HOW FAST - ANEMOMETER CALIBRATION ?

For a long time I have thought of building an anemometer - using 3 ball valve floats cut in half on their 5/16" brass arms, bearings etc to suit, spinning either a magnet and hall effect sensor as a revcounter or a small (12v) permag motor. The only thing stopping me was how to calibrate it with fair accuracy! I turn to the forum for HELP ! I could go out and buy one but I WANT TO MAKE ONE ! Any ideas most welcome ? Noel.

Hi there, Noel,

I don't want to 'rain on your parade' but with all that brass and copper your rotor would have quite a high moment of inertia. That might make it sluggish in responding to gusts.

Still, it's YOUR project so make it anyway and tell us how it works.

Best regards,

Swarf, Mostly!

If you have such big balls, drag and inertia will be 'damping' the results to an extent - but maybe that'll be a good thing, if you want a coherent impression of how far the wind is pushing things along expressed as distance/time.

I think once the speed has stabilised, you will have a fairly true picture of linear velocity by 'circumference /sec'

If you need to calibrate it, maybe a piece of flat open country, a ?balloon, a stopwatch and two or three helpers 100m apart, to see if you can time a linear 'run' of unfettered wind, would give you a figure compare the results with?

It'd be a fun precursor to going to the pub in any case, if you don't get arrested as hare-coursers..

Hold it out of a car window on a calm day. Use the speedo to calibrate it. Account for the fact that most cars have a speedo that under-reads by a couple of percent.

Martin C

Noel asked how to calibrate it once made. Obvious choice is to use a wind gauge, but have a look on the net for a simple pendulum type wind gauge perhaps. Go to your local sailing club and ask a few questions see if you could check your anemometer with the club's one ?

Kids wind gauge

Also Martin's idea seems like an interesting one too.

Bob

Gentlemen, thanks to those who have contributed so far ! The intention was to use plastic floats and on reflection ali rod would make good arms ! Some damping would help to stabilise readings ! Is there any theoretical way to calibrate the instrument or is it down to a still day and mount it on the roof rack,of the car, Knowing the speedo is 2MPH fast. As for hares there's more than a few round here, that's why there's a long barreled full choke 12 bore in the cupboard. When it was given to me I was warned not to shoot at anything I wanted to eat at less than 35yards. The local gun club stopped laughing when one day I used this weapon to stop clay pigeons just before they hit the ground, some considerable distance away ! Noel.

Edited By noel shelley on 18/02/2022 13:01:14

As Martin suggested but use the GPS spèed reading on one of the mapping apps such as Google maps or waze available on most smartphone. You may need to find a long bit of road where you can stay at a constant speed

I can't speak for calibration, but I've just taken one apart to repair a friends WiFi weather station.
The rpm sensor on that was a small magnet rotating close to a glass reed switch.

Also I've just repaired broken wire on my own hand held one, which uses a slotted brass disk rotating between a pair of sensors.

Unless you're planning to design and program the electronics yourself, I wonder about one of the little ebay magnetic rpm sensors used on lathes etc.

In both case, the 3 hemispherical cups are about 35mm and very light, to respond quickly to gusts.

I suspect trying to use a car speedo will be inaccurate, due to the vehicle aerodynamics affecting the speed of air flow over the car.
Maybe OK on a mast sticking out of the sunroof to get above the car and ground effect.

Bill

Sadly, won’t work - or at least very accurately - except at really low speed. Car will displace its frontal area and change the real speed. Speedo may provide even more error especially at low speed.

What you need is a pitot tube, to calibrate it properly.

I was looking in my copy of 'The Amateur Scientist', C L Stong, Heinneman Educational Books, 1962 when the gloom of yesterday's power cut stopped play! Thank you Storm Eunice, not.

As it's quite good on experimental physics I was hoping the articles on wind tunnels would cover calibration.

Not quite! Stong says:

Air speed through the tunnel was measured with two gauges: a calibrated pendulum vane and an anemometer of the Richards Type. Pressure in the tunnel during the calibration period was measured by a manometer arrangement, built by Phillips*, which utilised a pair of milk bottles. It was extremely accurate but abandoned after it was found too sensitive to temperature changes for prolonged use. (* Phillips was a member of the team, and a former MIT student.)

