Pic based Wind Speed meter

[Dave]

Bobbo_ZA, Why not use some OLD bell's from a couple of OLD phones. I did this years ago and it is still working as far as I know.

Dave Purola,
N8NTA
ALLSUNTRONEX.COM

[Charles Linquis]

Please note that a potentiometer has a limited life, even if it is a good one.

A "hot wire" anemometer requires a Whetstone bridge and an amplifier. The hot wire itself is generally fragile. I assure you that the PTC method I outlined above will be easier to implement.

[cncmachineguy]

Charles, I for one really like the wire method- no moving parts is always good. Is there anything needed to know about the wire mounting? Ie position, length...

[Charles Linquis]

I'm advocating that you NOT use the traditional hot wire approach. Most of the hot wire sensors use either a (special) purchased wire, or have you break a small incandescent bulb to get the wire. Both are relatively fragile. When I did this project I found some small (approx 3/8" diameter X 1/8" thick) PTCs in the DigiKey catalog. I don't see them there anymore. You could perhaps use one from

http://www.specsensors.com/custom/co...er.asp?thm=srh

The idea is that they have a low resistance so that when you apply 5V you can get enough power into them to easily heat them up to their "knee" temperature. The ones that I chose had a 5 ohm resistance and a knee temperature of 110C. When you apply 5V across them, they initially draw 1A (5Watts), which quickly heats them to their knee temperature, at which point their resistance rapidly increases and the current falls. If you try to cool them (by moving air across them), the current draw increases. Because of their self-regulating nature, they draw as much current as needed to keep their temperature at 110C. If you put a small (.1ohm or so) resistor in series with the PTC thermistor, the voltage drop across that resistor is proportional to the cooling effect of the air - which is proportional to the velocity. You do have to amplify that voltage in order to get it p to a good range for the PICs A/D converter.

The airspeed to voltage drop across this resistor is not a linear function. Also, be aware that you must compensate for ambient temperature, since a 20 MPH wind at 0C will cool a hot thermistor more effectively than a 20 MPH wind at 30C, so you need to add a second temperature sensor to measure ambient and use that in a calibration routine. I can't really give you the calibration code that I used because I'm certain your thermistor will be different than the one I used.

Still, with a little work and some calibration, the method outlined performs very well. I have used it to measure air velocities in electronic cabinets.

[Bobbo_ZA]

Dave, that would work, although I would expect it to need quite a stiff wind to get over the mass of the bells? what did you use for a pickup? are there any more details of your project posted anywhere?

I was thinking about it again the other night while playing on the breadboard, would it be possible to have the magnet cause an interupt, and then meassure the time ellapsed when the magnet triggers it the second time and then reset and wait for the magnet again?

I know this will likely result in a fairly "slow" refresh but im assuming we are talking milliseconds anyway, :-)

Any hints on where i can start looking to find info on timing the triggers?

[Dave]

Well Bobbo_ZA, I used 4 rare earth magnets about 1/4 inch square epoxyed to the shaft. The bottom of the shaft sat on a single ball bearing as the thrust bearing. the upper shaft outer bearing was made from a piece of teflon rod, drilled out to slightly biger than the shaft diameter. I calibrated it in the car, on some old country roads when there was no wind early in the monring. The main shaft was made out of stainless and the cross arms to hokd the cups were made of brass rod. I used a hall efect switch for the pickup and it was housed in an outside vertical junction box.

Dave Purola,
N8NTA

[Bobbo_ZA]

mmmm, this is all sounding way past my current / very limited PicBasicskills, sounded quite "easy" at first but seems to be very deceptive.

Had another thought that may make this a bit easier to handle, :-)

If i were to use a differential pressure sensor with the pitot tube mounted on a rotating platform that would point into the wind regardless of direction (weather vane of sorts), I should then be able to measure the airspeed / wind speed from that?

I think the mechanics of rotating the pitot tube will be a challenge but that part I’m quite comfortable with.

Your thoughts?? (Something like: http://www.picbasic.co.uk/forum/show...ressure+sensor - in this topic Melanie references an earlier topic of mine using a 3.9kpa sensor which should give me a reasonable speed range)