Topic: Davis VP2 anemometer: recommend "inexpensive" anemometer to calibrate?

[boss281]

I know there are several threads out there regarding anecdotal under reporting of wind speed from the VP2, but I didn't want my question lost in the threads.  Can't find anything under search either.

I bought an Ambient Weather system several years back because the home I purchased in 2010 and its locale seemed to have an unusually high frequency of high wind and wind gusts.  The weather system confirmed my suspicions, and I attribute that to local geography.  The highest gust I recorded on that system was 52mph during that derecho a few years back.  When the AWS died, I purchased the VP2 and have successfully got it working with WU and CWOPS over the past two weeks with help from my new friends on this forum.

The recent high wind storm that swept the mid Atlantic and east coast was the worst one I experienced.  I watch trees bend in half and snap, the entire house shook, and I was up all night because the noise was deafening.  Yet, wind gust that must have been in the 50+ range never recorded.  I was dumfounded with a 45mph max shown with the VP2.

Now, after the long winded story, the question: where can I get an inexpensive anemometer that would give me a very close approximation of wind speed so I can calibrate it with the VP2, and see if I need a new unit or part? I'm willing to spend $100, hopefully that's enough...

[Felix1]

Kestrel portable wind meters are widely used by shooters, firefighters, parachutists, etc.


Their 1000-series meters go for under $75 discounted. The more advanced meters are many times that.

[boss281]

Yeah, those seem rated well.  Just Amazon'd the 1000.  Thanks for the response.

[johnd]

The only reason that a 6410 anemometer is likely to under-record is if the speed bearing is binding. If the cups spin freely by hand with no grinding detectable then this is pretty unlikely. If the bearing isn't perfect then (in the current 6410 version) fit a new cartridge. Other than this the 6410 speeds are likely to be accurate for the anemometer's siting.

I'd estimate that 95% of the time that people think the wind speed readings are too low it's because of non-ideal siting of the anemometer and not appreciating how critical factors like height and all-round good exposure are to recording maximum wind speeds. I'm not quite sure how you're planning to compare hand-held and 6410 speeds but it won't be easy to get good consistent results.

[PaulMy]

This from an old thread http://www.wxforum.net/index.php?topic=2753.msg23357#msg23357
I have used that.

Enjoy,
Paul

[boss281]

The only reason that a 6410 anemometer is likely to under-record is if the speed bearing is binding. If the cups spin freely by hand with no grinding detectable then this is pretty unlikely. If the bearing isn't perfect then (in the current 6410 version) fit a new cartridge. Other than this the 6410 speeds are likely to be accurate for the anemometer's siting.

I'd estimate that 95% of the time that people think the wind speed readings are too low it's because of non-ideal siting of the anemometer and not appreciating how critical factors like height and all-round good exposure are to recording maximum wind speeds. I'm not quite sure how you're planning to compare hand-held and 6410 speeds but it won't be easy to get good consistent results.

johnd, you make a valid point.  And perhaps, I'm comparing apples and oranges (Davis VP2 and my old and now dead AmbientWeather 1001).  All I can tell you, I experienced directly the recorded value of 50+ on the Ambient, and the speeds coming off that last storm were much greater.  Not empirical and seat of the pants, but I'm telling you...

Anyhow, I moved the unit twice the distance from the house (estimated 150 feet).  I'll need to see if I can dig a pit deep enough to cement a 25 ft pole and get the anemometer up there...

[johnd]

All I can tell you, I experienced directly the recorded value of 50+ on the Ambient, and the speeds coming off that last storm were much greater.  Not empirical and seat of the pants, but I'm telling you...

I do understand, but eg were the Ambient and VP2 at exactly the same position and height? How sure can you be that the Ambient was correct? It's a common experience that despite strong winds, an anemometer may be in a partly sheltered environment if it's at all close to buildings, trees etc and may appear to be under-recording. It's a shame we don't have 3D ways of looking up into the air and seeing how the air currents swirl and blend due to ground friction, obstructions like buildings etc, but all we can realistically do is to get the anemometer up into clear, free-flowing air as far as possible.