Several patents exist for pendulum vane gauges, like this example, but the two I looked at don't explain how they are calibrated in the first place.

In principle, air velocity can be calculated by measuring the pressure applied to a hinged vane. The vane has a known area, and the force on it can be measured by causing it to lift a weight, or stretch a spring of known strength. Or a manometer could be arranged to measure pressure difference between the front and back of the vane. A pitot tube connected to a manometer might be convenient way of capturing the pressure difference.

How accurate is another matter! I suspect it's necessary to compensate for air pressure, temperature, humidity, friction. It's relatively easy to measure to about a thou/0.02mm in an ordinary workshop with a micrometer. Much more care is needed to measure accurately and reliably down to a tenth/0.002mm because tiny factors such as worn instruments, temperature, dirt and operator technique add significant errors. I suspect if I were to make a steel block of peculiar width, such as 10.139mm, and ask 100 forum members to measure it, I would get many different answers. Or I might get a cluster of wrong answers claiming 0.14mm because the human brain is biased in favour of round numbers! Same with making an anemometer: I guess ±5% accuracy isn't difficult, but ±0.5% is!

Another fascinating project. Noel gets a Gold Star from me, and I hope he keeps us updated.

Dave

I've read that the Wright bros tested aerofoil sections by having them mounted in front of a bicycle, presumably in undisturbed air. Down a decent hill you might manage 40mph,above that you need Windy or his cohorts.

Edited By duncan webster on 19/02/2022 12:33:24

Our old weather station just used a small magnet, supported on one small ball race, rotating above a reed switch. So the read out was really a pulse counter.

The replacement is probably the same. Because there are trees nearby, am convinced that they under read by a factor of about 3 or 4.

The three cups are only about 30 mm diameter plastic (Table tennis ball sized? )

I like the idea of calibrating from a moving car, probably need to be held well away from the bodywork to be clear of any strange eddies.

Storm Eunice en route to Noel and the North Sea, has provided some of the highest readings yet seen, well into double figures.

Howard

That is a great book. I've been looking for a decent copy since I saw it at the library.

I calibrated mine by holding it out of the car window - making sure Karen drove it in both directions along a quiet road. That way it was she who got done when we got caught doing 130mph...

MikeK - until you find a paper copy...

The Amateur Scientist

Thanks, Bill.

Mike

A very long time ago, I carried out (regular) dust testing in exhaust ducts (some inside buildings and others perched on a gantry about 20-30m up a 90m chimney - along with gas analysis - long before the modern techniques for fume control.

A pitot tube - yes, with the accompanying manometer - was the standard method for finding the flue gas velocity across a large duct. Plastic fan type anemometers are not much good at 200 degrees Celsius!

Pitots seem to work very well for lots of applications requiring proper, reliable velocity measurement. Aeroplanes are a typical common application.

Gas analysis was by chemical means using an Orsat apparatus. All good old-fashioned analyses, back then.

Now to some obvious options. Take it along to a weather station and compare results to the official machines, or borrow a commercial air speed monitor?

I’m not sure a) whether I can easily locate an air flow meter, and b) whether still functional (last used one about 10-15 years ago). In the back of my memory, there may also be a hot wire anemometer knocking around somewhere…

I can hunt around next week, to see if I can find one, if you are not in a hurry…

Many thanks NYIT ! No hurry at all, after writing my O/P It crossed my mind that many years ago I was given a beautifully engineered device that could well be an anemometer though if it worked I know not, it was in a nice wooded box and MAY be of the Richard freres type. will have to look in the back bedroom - it might be in there !

The mechanical side is easy and a hall effect device will read the revs, calibration may be easy given I live in the country, one road comes to mind as a test track, though readings above 70 in old diesel peugeot may have to be extrapalated form slower readings ! The rig would be mounted to one side, above and ahead of the screen to avoid as much as possible the effects of the slip stream. A marine satnav will be used plus the correction sat 120, in the hope of better accuracy.

To all who have commented Thank you, as things progress I will keep you all posted ! Noel.

Now for a pressure tube anemograph?