Check my logic? Anemometer rpm.

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I have a little one-pulse-per rev anemometer (wind speed meter). I'm trying to guesstimate the pulses per mph, and my answer is looking far too easy...

Approx dimension from the axle to the centre of the cups is 7cm. That gives a circumference at the cup centre of 44cm (in round numbers).

One mile is 1760 * 3 * 30.5cm = 161,040 cm

Dividing that by 44 = 161,040/44 = 3,660

And that looks far too close to 3,600 to be coincidence, and a 1.6% error is well inside my likely measurement error and estimation of pi... (hey, I was balanced on a fence to reach the damn thing to measure it. And it was raining.)

Which rather suggests that as we might expect the cups to move at wind speed, we can expect 3,600 pulses per hour per mile per hour. Which is one pulse per second per mile per hour. So if I average over, say, ten seconds and divide by ten, I'll get a direct reading in mph.

Sounds far too easy - any holes in my logic?

Neil

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Just one thought, more a question than a suggestion, and that maybe can be confirmed (or not) by your experimentation. I wish if the cups will really move at the same speed than wind. I doubt, since the cup that flows in the same direction has to move the drag that the same wind generates on the other cups that are moving in the opposite direction. That probably mean that there is some proportional factor that should be multiplied in order to have the right readings. Ie: read wind speed = 0.8 * real wind speed.

BTW, I remember another anemometer that doesn't have this issue, based on ultrasonic sensors, with an ATmega64 IIRC, by Monsieur Sylvain Bissonette, and that you can find posted at his nice web page http://www.microsyl.com/. Could be interesting to compare results.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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There are several styles of anemometer.

One is the vertical axis type that has cups that move in a plane parallel to the ground. Another is the horizontal type that has a propeller somewhat like that of an airplane. A third is the ultrasonic.

Both of the first two have some "slip" factor. I am trying to get this number from Davis Instruments for their vertical axis anemometers but have not gotten a response, so far. One of the difficulties with your measurement is what the "real" radius is; do you measure to the outer edge of the cups or to the center or to somewhere else. And, as Guillem points out, the cup moving forward has drag that cannot be ignored.

My uncle used to calibrate these things in an automobile. If you have an operating display unit that reads MPH, then build a circuit to "snoop" at the anemometer connection on the display, and measure it while noting the indicated wind speed. The calibration function MAY be non-linear, especially at very low wind speeds where bearing friction is an additional issue.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I recall that cup anemometers are drag wind turbines rather than airfoil wind turbines, and usually turn at windspeed/3. I guess this depends on the diam of the cup and the length of the arm, etc.

Imagecraft compiler user

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Yes, I figured there's going to be some slippage - though I note that a sphere (which is what is going *into* the wind) has the lowest drag at low speeds.

Oooer... looks complicated: http://www.wmo.int/pages/prog/ww...

But (a) I only care for speeds above about five mph and below twenty-five mph; and to be honest, consistency is better than accuracy, and (b) I can calibrate that against a number of hand-held meters.

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asdfg

Last Edited: Tue. Feb 24, 2009 - 08:40 PM
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When I did hangliding back in the 70's we usually just used a strip of dacron tied to a pole.If the strip wasn't moving away from the pole it was pretty dead.

At 45 degrees it was ok.Straight with the wind was great.If it bent the pole-good luck.

We also had used small little handheld ones that had a little ball that would rise up inside the tube with increasing wind speed.

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In paragliding we have a smaller range of windspeeds, depending whether we're looking to soar or thermal. More than 22-23mph and you'll be going backwards; even at 15mph on a suitable slope you might take off going forwards but facing backwards if you're not careful.

We use windsocks and threads and handheld meters, but I want a large display I can perch close to the launch and see from fifty yards away.

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44cm/sec -> 17 in/sec -> 1.44 ft/sec -> .98 mph. Hold it out the window at 30mph and see if it reads 10mph?

