Fine Offset Anemometer Factor.

[werribee_au]

Phew! Gina, when you first mentioned one pulse per rev I thought, Ummmm, ok. So I never questioned it.

Thank you for double checking. Now we can come up with some sort of calculation for revs per day/month/year etc

[Gina]

Phew! Gina, when you first mentioned one pulse per rev I thought, Ummmm, ok. So I never questioned it.

Yes, sorry about that.

Thank you for double checking. Now we can come up with some sort of calculation for revs per day/month/year etc

.Yes again, I'm planning to work that out too.

[Gina]

I found 1 pulse = 0.34 m/s, I agree with that. So 1m/s = 1/0.34 = 2.94 pulses (OK so that the mathematical number!). 10m/s = 29.4 etc.

P/s . . m/s . . mph
1 - - - 0.34 . . 0.76
2 - - - 0.68 . . 1.52
3 - - - 1.04 . . 2.28
5 - - - - 1.7 . . 3.80
10 - - - 3.4 . . 7.6
20 - - - 6.8 . . 15.2
30 - - 10.4 . . 22.8
40 - - 13.6 . . 30.4
50 - - 17 .. . . 38

Or to put it the other way round :-
m/s . . pps
10 - - - 29
20 - - - 59
30 - - - 88
40 - - 118
50 - - 147

mph . . pps
10 - - - 130
20 - - - 260
30 - - - 390
40 - - - 560
50 - - - 650

[brassing]

I found 1 pulse = 0.34 m/s, I agree with that. So 1m/s = 1/0.34 = 2.94 pulses (OK so that the mathematical number!). 10m/s = 29.4 etc.

P/s . . m/s . . mph
1 - - - 0.34 . . 0.76
2 - - - 0.68 . . 1.52
3 - - - 1.04 . . 2.28
5 - - - - 1.7 . . 3.80
10 - - - 3.4 . . 7.6
20 - - - 6.8 . . 15.2
30 - - 10.4 . . 22.8
40 - - 13.6 . . 30.4
50 - - 17 .. . . 38

Or to put it the other way round :-
m/s . . pps
10 - - - 29
20 - - - 59
30 - - - 88
40 - - 118
50 - - 147

mph . . pps
10 - - - 130
20 - - - 260
30 - - - 390
40 - - - 560
50 - - - 650

I think it is 1 count in 2 s = 0.34 m/s. So you need to multiply all the above speeds by 2.

[Gina]

I think it is 1 count in 2 s = 0.34 m/s. So you need to multiply all the above speeds by 2.

OK, I'll run a test with the F.O. unit to check.

[Gina]

Desk fan blowing air stream past anemometer and system showing about 10mph. Oscilloscope period measurement gives 135ms. That's 7.4 pulses per second. That doesn't agree with the calculations above! I'll check that again!

[mcrossley]

Gina

I remember you saying that the 'avg' speed reported by the FO was always lower than the 'gust' even in a constant speed wind (a fan). Do you have any numbers to quantify that? I'm thinking we could come up with a 'correction factor' to plug into Cumulus to account for this anomaly.

[Gina]

Yes, that's right. I have results from when I was testing the anemometer for temperature spike problems but I'm just setting up again to confirm those anemometer speed results and hopefully take more accurate readings.

[brassing]

Desk fan blowing air stream past anemometer and system showing about 10mph. Oscilloscope period measurement gives 135ms. That's 7.4 pulses per second. That doesn't agree with the calculations above! I'll check that again!

7.4 pulses per s = 14.8 in 2 s so speed should be 14.8 * 0.34 m/s = 5.03 m/s = 11.2 mph. So not far out. Difference could be due to rounding and resolution errors.

[Gina]

Fan providing air flow again. Console showing 5.4 and 6 mph alternately. One revolution with two pulses in 0.50 secs as measured on my scope using the time markers. Computer showing 2.7m/s average and 3.1m/s gust.

4 pulses per second = 8 in 2s 8 * 0.34 m/s = 2.72 m/s = 6.08 mph - now that fits well We seem to agree

test-Wind=b.png

[Gina]

1pps = 1.5mph

15mph = 10pps
30mph = 20pps
45mph = 30pps
60mph = 40pps

[Gina]

Another test with much lower air flow - around the lowest readable. Console showing 0.7 mph. One revolution with two pulses in 3.45 secs. Computer showing 0.3m/s average and 0.7m/s gust. One unit average and two units of gust (with occasional 3 units of gust, 0.34x3 = 1.02)

2 pulses in 3.45 seconds = 4/3.45 = 1.16 in 2s ... 1.16 * 0.34 m/s = 0.39 m/s = 0.87 mph.
Since it can't measure a fraction of a pulse, this is 1 pulse in 2s and 0.34 m/s = 0.7 mph

Plot of wind speed in m/s

test-Wind=c.png

[werribee_au]

Another test with much lower air flow - around the lowest readable. Console showing 0.7 mph. One revolution with two pulses in 3.45 secs. Computer showing 0.3m/s average and 0.7m/s gust. One unit average and two units of gust (with occasional 3 units of gust, 0.34x3 = 1.02)

2 pulses in 3.45 seconds = 4/3.45 = 1.16 in 2s ... 1.16 * 0.34 m/s = 0.39 m/s = 0.87 mph

So the magical figure is approx 0.39m/s for 1 rev/sec (2 pulses) which is 0.39m of wind run per anemometer rev.
Have I got that correct?

If I have, I can see millions of revolutions.

[Gina]

So the magical figure is approx 0.39m/s for 1 rev/sec (2 pulses) which is 0.39m of wind run per anemometer rev.
Have I got that correct?

Yes, but I should have added "since it can't measure a fraction of a pulse, this is 1 pulse in 2s and 0.34 m/s = 0.7 mph".

If I have, I can see millions of revolutions.

The only part subject to wear is a shielded ball bearing and these are rated for millions of revolutions - much more than this application requires. Unless the bearing gets contaminated with foreign matter, it should go on indefinitely. The design should reduce the likelihood of condensation getting into the bearing.

[Ned]

To count revolutions over long periods of time, I think the easiest way is to relate them to wind run, which Cumulus records. So I tried to derive the relationship as follows (in metric units). Please correct any errors!
Using Gina's recent table:
30mph = 20 pps
or 50km/hr = 20 p/s (roughly)
50km/hr = 20x60x60 p/hr = 72000p/hr
reducing, 50km = 72000 pulses or 36000 revolutions
therefore 1km = 36000/50 = 720 revs
Now extrapolating my wind run (since July) to a full year yields about 50,000km, equating to 36 million revolutions.

Digging up an old bearing manual, it states that bearing life (in millions of revolutions) is dependant on the relationship between the dynamic bearing rating and the equivalent bearing load, and the tables range from less than 1 million to over 30 billion revs, depending on the load factor. My assumption is that anemometer bearings are very lightly loaded, and should give a life of billions of revs, assuming high quality manufacture and excellent sealing against contamination. Just keep in mind the country of origin - there have been cases reported here of bearing failure.