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'[OT]: Wind Speed Formula??'
2001\02\10@194923 by hgraf

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    I'm thinking of adding a wind speed measurement device to my house
monitoring system and I was wondering if anybody can point me to a formula
that relates the wind speed to the RPM of the "vane". I am using the device
with several "cups" connected to a bar that rotates in response to wind.
Thanks for any pointers. TTYL

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2001\02\10@200144 by Sean H. Breheny

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Hi Herbert,

Warning: I'm not an aerodynamics expert, this is only a speculation and I
invite answers from people who know.

I think that it should just be the simple formula:

v = 2*pi*(RPM/60)*r

where r is the radius of the vane (from pivot point to some point in the
cup), and v is the speed in whatever units r is in, per second.

The idea is that some point on the vane must be going at the same speed as
the air, in the case where the air has been going at a constant speed for
long enough for the vane to settle down to a constant speed. I think this
is true because, otherwise, you would have a vane moving faster or slower
than the air going over it, and it would be equivalent to a vane spinning
in a calm wind.

The only troubling thing is how to measure r. If you had a very small cups
on the vane and extremely thin rods connecting them to the hub, then it
would be easy. Otherwise, there will be some point on the vane (almost
certainly in the cup) which acts like a centroid (similar to the center of
pressure in the rocket discussion today). This point may change slightly
with wind speed, too.

So, my guess is that if you go with a very lightweight vane(so it doesn't
have much inertia) with small cups and very thin rods connecting them, you
can use the formula above.

Sean

At 07:49 PM 2/10/01 -0500, Herbert Graf wrote:
>      I'm thinking of adding a wind speed measurement device to my house
>monitoring system and I was wondering if anybody can point me to a formula
>that relates the wind speed to the RPM of the "vane". I am using the device
>with several "cups" connected to a bar that rotates in response to wind.
>Thanks for any pointers. TTYL
>
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2001\02\10@200955 by Sean H. Breheny

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Just wanted to add one thing to my last post: I was assuming that you have
very good bearings and sensor, therefore negligible mechanical friction.

Sean

At 07:49 PM 2/10/01 -0500, Herbert Graf wrote:
>      I'm thinking of adding a wind speed measurement device to my house
>monitoring system and I was wondering if anybody can point me to a formula
>that relates the wind speed to the RPM of the "vane". I am using the device
>with several "cups" connected to a bar that rotates in response to wind.
>Thanks for any pointers. TTYL
>
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2001\02\11@051529 by Chris Carr

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face
I don't think there is a formula as such because the speed of the shaft
will
depend on the construction of the cones and the bearings, weight of the
rotating part etc.

Do what I do. Construct the thing, get in your car and calibrate against
the
speedometer or better still a GPS Receiver. I find it easier to do if
someone else is driving the car   :-)

Regards
Chris

>      I'm thinking of adding a wind speed measurement device to my house
> monitoring system and I was wondering if anybody can point me to a
> formula
> that relates the wind speed to the RPM of the "vane". I am using the
> device
> with several "cups" connected to a bar that rotates in response to wind.
> Thanks for any pointers. TTYL
>

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2001\02\11@054650 by Jinx
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Some anemometers use a small motor attached to the
shaft. As the vanes turn they generate a voltage that can
be measured with A2D. You'd need a free-running motor
that's well protected from the weather. Possibly over-
engineering compared to the optical method and might
not give such good results at very low wind speed

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2001\02\11@111630 by Thomas McGahee

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    Velocity=distance travelled/time
Since the path is circular,
    Velocity=pi*diameter/(time for 1 revolution)
This is essentially what Sean has in his formula:
    v = 2*pi*(RPM/60)*r
Only the way it is expressed is different.

Now, the above assumes perfect conditions. But
the system is FAR from perfect. Sources of error
include:

    Nonlinearity (of generator. Opto method is linear)
    Bearing friction
    Magnetic opposition (if driving a generator)
    Air friction due to the support for the cups
    Air friction due to the geometry of the cups

This last source of error can be quite large! When
the cup makes the round trip, it comes back INTO
the wind, and the contributed error has an error
that is somewhat "sine" in nature, due to the
source of the error being based on a circular function.
The error is not perfectly sinusoidal, and is also
very dependent on the number of vanes used.

