert [xdavid at cis-broadband.com]
>>> Sent: Monday, April 28, 2014 12:35 PM
>>> To: towertalk at 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 at contesting.com] On Behalf Of
>>>> David Gilbert [xdavid at cis-broadband.com]
>>>> Sent: Monday, April 28, 2014 1:26 AM
>>>> To: towertalk at 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 at 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 at sbcglobal.net]
>>>>>> Sent: Sunday, April 27, 2014 8:08 PM
>>>>>> To: tomkd8deg
>>>>>> Cc: EZ Rhino; <towertalk at 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 at 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 at contesting.com] On
>>>>>> Behalf Of EZ Rhino
>>>>>> Sent: Sunday, April 27, 2014 4:10 PM
>>>>>> To: towertalk at 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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>>>>>>
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>>>>>>
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>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
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>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
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