I just figured out how the AcuRite VN1TX 5-in-1 sensor reports wind speed in digital form (both over the radio link and through the AcuRite 02032C console). I was long puzzled by what seemed to be a weird relationship between the data values, and wind speed displayed by the VIS reader but last night it finally dawned on me what is going on. I confirmed my guess today by creating a table of digital data versus wind speed on the VIS reader.
Each wind speed report contains an integer number, which is the number of full cup rotations in a four-second period (others have reported that this is actually the highest 4-second count measured in an 18-second window).
By comparing reported integer values to wind speeds on the VIS reader the following relationship emerged:
V(km/hr) = 0.8278 n + 1.00,
where "n" is the reported data value and V is in km/hr. This a well known industry standard anemometer equation. This equation fits the data I gathered to within +-0.005 km/hr, so I'm pretty sure the equation is correct.
Since VIS is working cooperatively with AcuRite I assume this equation came directly from AcuRite.
What about when "n" is zero? That means the cups are not moving. Speed is zero. When "n" goes to one (i.e. one cup revolution every four seconds), it indicates a wind speed of 1.83 km/hr -- so that is the anemometer's minimum reportable wind speed.
For those anemometer gurus out there, and just for fun: From this we can calculate the "K" factor. I measured the radius from the pivot to the center of the cups (called "Rrc") and it came out to 5.2cm. To determine "K" we must have the equation above in different units. That is, frequency of cup rotation in revolutions per second and wind speed speed in cm/sec instead of km/hr. You can check my math, but I came up with this:
V(cm/sec) = 91.97 f + 28,
where "f" is now the numer of cup rotations per second. Now we can compute "K":
K = V / (2 x pi x f x Rrc) = (91.97 x f) / (2 x pi x f x 5.2) + 28 / (2 x pi x f x 5.2)
Here is a table of K at different wind speeds:
N KPH K
======================
4 4.3 3.7
11 10.1 3.1
23 20.0 3.0
35 30.0 2.9
59 49.8 2.9
Of course, normally "K" is determined by actual measurement and I've just calculated it here from the anemometer wind speed formula. This puts K in the normal range for anemometers (about 2.6 to 4 or so).
For those interested in more detail, here are links to two technical papers on anemometers:
Pindado, Cubas and Sorribes-Palmer, "The Cup Anemometer, a Fundamental Meteorological Instrument..."
Issue 2014/14 of the Journal "Sensors",
www.mdpi.com/journal/sensors, pp21418-21452.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4279541/pdf/sensors-14-21418.pdfKristensen, "The Perennial Cup Anemometer", Issue 2-1999 of Wind Energy, pp59-75
http://www.windsensor.com/technical/The%20Perennial%20Cup%20Anemometer.pdf