Short radial rods work better than long radial rods.
Try to keep the radial rods as thin as possible while
still maintaining the necessary rigidity. The cups
should be as aerodynamically shaped as possible.
An egg-shape works better than a spherical shape.

Bottom line: you have to calibrate the thing. One
fairly easy method is to measure the output with the
device mounted to the top of your car. But you have
to eliminate any error that might be due to the wind
blowing. So the usual technique is to first take a reading
with the car aiming directly INTO the wind, and then
take another reading with the car going in the exact
opposite direction. Add the two readings and divide by
two. (car velocity+wind)+(car velocity-wind) = 2*(car velocity)

2*(car velocity)/2 = car velocity.

The two readings should be taken as close together in time
as possible, and only when the wind speed is fairly constant
both in velocity and in direction.

Large mall parking lots make good places to do the test runs
if you can find a time when they are somewhat deserted.
Always have one person driving and another supervising
the experiment to avoid an accident.

Fr. Tom McGahee

{Original Message removed}

2001\02\11@113128 by David VanHorn

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>
>Short radial rods work better than long radial rods.
>Try to keep the radial rods as thin as possible while
>still maintaining the necessary rigidity. The cups
>should be as aerodynamically shaped as possible.
>An egg-shape works better than a spherical shape.

You can take the rod length to zero.
Look up "Savonius rotor"
Picture tin can cut in half vertically, displaced about 2/3 it's width.

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2001\02\12@114409 by Barry King

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face
Sean is right, to a first order approximation, but that's not good
enough to have a useful calibration for a typical cup anemometer.

The second order effects are too large, so that the "slip" between
the airstream and the cup speed is always significant, and depends
greatly on the shape of the cups, the friction, and more.

The simplest answer is to calibrate.  If you need accuracy better
than 10% or so, your car speedometer may not do it.

When we calibrate commercial units, we run them side-by-side with
traceable calibrated anemometers.

Barry.
--------------------
Barry King
NRG Systems "Measuring the Wind's Energy"
http://www.nrgsystems.com

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2001\02\12@124649 by Chris Carr

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> The simplest answer is to calibrate.  If you need accuracy better
> than 10% or so, your car speedometer may not do it.
>
Hmm..could that be why I suggested using GPS.

Chris

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2001\02\12@131956 by mmucker

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face
GPS won't do you much good if there's wind as you're driving.  You'll need
to do two (or more) passes in different directions to cancel out the effect
of wind.

> {Original Message removed}

2001\02\12@135936 by Chris Carr

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face
> GPS won't do you much good if there's wind as you're driving.  You'll need
> to do two (or more) passes in different directions to cancel out the
effect
> of wind.
>
Plus a large flat open space such an Airfield (best when large flying
objects are not taking off and landing) Plus the Anemometer Head needs to be
on a pole at least 10 foot above the vehicle, however if you have a vehicle
that has the aerodynamic characteristics of a LandRover then a 20 foot pole
is better.

Chris

{Quote hidden}

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2001\02\12@153952 by Herbert Graf

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       Thank to all for the suggestions. I have decided on just using the "dc
motor" approach suggested here. I found a nice one from an old VCR that
gives a nice output. As for calibration, I guess I won't for the moment, I
don't really need the actual speed that much, just the relative differences,
when I get a chance (ie. when it gets WARMER!) I'll use the car method to
calibrate it. Thanks again. TTYL.

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2001\02\12@165708 by David VanHorn

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At 03:38 PM 2/12/01 -0500, Herbert Graf wrote:
>         Thank to all for the suggestions. I have decided on just using
> the "dc
>motor" approach suggested here. I found a nice one from an old VCR that
>gives a nice output. As for calibration, I guess I won't for the moment, I
>don't really need the actual speed that much, just the relative differences,
>when I get a chance (ie. when it gets WARMER!) I'll use the car method to
>calibrate it. Thanks again. TTYL.

The VCR Head bearing makes a nice one for a wind speed indicator.
I did one of these a while back, S-Rotor optically sensed.

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2001\02\12@172341 by Chris Carr

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>         Thank to all for the suggestions. I have decided on just using the
"dc
> motor" approach suggested here. I found a nice one from an old VCR that
> gives a nice output. As for calibration, I guess I won't for the moment, I
> don't really need the actual speed that much, just the relative
differences,
> when I get a chance (ie. when it gets WARMER!) I'll use the car method to
> calibrate it. Thanks again. TTYL.
>
Good Luck Herbert.