Imagecraft compiler user

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Quote:

Hold it out the window at 30mph

Yah know, I was thinking how to calibrate the darn things while reading down the thread. I think Bob hit it--mount the darned thing on a mast [to get it out of vehicle turbulence] on the top of your pickup truck on a calm day, and make a few trips back-and-forth on an open road at various speeds, and Bob's your uncle. No tape measure or algebra needed.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Ah, my 'pickup truck'...

That was my first thought; I was just wondering what sort of range of values I'd get. I seem to have the maths right, anyway; I think it's fair to say that one tick per second = no more than 1mph, so apply a fudge factor to that (possibly non-linear).

Thanks!

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Hello Neil, Bob, Jim et al.

Thanks for posting the link to Dr. John Gorman's instrument Test Report. It saved me digging it out. I used to work at the Bureau where John still works. I used to be at various times the officer who bore those odd position titles of SRUM and SROO on the distribution list of John's report.

The 3 cup anemometer (with 120 degree angular separations) is indeed a strange beast. At the same time that the orthogonally exposed cup is being pushed by the wind, the other two cups are pushing against the same wind. And the three circular cross-sectioned arms are presenting less than ideal drag exposure to the wind. (aside: I am sure you will have heard how the Wright Bros would have flown further if they had not used drag producing circular cross-sectioned wing struts in their aircraft). All of the above is a simple explanation of why there is not a 1:1 relationship as implied in your (Neil) initial calculation. (another aside: John's exposure on the roof of his house fails WMO's test standards unless he is now 3 metres tall. Structure-induced vertical wind turbulence will contaminate all readings but not necessarily in an equal way. I will contact him tomorrow about that :lol: )

When working for the Saudi Met Service in the early 1980s, I was asked to do acceptance testing on a delivery of anemometers from their German manufacturer. Saudi did not have a wind tunnel, so I used my car with a boom holding the instruments ahead of the vehicle and had my 12 year old son log the readings while I drove back and forth along a test range on a still day at various speeds. The precise wind speed was not important because the spread of readings in the batch was enough to reject them all as unfit for service. I am confident that you could replicate such a precise "test range" :roll:

So you need a system visible from 50 yards away. I think you and Bob should get together and design a mini-AWOS, but perhaps have the speed being transmitted without need for the push to talk button.

I might even suggest a pitot tube (there you go Jim a fourth style) mounted inside a wind direction vane.

Ain't the weather a constant talking point !!

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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barnacle wrote:
In paragliding ...

Neil,

Are you looking to know the wind speed across the hillside surface or at some height above it? Orographic lifting, as I am certain you understand far better than me, can invalidate surface readings very quickly.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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He might not know as much about it as you give him credit for.... I'm surprised he is even considering measuring the darn wind speed. I recommend something tamer like knitting.

Imagecraft compiler user

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Quote:

...so I used my car with a boom holding the instruments ahead of the vehicle ...

Then one could say that you were facing the "rear" of the unit-under-test which was out front breaking wind.

I was pretty close, then, with my idea of a mast on the top of the pickup truck (smaller cab already raised from the bulk of the body) to do it "right".

Another thought that I had was that nowadays the vehicle speedometer (and its accuracy) is not of primary importance anymore, given the advanced features of speed/average speed/distance on even basic GPS units.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Ross, where we take off, the wind speed is *always* higher a few feet above the surface than the ground speed in front of the hill; I care really about the speed at the height of the wing before take off. Normal technique is to hold a hand-held anemometer as high as possible for as long as possible (to get trends). I want to replace that with something I can watch from a distance.

http://www.youtube.com/watch?v=c... (not me!)

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Quote:

I want to replace that with something I can watch from a distance.

Do it right, and get a Raven kit. Keep one with you, and have it beep/flash in proportion to the measured wind speed at the "home base".

(Now, you appear to have a lot of faith in your readings anyway. Given that the important part is the wind speed at the wing where you are taking off, what if it is different than that measured "from a distance"?!?)

