Anemometer Calculations

A project log for Climate & Environment Monitoring Station

Create a monitoring station to measure weather, soil, seismic, solar, magnetic, and gravity conditions - focus best accuracy/dollar w/COTS.

sparks.ronsparks.ron 02/21/2017 at 18:240 Comments

I did a bit more thinking about the anemometer. The way the units I have (and have seen) work is by closing a reed switch with a magnet on the rotating cups. Both of them use the rotation speed to convert to wind speed. The interesting thing about wind speed versus rotation speed in an anemometer is that it is a "segmented" equation. In my mind I see it as analogous to a pump curve in fluid flow. What you see in both cases is a low flow regime where the inertia and friction forces dominate. In the middle region the movement is controlled by a "positive displacement" line. Then at the higher speeds you begin to see the effects of drag and edge spillage dominate.

The anemometer I am going to re-purpose is from Peet Bros., but the concepts and equations would be the same for my old Davis unit or even one that was DIY. Only the constants would change. For my anemometer the change from "slow" to "middle" happens about 8 MPH. The shift from "middle" to "high" happens at 135-ish MPH. However, at that speed the anemometer would have long since self-destructed. Therefore I am only going to look at the range from calm to 110 MPH.

When I do the math from the anemometer equations (which I will upload later in the project when I have cleaned up my ugly spreadsheet), I find that across the wind range of 1.5 MPH to 110 MPH, the output will vary between about 0.5 pulses per second (PPS) up to 145 PPS. That is too slow to easily measure as a frequency but will be easy to do as a period measurement which is then inverted in software to be a frequency.

The bottom of the range at 1.5 MPH is likely below the speed required for the wind to overcome the friction in the anemometer and start it turning. 0.5 seconds is 2000mS and on the upper end of 145 PPS, that would be about 7 mS. That is a very large range for a sensor, but not a problem for software. Using a non-blocking interrupt counter on the Arduino will likely be all that is needed, apart from input conditioning.

On the other hand, since the input will need some passives and a Schmitt trigger to condition the input, it would be very easy to add a gated counter and handle the counting at the input before "bothering the Arduino" with an interrupt. I need to look at the pros/cons of that. Any method used will need to gracefully handle a rollover condition in the counter for times when the wind is calm or too slow to turn the anemometer.