Remember that wind is not a constant speed so some form of averaging is
usually necessary. My advice would be to keep the moving parts as light as
possible, the bearings as friction free as possible and keep a light load on
the motor (generator) and do the averaging in the electronics. That way you
can also get a fairly good indication of peak wind speed.

Plus location is important, no I'll change that, location is critical. As I
have intimated with regards to calibration, the wind flow is highly modified
by nearby (and frequently not so nearby) objects. Modified wind flow equals
a change in wind speed giving false results. For instance, if you intend
mounting your measuring unit on a house with a ridged roof, mount it as high
as possible above the ridge otherwise you will measure different wind
velocities with regards to wind direction even though the wind speed is
identical.

I believe this thread started off with the request for a formula :-)

Regards

Chris

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2001\02\12@174832 by Fredrik

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I have a comment about the sensor. I buld one of an old CPU - fan and got
one pulse of about 50mV/turn form the fan motor. And after that in to a
opamp(LM358), single +5V to get the right levels for the pic. This works
fine for me. The programing is simple, just to count the pulses over some
time...

regards

Fredrik







At 23:20 2001-02-12, you wrote:
{Quote hidden}

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2001\02\12@184440 by Herbert Graf

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{Quote hidden}

    Well I'll only be reading the sensor at 4 minute intervals so I didn't
really want peak speed indication. I actually purposely built it heavier so
that there would be alot of inertia (I used tennis balls! :) ).

{Quote hidden}

       Actually I think I'll be mounting it a about the WORST place possible! :(
My balcony. Don't really have any other choice, the roof of my building
isn't accessible!

> I believe this thread started off with the request for a formula :-)

       Yup, it's amazing how a thread can go on so many different and useful
tangents. Thanks for the help. TTYL

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2001\02\13@053039 by Jeszs

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what about using a pitot tube and a pressure transducer?
this worked fine in my motorbike but I did not attach the signal to a pic
but a gauge.

the formula for the velocity given the dynamic pressure is a simple square
root and a couple of divisions (considering constant air density and/or
known temperature). Ask Mr. Bernouilli if you need more data.

ciao

--------------------
Jeszs Gonzalo
Lesn (SPAIN)
--------------------

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2001\02\13@103812 by Herbert Graf

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face
> what about using a pitot tube and a pressure transducer?
> this worked fine in my motorbike but I did not attach the signal to a pic
> but a gauge.
>
> the formula for the velocity given the dynamic pressure is a simple square
> root and a couple of divisions (considering constant air density and/or
> known temperature). Ask Mr. Bernouilli if you need more data.

    Unfortunately that is directional isn't it? TTYL

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2001\02\13@105239 by Sean H. Breheny

face picon face
Yes, but you could probably mount it on a rotatable vane with a tail on it
to keep it pointed into the wind. I don't think that this would be any
easier than what you are currently planning on doing, though (the rotating
cups), except that you could get direction out of it,too.

Sean

At 10:38 AM 2/13/01 -0500, you wrote:
{Quote hidden}

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2001\02\13@105950 by t F. Touchton

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face
This might be a little far out, but why not use relative humidity measurements
in reverse?  Seems as if the temperature measured would be directly related to
humidity and wind speed (wet bulb).  Just a thought.

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2001\02\13@121601 by Jeszs

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> > what about using a pitot tube and a pressure transducer?
> > this worked fine in my motorbike but I did not attach the signal to a
pic
> > but a gauge.
> >
> > the formula for the velocity given the dynamic pressure is a simple
square
> > root and a couple of divisions (considering constant air density and/or
> > known temperature). Ask Mr. Bernouilli if you need more data.
>
>      Unfortunately that is directional isn't it? TTYL
>

Sure, my bike normally runs ahead.
TTBOMN, a kind of omnidirectional Venturi device could be built from two
horizontal, face-opposed, music plates with the pressure sensor at the
center. IMHO this could work, and also be decorative in the rook of a house.
TTYL.

