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My question is not 100% related to electronics design but it might be because I don't know the answer.

I'm writing a simple research on anemometer calibration. The thing is all the resources I found are explaining the procedure as measurement setup and comparing with a reference but not fixing. For the sake of clarity I posed my question with the following scenario:

Let's say I followed the setup procedure, collected all the data and measured the anemometer's speed changes with respect to a reference and drew a regression line for the anemometer. But how will then actually fix this anemometer mechanically or electrically? The papers don't mention it. Is measurement and data acquisition enough for calibration? How will the anemometer function in a different (proper) way after these steps if there is no mechanical or electronic adjustment? Will the adjustment (fixing) be on software or electronics circuitry?

I mean: What will be the actual adjustment for an anemometer after collecting the data with respect to a reference?

I'm really confused. Any ideas?

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The hard part is figuring out how exactly your anemometer reacts to different conditions. The reason papers don't mention how to use this data is because it is obvious. The calibration procedure gives you a function of input to output. You reverse this to get the output to input function, then run any measured data thru that. Again, this really should be obvious. – Olin Lathrop Nov 22 '12 at 15:22
up vote 5 down vote accepted

One way to use the calibration is to make no physical adjustments to the device. Simply do a mathematical adjustment to all the measurements you take with the device.

For example, an anemometer measures windspeed. Say you have a 5 mph reference flow and a 10 mph reference flow. Say in the calibration measurement, the 5 mph flow measures as 6.5 mph and the 10 mph flow measures as 12 mph. You then determine that the measured speed (I'll call it vm) is related to the real speed (v) by

\$v_m = 1.1v + 1\$

For your "live" measurements you'll reverse this formula to get the real speed in terms of the raw measurement:

\$v = (v_m - 1) / 1.1\$

So if you get a raw measurement of 8 mph, you use your calibration to estimate that the real speed was about 6.4 mph.

Of course the correction I described is based on a very simple, linear model of the instrument errors. In the real world, you might need a more complex model to get a correction formula that gives really accurate measurements.

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many thanks very clear. now i got it! but just one more question to you: When the anemometers are manufactured don't they already know the characteristics of their products before selling them? Or is calibration is for used anemometers which lost their accuracy? MANY THANKS FOR UR CLEAR NICE EXPLANATION! – user16307 Nov 22 '12 at 0:34
Sorry, I have no idea about anemometers actually; my answer is purely on the basics of calibration as it applies to any kind of sensor. I can imagine though, that anemometer errors could depend on lots of things in the environment where they're installed: structures that change the air flow or the temperature, for example. It could also change over time as the moving parts wear. If the user is going to have to do a regular calibration anyway, it reduces cost for the maker to not make the device so precisely, and have you fold any built in errors into the cal. – The Photon Nov 22 '12 at 4:34

If calibration involves measurement only, then it must be the intent that the data collected with the instrument must be adjusted according to the measurement.

Data gathering without any action being taken changes nothing.

For example if we calibrated a weighing scale simply by noting the weight that it reads when unloaded, then we would have to use that value by subtracting it from all subsequent weight measurements. Of course, we are not then calibrating the scale itself, just our system of measurement which is based on that scale!

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but what is actually adjusted in my case? lets say one obtained a rmp speed voltage graphic for an anemometer. with this graphic what will the actually user do with this anemometer when in use? u mean he will upload this data to his computer? – user16307 Nov 21 '12 at 23:59
There has to be some place to enter the calibration info, otherwise the computer will be running with figures which are off, and whoever is interpreting the results will have to know that and take it into account. – Kaz Nov 22 '12 at 1:25

Answering this point:

Just one more question to you: When the anemometers are manufactured don't they already know the characteristics of their products before selling them? Or is calibration is for used anemometers which lost their accuracy?

It is likely that the anemometer calibration is correct for the speed of the anemometer cup - it'll read 80mph when the cups are spinning around at 80mph. Calibration is necessary to discover the windspeed - which won't be less than this but will be somewhat greater, say 90mph to spin the cups at 80mph. Let's call the difference "slip speed" or calibration error.

Why don't the manufacturers build this difference into the readings? They can't - at least not accurately. Calibrate at sea level and the calibration will be wrong at 5000 feet. For ultimate accuracy you probably have to compensate for air temperature and barometric pressure too.

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