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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barnacle wrote:
Ross, where we take off, the wind speed is *always* higher a few feet above the surface than the ground speed in front of the hill

... because of "surface roughness". There is an involved formula for calculating the wind speed at the standard 10m height based upon knowing the speed at a lesser height and the nature of the surface, eg mown grass, sand, etc. But that will not apply in your hillside situation.

barnacle wrote:
I care really about the speed at the height of the wing before take off. Normal technique is to hold a hand-held anemometer as high as possible for as long as possible (to get trends). I want to replace that with something I can watch from a distance.

http://www.youtube.com/watch?v=c... (not me!)

So with the partially inflated wing and you still on the ground, you want to be able to look at an indicator of the wind speed at the height that the wing would be when you are being lifted off the ground. Correct?

The "Dines" anemometer invented by one of your fellow countrymen in the late 19th century uses a pitot tube. Australia still used them up until the 1970s at some sites. I have also seen a similar handheld instrument for measuring air flow in air conditioning ducts. There are also several cheap anemometers used by the "sailing crowd" (yachties). Either of these could be easily exposed appropriately. The indicator is the interesting bit as far as I am concerned.

Musing in the shower (inspiration-central), it occurred to me that a vertical totem pole painted in bands using eg the resistor colour code, to indicate decades of speed unit could be seen from all vantage points. The bands could be say one or 2 feet high. The "pointer" could be an outer black cylindrical sleeve that slides up and down the totem driven by your sensor/indicator interface. I would probably use a tensioned string drive (like the old radio tuning dials) with the motor battery etc at the ground level. The totem could be cardboard tubing to minimise crashing hazards to other flyers.

All rights reserved by Shower Enterprises.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Wiki anemometer. Here .

Quote:
When Robinson first designed his anemometer, he wrongly claimed that no matter how big the cups or how long the arms, the cups always moved with one-third of the speed of the wind. This was apparently confirmed by some early independent experiments, but it was very far from the truth.

3 cup vs 4 cup discussion, and several other types of anemeters discussed.

Interesting reading, but I'm not sure it helps solve your current dilemma.

JC

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DocJC wrote:
Wiki anemometer. Here .

Quote:
When Robinson first designed his anemometer, he wrongly claimed that no matter how big the cups or how long the arms, the cups always moved with one-third of the speed of the wind. This was apparently confirmed by some early independent experiments, but it was very far from the truth.

3 cup vs 4 cup discussion, and several other types of anemeters discussed.

Interesting reading, but I'm not sure it helps solve your current dilemma.

JC

Interesting indeed. The section on sonic anemometers claims the first such device was developed in the 1970s. This is wrong because in 1973 I was reading journals from the mid 1950s that described a room sized instrument produced in Japan. Unfortunately I did not keep my papers from that time but seem to recall that it operated at around 50KHz and used valves (hence the room size needs). I went on to design (circa 1978) an ultrasonic anemometer with air temperature measurement for use on drifting ocean buoys when I was at the Australian Bureau of Meteorology.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Ross,

If it gets temporarily tipped upside down by a big wave it no doubt also serves as an ultrasonic fish finder!

JC

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Lee, the place where I inflate the wing is not necessarily the place where I take off - it depends on the site - though it will not be far away. The air flow around the actual edge where I take off can be complex and that's why I want to see what's happening.

Ross, I need to be able to see speeds with some accuracy - one of the places it will be used will be a school and the students need to learn how strong the wind is and how strong it feels - there are completely different take-off procedures for speeds below about eight mph, below fifteen mph, and above fifteen mph - though there is a wide overlap. It's just more information...

I haven't made myself clear: I want a system whereby I can measure and display the wind close to the takeoff point and which both I and any other observers can monitor both reasonably instantaneous speeds, and also observe trends.

Hence the big display.

The actual position of the anemometer itself is relatively immaterial, because anyone flying is making their own judgement based on experience - but ideally without having to hang around on the edge with an arm in the air.

Ultrasonics sound an excellent way of doing it - but I *have* the twirly-roundy-cups thing. And some canes!

And it seems that the adjustment factor will need to be between two and three-and-a-bit, to be confirmed by measurement.

Neil