--------------------
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Lesn (SPAIN)
--------------------

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2001\02\13@124416 by Bourdon, Bruce

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face
This reminds me:

I remember an Electronic Design article where Bob Pease made a wind speed
detector out of a small light bulb with the glass broken away.

He had a constant current circuit setup and (I believe) measured the voltage
required to generate the current. Also took into account temperature with a
thermistor.

This resulted in the airspeed, as the resistance of the bulbs filament
depends on temperature - and the faster heat was carried away by the moving
air the higher the resulting voltage would be.

Specifics may be way off, but that was the idea.

Maybe start your search here:
http://www.elecdesign.com/

Bruce.

{Original Message removed}

2001\02\13@130704 by David VanHorn

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>
>This resulted in the airspeed, as the resistance of the bulbs filament
>depends on temperature - and the faster heat was carried away by the moving
>air the higher the resulting voltage would be.

Would vary with humidity too, I think, but maybe not a large effect..

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2001\02\13@132926 by Jeszs

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face
> He had a constant current circuit setup and (I believe) measured the
voltage
> required to generate the current. Also took into account temperature with
a
> thermistor.
>

Yes, that is called 'hot thread anemometry' (sp?).

And this discussion would not be complete if we had not speak about 'laser
anemometry', i.e. to measure the doppler effect on a laser beam due to
particle movement; lots of averages required, of course, and quite delicate
equipment.

--------------------
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Lesn (SPAIN)
--------------------

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2001\02\13@145540 by Ken Gasper

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From my experience this is exactly how most modern Mass Air Flow sensors
used in automobiles work.  The sensor body has a filament and an integrated
temperature sensor.





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11:25:46 AM

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Subject:  Re: [OT]: Wind Speed Formula??


This reminds me:

I remember an Electronic Design article where Bob Pease made a wind speed
detector out of a small light bulb with the glass broken away.

He had a constant current circuit setup and (I believe) measured the
voltage
required to generate the current. Also took into account temperature with a
thermistor.

This resulted in the airspeed, as the resistance of the bulbs filament
depends on temperature - and the faster heat was carried away by the moving
air the higher the resulting voltage would be.

Specifics may be way off, but that was the idea.

Maybe start your search here:
http://www.elecdesign.com/

Bruce.

{Original Message removed}

2001\02\13@163447 by Dale Botkin

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On Tue, 13 Feb 2001, Ken Gasper wrote:

> >From my experience this is exactly how most modern Mass Air Flow sensors
> used in automobiles work.  The sensor body has a filament and an integrated
> temperature sensor.

You are correct.  One comon cause of poor performance with hot-rodders is
over-oiling the K&N air filter, causing oil to be deposited on the
filament in the MAF sensor.

IBM used this method also, on their 4300 series of processors, to detect
AMD (Air Moving Device - I swear this is what they called squirrel cage
blowers!) failures.

Dale
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2001\02\14@043109 by Alan B. Pearce

face picon face
>IBM used this method also, on their 4300 series of processors, to detect
>AMD (Air Moving Device - I swear this is what they called squirrel cage
>blowers!) failures.

One technique I have seen used for this is to have a high wattage resistor (5 to
10W) beside a thermal switch. If the air stops moving the resistor gets hot
enough to open the switch and turn other things off. Crude but effective when
actual temperature measurement not required.

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2001\02\14@172346 by Peter L. Peres

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>Unfortunately that is directional isn't it? TTYL

Actually there is a form of Venturi tube that does not require a static
port and is not directional. It is called a chimney usually. I happen to
know that someone once made an automatic device for wood fired chimneys
that sensed the draft in the chimney and closed a vane if it was cold and
too strong. It was mechanical (including the temperature sensor == bimetal
device).

Peter

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2001\02\14@172441 by Peter L. Peres

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Imho give up on the vanes and look into hot wire/hot thermistor
anemometry. Much more PICable. Based on some experiments I've made it
should be possible to make a wind measuring device (speed and direction)
using two equal lengths of normal copper wire about 1 meter long, each,
spanned horizontally and at 90degrees from each other. Think like a large
vertical fork with the 3 pins (?) bent out so their tips form a 90 degree
triangle with the wires in the two edges adjacent to the 90 degree angle.
I'd try this using 0.15 mm CuEm winding wire loosely wound on fishing line
for mechanical support.

Peter

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