What the Latest, Gust, Average and PeakGust wind speeds mean

[jim-easterbrook]

I had an email yesterday from someone who says he asked Fine Offset about its wind measurements and how they work, and got the following answer. As we're already led to believe, it makes 24 2-second samples over the 48 second measuring period, and the highest of these it reports as 'gust'. It then averages the 24 samples. And here's the strange part: it doesn't report that value as the 'speed'. Instead it adds that value to the 'gust' value and divides by two, and reports that as the 'speed'.

If that's true, then (ignoring rounding errors etc.) the 'true' average value is simply ((reported average * 2) - gust). To test this I tried plotting it for my last 24 hours of raw data (attached). The 'adjusted' data is mostly zero or less, which must be wrong.

[brassing]

I had an email yesterday from someone who says he asked Fine Offset about its wind measurements and how they work, and got the following answer. As we're already led to believe, it makes 24 2-second samples over the 48 second measuring period, and the highest of these it reports as 'gust'. It then averages the 24 samples. And here's the strange part: it doesn't report that value as the 'speed'. Instead it adds that value to the 'gust' value and divides by two, and reports that as the 'speed'.

If that's true, then (ignoring rounding errors etc.) the 'true' average value is simply ((reported average * 2) - gust). To test this I tried plotting it for my last 24 hours of raw data (attached). The 'adjusted' data is mostly zero or less, which must be wrong.

I agree - the information must be wrong. If it were true then the average speed could never be less than half the gust speed and it frequently is for my home station.

[steve]

I agree - the information must be wrong.

Do we believe the other part of the 'information'? Have your tests proven conclusively that it does take 24 2-second samples?

[MickinMoulden]

I agree - the information must be wrong.

Do we believe the other part of the 'information'? Have your tests proven conclusively that it does take 24 2-second samples?

I just did bench testing on the weekend. It does not take 24 2-second samples. I had a 150w fan set up and found out a few things:
* My fan reached 49 kmph! (Ok, so it was only pushing one side of the cups, so it doesn't count )
* When the cups are fully in the stream it measures av=30kmph.
* When putting half the cup in the wind tream (remember max is 49kmph), this gust speed would not register if held in the stream for 1, 2 or 3 seconds, only 4 or more (3 seconds got approx 30kmph).
* At anytime during the 48 sec cycle, that gust speed could be reached if held for 4 seconds.

On wind obstruction:
* The cups do little in the way of obstructing the wind to the wind vane,
* The wind vane obstructs the wind cups by a huge margin. ie. With gusts to 27 kmph (I had to hod the anometer further away from the fan to allow the wind vane to rotate on it's mounting around the anometer), I had the wind speed drop to 0 and even went backwards!

I promptly added 500mm to the height of the anometer.
I could attach the graphs, however I deleted alot of the data so unfortunately I wouldn't trust them......oh what the heck, I'll let you guys be the judge....

[MickinMoulden]

one more thing, the wind vane does not take any average measurement. The measurement it displays is excactly where it is pointing, strangly enough, exactly 4 seconds prior to sending the information.