Fine Offset Anemometer Factor.

[brassing]

It seems that the Fine Offset measures wind speed over 24 2 s intervals during its 48 s reporting period. The maximum of these measurements is reported as the gust speed. If you examine the gust speeds you see that they are all multiples of 0.34 m/s (or the equivalent in other units). Thus 0.34 m/s must presumably correspond to 1 pulse from the anemometer in 2 s. Since the anemometer has only one magnet and one sensor it gives one pulse per rev. The diameter of the anemometer is 140 mm (at the cup centres) so a bit of maths shows that at 0.34 m/s wind speed the cups are rotating at Pi x 140 / 2 /1000 m/s = 0.22 m/s.

I know very little about anemometer theory (just what I found in 30 min web research) but it seems the ratio of wind speed to cup speed is known as the anemometer factor and for 3 cup anemometers it varies between 2 and just over 3. The figures above give an anemometer factor of 0.34 / 0.22 = 1.56 which is well outside the expected range.

Either the Fine Offset anemometer is unusually efficient or there is something wrong with my logic. Does anyone have any idea where the discrepancy comes from?

(I have to admit that the value of 2 to just over 3 is found on various websites but without any source. The wording is almost identical on each site which indicates a single primary source, which might well be unreliable.)

[Charlie]

Yes, I chased this topic a couple days ago in the homebuilt section.
There has to be a formula, but I can't find it either.

[mcrossley]

Unless it needs two pulses to register the minimum 0.34 m/s, as one pulse is anywhere from zero up to 0.34 m/s. That would put the factor at 3.12 ? It should be easy to test if one pulse registers or not?

[beteljuice]

... Since the anemometer has only one magnet and one sensor it gives one pulse per rev. The diameter of the anemometer is 140 mm (at the cup centres) so a bit of maths shows that at 0.34 m/s wind speed the cups are rotating at Pi x 140 / 2 /1000 m/s = 0.22 m/s.

.... The figures above give an anemometer factor of 0.34 / 0.22 = 1.56 which is well outside the expected range.

You have used 2*Pi*D intead of 2*P1*r OR Pi*D ie. your answer should be 0.44ms not 0.22ms, which inturn ( ) doubles your ratio to 3.12:1

[brassing]

Unless it needs two pulses to register the minimum 0.34 m/s, as one pulse is anywhere from zero up to 0.34 m/s. That would put the factor at 3.12 ? It should be easy to test if one pulse registers or not?

Actually it works the other way. If it is 2 pulses per 0.34 m/s then that means the rotor is going twice as fast so the anemometer factor is halved to 0.78. I did wonder if there might be less than 1 pulse per 0.34 m/s i.e. that there is a second level of digitisation, but that would mean that some multiples of 0.34 would not be represented in the data which is not the case.

[Gina]

I'll check this out later on my spare (testing) kit. ATM the dining room table is booked for Christmas dinner.

[brassing]

... Since the anemometer has only one magnet and one sensor it gives one pulse per rev. The diameter of the anemometer is 140 mm (at the cup centres) so a bit of maths shows that at 0.34 m/s wind speed the cups are rotating at Pi x 140 / 2 /1000 m/s = 0.22 m/s.

.... The figures above give an anemometer factor of 0.34 / 0.22 = 1.56 which is well outside the expected range.

You have used 2*Pi*D intead of 2*P1*r OR Pi*D ie. your answer should be 0.44ms not 0.22ms, which inturn ( ) doubles your ratio to 3.12:1

No - I have used Pi x D. The 2 is the 2 s counting interval and it is a divisor not a multiplier.

The evidence that the counting interval is 2 s comes from the Signatrol site. I emailed them to check and they say they got it directly from Fine Offset. You would expect that to be definitive but manufacturers have been know to give out wrong figures. If the measuring interval were 3 s then that would make the anemometer factor 2.34 which is close to what you might expect.

Since there is a similar thread over on the homebuilt section of this forum I suggest we continue this topic over there.

[Gina]

Since there is a similar thread over on the homebuilt section of this forum I suggest we continue this topic over there.

I disagree, this is F.O. specific.

[brassing]

I think I have solved the mystery of the apparently low value of the anemometer factor. I had assumed (based on photos of a disassembled sensor) that it gave one pulse per rev. I have now seen the post about anemometer repair by DanielFon the homebuilt forum which shows a different photo and gives the opinion that the anemometer gives 2 pulses per rev. It seems there is only one magnet and one reed switch but the reed switch is cunningly positioned so the magnet operates it twice in a revolution. (The Fine Offset designer seems to like using reed switches in rather unconventional ways).

If it is 2 pulse per rev then that changes the calculation and makes the anemometer factor 3.12 which is a more reasonable value.

[Gina]

Mine produce one pulse per revolution. There is no mechanism to produce two.

[mcrossley]

I found one paper that attempted to derive a formula for (one example) of cup anemometer here:
http://mech.eng.unimelb.edu.au/people/s ... Ebert2.pdf

[Gina]

I found one paper that attempted to derive a formula for (one example) of cup anemometer here:
http://mech.eng.unimelb.edu.au/people/s ... Ebert2.pdf

Wow, there's some complicated maths in that!!

[werribee_au]

I always thought there were 2 pulses per revolution as I remember the year before last when I returned my anemometer for replacement due to a sticky spot, that when I was slowly rotating the cups by hand I could hear the reed switch being activated twice per revolution.

In hindsight I should have put the multimeter on to confirm it.
Next time the mast comes down I'll do just that.

Until then, the debate continues I guess.

[Gina]

I have now carried out the definitive test. The answer is TWO pulses per revolution. I was wrong. I hadn't actually done the test carefully enough - I had assumed 1 magnet and 1 reed switch meant 1 pulse per rev.

Time defeated me today and I haven't been able to check revs or pulses per second against recorded wind speed.

[beteljuice]

Gina ... wrong

It does seem very odd.
Looking at the posted picture, the rod magnet passes axially over the reed, which seems a little 'iffy' anyway. There does not appear to be any kind of magnetic shield or 'keep', so how does it operate twice ?

....... unless ...

The reed does NOT operate when the magnet is axially over it, but at the 90 degree positions. hmm ......