[time-nuts] Weather/units question for European members

Mark Sims holrum at hotmail.com
Fri May 23 23:12:21 EDT 2014

>The project sounds like a fun hack -- I would be curious as to the >resolution you achieve with these modules.-------
The best description on the net about building a sonic anemometer is one by Hardy Lau:http://www.technik.dhbw-ravensburg.de/~lau/ultrasonic-anemometer.html

I have also built one that I call the "Cheap Ass Sonic Anemometer"   It is part of a portable weather sensor for use at launches of large model rockets (some reach over 100,000 feet).  It is built out of very inexpensive, off-the-shelf components.  The only wiring involved is hooking power, ground, clock, and data lines to small off-the-shelf circuit boards (mostly from China off of Ebay).  I did lay out a custom circuit board so I don't even have to do that.   A commercial sonic anememeter can set you back $5000.
It uses 4 HC-SR04 ultrasonic distance sensors ($1 each!) as the wind sensors.  These are small modules that you drive with a 10 microsecond "ping" signal and they output a pulse width dependent upon the distance to an object.  I use two modules facing each other on each axis and fire them in pairs.  Instead of picking up an echo,  they receive the ping from the opposite module.   You can fire them around 30 times per second.
I use an $5 ATMEGA328 chip based CPU board (you could use an Arduino)  to measure the pulses and calculate the wind direction,  speed,  and temperature.  I was originally using a more powerful processor (ATMEGA128) that has two 16 bit counters with input capture capability to measure the pulses with 65 nanosecond resolution,  but to make the device more accessible for other people to build,  I switched to the ATMEGA328 and time the pulses in software to around 1 microsecond resolution.  I was measuring all four directions at the same time,  but started seeing some cross-axis interference with some of the SR04 modules and switched to simultaneously measuring the N-S and S-N times followed 3 milliseconds later by the E-W and W-E times.  Surprisingly the downgraded pulse measurement capability did not affect the results to any significant degree.
The structure is built out of 1 inch inside diameter PVC plumbing pipe and couplers/caps...  around $5 total.   The ultrasonic sensors are spaced around 0.65 meters.  The D/L ratio is around 20.  The larger the D/L ratio,  the smaller the errors caused by the structure interfering with wind flow.
The device also contains a 10 degree-of-freedom inertial measurement unit board ($11) that is used as a tilt compensated magnetic compass and highly accurate barometer.  It also has a DHT-22 humidity sensor ($5) and compensates the sound measurements for humidity and air pressure (which can be quite high a high temperatures and humidity levels).  With the compass measurements you can deploy the sensor in any orientation and it adjusts the wind readings for however it is aligned.  Also,  you can buy the IMU boards for less than the price of the pressure sensor alone!  The software auto-detects and supports 5 different pressure sensors,  4 different accelerometers,  and the popular HMC5883 magnetometer chip (also used in the popular ST LSM303 integrated magnetometer/accelerometer.
It also has an AS3935 based lightning sensor board ($20)...  but they don't seem to work very well.  I live next to a golf course that has an elaborate lightning warning system and the AS3935 has never triggered even though the golf course waring sirens have gone off several times.  It also has a UV light sensor ($13 from Sparkfun) for reporting a UV index.
I get temperature measurements with around 0.5 degree F accuracy and 0.25 F noise (averaging readings over 3 seconds).  I am still characterizing the wind speed/direction measurements,  but they seem to agree quite well with a cup/vane anemometer...   basically if your temperature results are accurate your wind measurements will be spot on.  The nice thing about measuring temperature via sonic measurements is that the measurements are unaffected by solar heating of the apparatus... it does not need to be in the shade.  Also

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