The thing about cup anemometers like the VP2 one is that there's not too much to go wrong with them that would make them under-record other than poor exposure. Provided the cups are spinning freely on their spindle then the speed of rotation is determined purely by the geometry of the cups, which obviously isn't going to change over time. And, certainly for the VP2, this has all been checked in wind tunnel tests.

Other than that, the only other possible source of error is under-counting of the revolutions. In older 6410 units with a reed switch then a faulty switch certainly could miss revs, but for a solid state switch in anemometers made in the past 4-5 years that's unlikely now. I guess the counter circuit on the SIM is another possible source of error, but again that seems pretty unlikely - these things tend either to work correctly or fail completely.

So, my money would be on limited exposure.

[boss281]

John, again, thanks for the thoughtful discussion.

I'm ready to concede the AW system might have been flawed.  It was also 50 feet from the house for years and data was likely skewed from the NW to ENE (house in the way).  During the latest windstorm my Davis unit was in the same spot, but I've since moved it to 100ft at the property border behind the house.

The attached shows my property.  NW is to the top of the image.  In general, the topography slopes DOWN right to left, and DOWN bottom to top.  Looking NW, the slope continues down perhaps another 100 feet for about 200 yards, then slopes UPWARD another 50 feet above my houses elevation.  I'm in a slight valley that is roughly east to west, on the SW side of the ridge.

I'll toss it up to location, and work on getting the anemometer on a pole as high as possible (it sits but 6ft off the lawn at this time.

[dalecoy]

..... Provided the cups are spinning freely on their spindle then the speed of rotation is determined purely by the geometry of the cups, which obviously isn't going to change over time. And, certainly for the VP2, this has all been checked in wind tunnel tests.

Not that it has any great bearing on the discussion in this thread (location location location)

But your statement is only correct for the "normal" steady-state condition.  That is, reasonably constant wind.  The check in the wind tunnel is done using contant air velocities.

Wind cup assemblies will vary a bit under situations that could be called "impulse" and "coasting", because of several factors (inertia, back-side resistance, etc.).  Consider what happens when a sudden "strong puff of wind" occurs - the assembly has to suddenly speed up - and may not finally reach the highest speed of the "puff" - or might actually reach a higher speed for a brief time.  And if the wind suddenly stops, the assembly doesn't suddenly halt - it coasts down in speed.

That's not a big deal - it's just a minor detail.

[johnd]

But your statement is only correct for the "normal" steady-state condition.  That is, reasonably constant wind. 

Sure, agreed - you're talking about angular momentum effects, though I'm guessing that a relatively lightweight set of plastic cups will suffer from these effects less than potentially heavier metal cups (though maybe some thin lightweight alloy cups could be lighter still than plastic while still being strong enough to have a good working life?).

And there are obviously other second-order effects like which direction the wind is blowing from, relative to the anemometer mounting pole and arm structure (though that's compensated for in the VP2 console). But another important factor has to be the gating interval of the measurement - a different type of anemometer like a handheld unit that might use eg a 1-second or a 4-second measurement interval rather than the VP2's 2.25 secs is going to read significantly different gust speeds even with identical exposure.

But all I was trying to say was that other than potential bearing issues, there's little in the way that the 6410 unit intrinsically works that is likely to make it under-record. But it's not necessarily going to read identical values to another good anemometer for all of these secondary reasons.

Incidentally, I should have added the usual point about anemometer exposure, ie that it's recommended to be distanced from any nearby object by at least 10x the height of that object.

[dalecoy]

..... But another important factor has to be the gating interval of the measurement - a different type of anemometer like a handheld unit that might use eg a 1-second or a 4-second measurement interval rather than the VP2's 2.25 secs is going to read significantly different gust speeds even with identical exposure.

An excellent point, of course.  Shorter intervals would read higher gust speed for brief gusts.