WH1081PC wind-speed plateauing

[DanielF]

My (cheap-and-nasty) weather station has no brand marked anywhere on it, but I believe it's made by Fine Offset.

In the nine months I've been running it, we've had several (non-contiguous) days of 'F9 Strong gale', and on almost all of those days the maximum gust recorded by Cumulus is 91.8 km/h. This is too much of a coincidence to be believable, and I strongly suspect something is limiting the maximum reading (though the WH1081PC is rated for wind-speeds to 160 km/h).

It could be something electronic - though I'm an electronics engineer, I haven't yet looked into how the anemometer works - can anyone tell me the system used? Does the anemometer send a DC signal proportional to speed, or an AC fequency proportional to speed to the wireless transmitter unit? Or does it process internally and send a digital data stream to the transmitter (as I understand some other cheapies do)?

It could also be something physical. Could the anemometer cups have some maximum rotation speed? Although mounted about 1 metre above the peak of my tiled roof, could there be some laminar flow situation that limits the maximum flow speed regardless of actual wind speed in the vicinity?

Has anyone else experienced this problem? Can anyone shed some light on the possible cause, and thence a cure? Also, if anyone has solved the problem of false rain impulses in these high winds (Cumulus has recorded 256mm today [with F9 winds], though no actual rain!), a cure for that would be of great interest. Any advice would be much appreciated.

[steve]

There was an error in the unofficial spec of the Fine Offset memory map that I was working from. I worked out a correction, and the 1.8.8 beta has the fix - see the announcements section for the link.

[DanielF]

Thanks Steve. I've installed 1.8.8beta and I'll watch it over the next few weeks for any change. Unfortunately the wind is now abating (down to F8 currently), so I won't be able to check the 'plateau' till we get another good wind (maybe next week).

Can you give me an idea of what the previous error was, and how it should affect subsequent readings relative to what was previously shown?

I notice that none of the wind speeds (current/gust/average) in Cumulus 1.8.8b agree with the display on the weather station (which reads 10-20km/h lower than your average). Is your 'average' a Cumulus average or is it read from the station? If the former, the discrepancy is possibly due to a different averaging period, though the difference is still bigger than I would expect.

The first thing I noticed different with 1.8.8b is the 'crowded' wind gauge. The numbers (when auto-scaled to 100 max) overlap and partly obscure each other (I run a 1024x768 screen). It would be sufficient to show numbers just for the tens, without the intermediate fives.

[steve]

Can you give me an idea of what the previous error was, and how it should affect subsequent readings relative to what was previously shown?

The memory map said that the gust was in two bytes and the wind speed in one, while in fact the second byte of the gust is actually split in two to give the most significant part of both. So as it was, the speed could never be more than 25.5 m/s, and if the gust did go over 25.5 m/s, it triggered the 'bad data' check because it's the top half of the second byte which is the overflow for gust.

I notice that none of the wind speeds (current/gust/average) in Cumulus 1.8.8b agree with the display on the weather station (which reads 10-20km/h lower than your average). Is your 'average' a Cumulus average or is it read from the station?

It depends on whether you've told Cumulus to calculate the average or not - http://wiki.sandaysoft.com/index.php?ti ... tations.3F

Either way, the 'latest' figure in Cumulus should still correspond to the 'gust' on the console, within a few seconds. If not, and you haven't set a calibration, there's something wrong somewhere.

The first thing I noticed different with 1.8.8b is the 'crowded' wind gauge. The numbers (when auto-scaled to 100 max) overlap and partly obscure each other (I run a 1024x768 screen). It would be sufficient to show numbers just for the tens, without the intermediate fives.

Yes - I'm not sure why I did that - I'm aiming for a constant interval of 5, when I should probably just leave the number of ticks at 10. Below 20 (if you have it set to go that low) I use an interval of 1, which is probably OK.

[Super-T]

Daniel
It will be interesting to see how a dumping of dust affects your rainfall figures?

[DanielF]

Daniel
It will be interesting to see how a dumping of dust affects your rainfall figures?

Terry, unfortunately the dust-storm coincided with the 'Strong gale' conditions (well, that's what caused the dust-storm!), so my rain gauge went mad (as it always does in >=F8 conditions, presumably from wind turbulence inside the gauge housing tipping the 'see-saw') and recorded 294mm (there was actually none!).

Steve, my console was set for 'average', so I changed it to 'gust' and its reading now agrees with your 'current' display. So that's good! I also checked, and Cumulus is calculating its own 10-min average, so that explains the observed discrepancy.

So all is well with the wind measurements, thank you, and I'm looking forward to the next >=F9 day to see what gust peaks we actually get here! Could be as early as next week from the BOM forecast.

[Super-T]

Nice looking beach you have at Gerroa Daniel.
Should be over that way sometime.....have cousins in Merimbula.
We're expecting to see some Aussie dust in the sunsets in the next few days in NZ

[DanielF]

Woohoo! We finally cracked the ton!

Our BoM predicted a breezy weekend, and sure enough Cumulus recorded a peak gust of 116km/h this morning. And for the first time I've been able to observe the wind gauge auto-scaling above 100km/h (to 150). Of course I couldn't read the digits on the gauge (as discussed), but no doubt that will be fixed in a later Cumulus build.

So thanks Steve, all is well now, and I'm happy that I can now (vicariously) experience some 'real' winds (sitting inside my cosy house)!

Nice looking beach you have at Gerroa Daniel.

Yes, it's a great spot, known to the locals as 'Paradise'. You should drop in on your way to Merimbula. But check the weather first http://www.users.on.net/~idmills.

[steve]

Of course I couldn't read the digits on the gauge (as discussed), but no doubt that will be fixed in a later Cumulus build.

I realised why I went for an interval of 5 rather than a fixed number of ticks; if you have the max at 30, for example, it looks a bit strange with the labels going up in threes. I think I need to stick to the interval setting, but change the interval as the max increases. I've tried it at an interval of 1 below 20, 5 below 100, and 10 for the rest. I think they all look OK - what do you think?

scale15.png

scale50.png

scale75.png

scale100.png

scale150.png

So thanks Steve, all is well now, and I'm happy that I can now (vicariously) experience some 'real' winds (sitting inside my cosy house)!

Thanks for confirming that it works; as soon as I'd done the fix, the wind speeds here dropped below the required 25.5 m/s and haven't been back up since, so I wasn't able to test it properly.

[DanielF]

I've tried it at an interval of 1 below 20, 5 below 100, and 10 for the rest. I think they all look OK - what do you think?

Steve, they look excellent to me. Well done, and thanks!

[steve]

OK - I'll upload a new build with the fix later today.