Pic based Wind Speed meter

[Bobbo_ZA]

Hi Dave,

:-), well, the 20hz refresh was a number i was thinking of, realistically i dont think i would need such a fine resolution. I know it has been done with a simple bicycle speedo which also uses the same technology, problem is the bicycle sensor only gives me kph and I'm looking of m/s. The other reason is I would like to add my previous projects using temp sensors and eventually a pressure sensor, :-)

Kind regards
Rob

[Charles Linquis]

The three common ways to measure wind speed are :

1. Hot wire
2. Rotating cups or fan
3. Vane

The only one that doesn't depend on wind direction is rotating cups.
A hot wire doesn't have to be directional, but most are.
A vane definitely has to turn with the wind.

I once made rotating cups from old LEGGS pantyhose containers. They are plastic and egg-shaped. When opened, they are fairly hemispherical plastic. I epoxied 3 of them to stainless steel rods about 15 inches long. Those rods were press-fitted into a machined aluminum cylinder that was attached to an old 5.25" disk-drive motor. The motor provided the bearings and the speed indicator. The motor had a separate, three-wire (amplified hall-effect) tachometer that didn't require powering the motor to provide an output of 5V P-P.
It worked for awhile, but the epoxy and plastic LEGGS containers couldn't withstand the elements. The sun's ultraviolet rays made the plastic brittle, and the rain and the ultraviolet rays made the epoxy soft. You could probably devise a better way. By the way. I calibrated this device by sticking it out a car window and driving down a road at various speeds, and then down the same road in the opposite direction at various speeds and averaging the equivalents.

Another approach that I used (this one worked) was using a PTC (Positive Temperature Coefficient) thermistor. I chose one that had a knee temperature of about 70C and would reach that temperature when powered by 12V. I put a small (10 ohm) resistor in series and boosted the voltage drop across that resistor with an op amp.
A PTC thermistor is like a self-regulating heater. If you apply sufficient voltage, it will heat up (due to its resistance) up to the knee temperature. At that point, the resistance goes up rapidly. The increasing resistance lowers the current through the device and it cools back down again. In other words, it will maintain a given temperature all by itself by regulating its own current. If you blow air across it, the air will cool it down. The PTC thermistor will immediately sense this and lower its resistance in order to maintain its temperature (at the knee value). If you measure the current draw of the thermistor, you will get a reading that is dependent on the cooling effect of the air - which depends on two factors: #1. The ambient air temperature and #2. THE VELOCITY OF THE AIR. The cooling effect of air is proportional to a constant times plus the square root of the velocity. So, if your PIC knows the ambient temperature and the current through the PTC, it can calculate the wind velocity. PTCs are generally disk shaped, however, so they are somewhat wind-direction dependent.

I always thought a vane would work but never built one. I was going to hang a piece of metal from a low-friction bearing and measure the angle of deflection.
using an optical method.

[Bill Legge]

I've always wanted to have a go at a hot wire anenometer. I presume they work by the cooling effect of the wind changing the resistance of the heated wire? Gets around all the mechanical and weather-proofing problems?
Apart from that, I'd:
1. If it's for the fun/learning experience, try the slotted/disk method.
2. If you want it to work properly, outside and for a long period - byte the bullet and purchase the Davis sensor at about $125.

Regards Bill Legge

[Bobbo_ZA]

Agreed, the weather proofing of a homemade "stick and cup" versionis an issue, however I have managed to source a commercial replacement set thatis UV treated etc., so that should solve the longevity issue.

I was thinking last night, would it not be possible to still use the magnetand read switch and then use pulsein. From what i was thinking pulsein shouldgive me a frequency, i don’t know if it’s possible to convert the frequency toa usable value that i can do some calculations on.

Also, in this I'm also not 100% sure on the wiring for getting it to give mea frequency, I know the read switch is normally open, so would i connect it topull the pin high (v+) or connect the switch to ground?

[kellyseye]

Commercially available wind speed and direction (WS&D) indicators are common-place onboard sailing craft of all sizes. The wind speed is invariably measured by 'cups' on a rotating shaft passing a magnet across a hall effect sensor. One device I recently repaired had the cup unit changed and the cups were rotating on a radius of some 50mm so if manufacturing one yourself then don't go 'too big'.
Similarly, from what I could gather from the internals of a WS&D I recognised a Bournes 'potentiometer' that (I think) operated using hall effect. I keep meaning to look this device up but I can only assume it outputs a voltage proportional to the angle of the potentiometer shaft (which rotates through 360 degrees) - perhaps a search at the Bournes website might reveal the actual devices used?

edit - a quick search at Bournes reveal rotary (360 degree) position sensors but nothing based on hall effect that I can see immediately. Anyone else seen/heard of devices like this?

[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...