Wind speed and direction idea

Get 5 of those BMP085 barometric pressure sensors. Place 4 of them on the 4 vertical sides of a cube. Place the 5th on the top surface or even inside the cube with a small hole on to allow pressure equalization. Measure the difference between the sensors and calculate/interpolate the windspeed and direction. The sensors are said to be accurate to 3 Pa. I have no idea what change in pressure a given wind speed causes, but Bernoulli's Principle should enable its calculation.

Problems: weatherproofing; addressing each sensor on same i2c bus.

Thouhts?

Paul

addressing each sensor on same i2c bus.

You could switch on/off the power supply of the sensors with a digital pin. Don't know the reset time of the BMP085.

If you hold the BMP085 in your hand, keeping it high and then low, the value already changes due to the difference in heigth. It is that sensitive.

I had the same idea as you, but I don't think it can be done.

To get a difference in pressure, perhaps a short tube in front of the sensor is needed. But with the tubes, 4 sensors in 4 directions is not enough, perhaps 12 sensors.

Erdin: I had the same idea as you, but I don't think it can be done.

To get a difference in pressure, perhaps a short tube in front of the sensor is needed. But with the tubes, 4 sensors in 4 directions is not enough, perhaps 12 sensors.

Please go on: if it might work with 12, why not 4?

robtillaart:

addressing each sensor on same i2c bus.

You could switch on/off the power supply of the sensors with a digital pin. Don't know the reset time of the BMP085.

Yes, I suppose that might work, or use those 4 pins to connect to an enable pin, if bmp085 has those, or some gates or transistors to switch the sda connection to one at a time.

Perhaps using a wider shorter tube when using only 4 sensors.

The pinhole in the BMP085 doesn't have to 'look' in the wind direction. It can be mounted to the top inside the tube, with the tube closed at the end.

Perhaps some kind of perforated membrane, to keep spiders out.

I still think it will not be practical. The measurements would be changing a lot, so some kind of average should be used. That would make it hard to make it battery operated.

Any turbulence at all will likely defeat you. The Pitot tube pressure sensing method (tube pointing into the wind) works at aircraft speeds, but Its sounds unlikely to have much success at low speeds.

The main problem is that pressure goes up as the square of the windspeed, so low speeds mean very small pressures. Ultrasonic anemometers are used these days, and the time difference in ultrasonic pulse propagation is linear in windspeed.

A google search revealed a bernouilli effect pressure system for measuring extreme winds - so clearly for high winds pressure is a workable method. It might be an interesting experiment to see how much signal to noise you get for normal windspeeds using something like pitot tubes or bernouilli venturi...

You couldn’t be more right.

I placed a tube over the BMP085, 1cm wide, 2cm long.
I blew right into the tube it to see what the value did.
That increased the kPa by about 1.0
However, if that was real wind, it would be a medium wind.

Normal values (with noise) are 1029.59 to 1029.72 kPa.

A gentle breeze will be very hard to detect.
And if one sensor changes more with temperature changes than others, the direction is impossible to determine.
It is also not possible to calibrate the sensors once they are installed outdoors. To calibrate, the wind must be zero, but that is not possible to create.

Erdin:
You couldn’t be more right.

I placed a tube over the BMP085, 1cm wide, 2cm long.
I blew right into the tube it to see what the value did.
That increased the kPa by about 1.0

Blowing right into the tube isn’t what happens with the actual wind. You need to hold the tube in a stream of moving air, not
anything like the same situation.

To get an idea of what the working pressure take your pressure sensors and mount them on the roof of the car on a mast (say 12 inches) and drive it on a calm day and see what how the readings correlate to the speedometer readings.

Would it be possible to use some kind of tube with a certain opening that would cause vibrations in the wind. And with an electret mic on every tube to detect it. The one with the highest amplitude is the one where the wind comes from.

Erdin:
Would it be possible to use some kind of tube with a certain opening that would cause vibrations in the wind. And with an electret mic on every tube to detect it. The one with the highest amplitude is the one where the wind comes from.

That would be a directional aeolian harp perhaps? Sounds worth doing for the fun of it even if its totally impractical
for measurement!

One effect that might be useful is measuring the cooling effect of the wind - a resistor to heat and a thermistor
to measure the temperature mounted together in a probe, sticking into a tube?

Using ultrasound is practical, but not easy to make DIY. And a distance is needed for the sound. So it can not be made very small.

Using something heated up, and the wind that cools it down, is done many times. There are even microphone made with it, and there a chips that do that.

I have one more idea: There are x-y force sensors. They consist a small standing metal rod (perhaps 2 or 3 cm heigth) on a base plate. The metal rod doesn't move, but can detect force in x-y direction. Add a light weigth round ball on top of it, and ready is the wind sensor. I'm looking for that sensor on the internet, but can't find it yet.

Using ultrasound is practical, but not easy to make DIY. And a distance is needed for the sound. So it can not be made very small.

check this thread - http://forum.arduino.cc/index.php?topic=53569.0 -

Instead of electret mics, also piezo elements can sense pressure. The idea to have 12 piezo pressure sensors around, has been patented in 1985. http://www.google.com/patents/US4615214

I still have not found a (cheap) x-y pressure gauge with a small steed rod on a plate. I do know they exist.

Check that too: http://forum.arduino.cc/index.php?topic=177670.msg1318933#msg1318933 Compact solid state anemometer, wind vane

I finally found what I was looking for.

It is called a "Strain Gauge Joystick" or "Force Joystick". These are joysticks that don't move, but measure the force like a load cell. They measure in X and Y direction.

If I could buy one for 20 dollars/euros and place a ball on top of about 10 cm diameter, I could measure the wind speed and direction. With a good ADC I should be able to measure a light breeze. Since the value would change a lot due to wind fluctuations, the Arduino should perhaps sample continuesly between 10 to 300 samples per second and calculate the average.

Here are some interesting links: http://www.smdsensors.com/Products/productView.asp?productID=22&categoryID=165 http://www.google.com/patents/US5228348 http://www.google.com/patents/US7516675 http://www.traxsys.com/OEMTechnology/JoysticksOtherPointingDevices/ForceJoystick/tabid/1366/Default.aspx

I found two patents, but there are more.

Since the value would change a lot due to wind fluctuations, the Arduino should perhaps sample continuesly between 10 to 300 samples per second and calculate the average.

and the min_speed and the max_speed, and the min_angle, max_angle and average angle // this one is more difficult, its an average modulo 360 degrees or so

Seems easy to me, it's only 2D, and the Arduino would have not much else to do. But I don't know where I can buy of those 2 axis strain gauge sensors to try it.

But I don't know where I can buy of those 2 axis strain gauge sensors to try it.

At least you can write and test code with some simulation.

Which country are you looking for the sensor? - http://www.smdsensors.com/Content/Contact_Us.asp - offices in US and UK... mail them