WXforum.net
Miscellaneous Debris => Weather sensors, design, analysis, discussion, home brew => Tech Corner => Anemometers => Topic started by: kmahler on April 07, 2017, 11:43:03 PM
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I see lots of discussions about calibrating sensors but nothing about calibrating anemometers. I really want to do a write-up comparing wx stations based on accuracy rather than features. I purchased two NIST traceable mercury thermometers for temp and wet bulb checks. I have read up on the barometer procedures published in this section and think I can reproduce the protocols. The rain gauges will be simple enough based on what I've read here. But, I haven't seen anything on testing the accuracy of anemometers.
Does anyone have a good method? My thought was to build a temporary wind tunnel large enough to place the sensor in and test them that way. But, I need an accurate reference. I've considered one of the Kestral Handheld meters that are NIST traceable but, I wonder if that's necessary. There must be another way.
I'm open to ideas and brainstorming.
Thanks,
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Possibly a passenger to hold it out the window while you drive using your speedometer to compare? Only thing is the anny would have to be away from the car's deflection of oncoming air flow. Might rule it out right there.
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Possibly a passenger to hold it out the window while you drive using your speedometer to compare? Only thing is the anny would have to be away from the car's deflection of oncoming air flow. Might rule it out right there.
I'm looking for something far more scientific. That method has so many unknown variables, I wouldn't consider it for even a estimate of accuracy.
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A home-made wind tunnel sounds like fun. You will likely need some straightening vanes between the fan or blower to laminarize the flow. As far as calibrating the tunnel, you could look at hot-wire anemometers, or maybe the HVAC industry has something you could use.
Here are some things I've learned about cup anemometers: In theory, the swept circumference of the cups travels at the same speed of the wind. It helps if you think of the cups as a wheel, and the wind as a road.
In practice, the cups always spin slower than the wind due to aerodynamic drag and bearing friction (except for the case of over spin caused by a big gust followed by a lull or such). Well characterized anemometers have a correction factor to get the "true" wind speed from the indicated one. Davis mentions theirs in a app note.
Back to the first point... The larger the diameter of the cup wheel, the slower the revolutions for the same wind.
The larger the cups, the lower the threshold. My copper add-a-mometer has a lower threshold than the Davis right below it due to cups (4" diameter vs 1.5")
Cup geometry has a small effect. I read a study from the 40s I believe, that elliptical cups were better than hemispheres.
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I still use an old Dwyer Portable in the field which holds up to accuracy next to my Kestrel.
(http://i.imgur.com/gSEWnEn.jpg)
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Ultra sonic reference anemometer would be the way to go $$$