Anemometer errors

[raym]

Hi,

New here, but have searched the forum a bit.

I've a wh2080, received beginning of Jan 2013. I had to replace the anemometer bearing within 3 days - it seized solid. It is not the quality of engineering I am used to, but thanks to the info given on this forum, it was easy enough to fix. However, folk have said this type of anemometer tends to read low. Not wanting to fix it on top of my car/whatever, I looked for a formula to relate wind speed to number of revolutions. I could not find a definitve answer. Now, those of you that have calibrated your anemometers, what values did you use?

A basic defect with much of this type of system is that when you want the more extreme results, you can't get them. A strong wind, with sleet/snow, fills the anemometer cups, so it will false results, similarly, I guess, with horizontal rain . Another defect is that gusts tend to be exaggerated, due tio inertia in the system. However, it is good enough, as is, for comparison purposes here.

Snow gives a heavy rainfall measurement when it thaws (and generally clogs up everything else, of course). Not complaining, but I would be wary of believing every reading obtained at this time of year.

Best wishes,

Ray

[beteljuice]

Try: https://cumulus.hosiene.co.uk/viewtopic.p ... 8&start=10

also: https://cumulus.hosiene.co.uk/viewtopic.p ... 30&start=0

[raym]

Hi,

Thanks for the links - I hadn't searched far enough. I will have to study that and do some sums, but I'd be grateful if folk could say what calibration multipliers they are applying to the values they are getting from the FineOffset/easyweather anemometers within cumulus.

Best wishes,

Ray

[AllyCat]

Another defect is that gusts tend to be exaggerated, due tio inertia in the system.

Hi Ray,

If anything, I would have thought that inertia might cause gusts to be under-read. But cup anemometers are used almost universally even in "professional" applications, with the more "exotic" types such as "acoustic time of flight" reserved for ultra-harsh environments such as high mountain tops.

In Cumulus station settings, if "Calculate 10-min wind average" is ticked and "Use 'speed' for avge calculation" is NOT ticked, then Cumulus uses the "Gust" value as the "Average". Since gusts are typically reported as 60% - 100% greater than the average, this is effectively introducing a "wind calibration multiplier" of between 1.6 and 2.0 (although I doubt if many users of that configuration realise so), which IMHO is far too high.

Certainly my FO wind speeds do appear to be low, but I'm located in an urban environment so wind speeds may indeed be (much) lower than in open country. I haven't tested the anemometer on my car yet, but I have put it at the end of a 5 foot pole forming a "roundabout" spinning at up to 0.5 rev/sec (which corresponds to about 10 mph) and have NOT been able to PROVE any major errors in the FO calibration.

The Console truncates the displayed/reported average values so does potentially under-read by up to 0.75 mph. But this is hardly significant and can be compensated by adding about 9 miles to the daily wind run (assuming that there have been no long periods of truly zero windspeed). Also, I ran an extended test (many weeks) with the FO anemometer driving both the transmitter and a pulse counter and the Cumulus wind run was indeed around 10 miles per day less than that calculated from the pulse count (assuming the same anemometer factor).

So the Console appears to work reasonably accurately, but the question remains: has anybody actually managed to definitively "calibrate" the FO anemometer against a known range of wind speeds?

Cheers, Alan.

[raym]

Hi Alan,

Thanks for your reply. wrt overspeeding, I can't find the original article I had read , but from from http://faculty.eas.ualberta.ca/jdwilson ... 7wind.html it says -

.... But it can be shown that a cup anemometer in turbulent flow overestimates the mean speed ("overspeeds") because it accelerates more rapidly in gusts than it decelerates in lulls (example of cup overspeeding error; observations from an open flat field at Ellerslie, Alberta, May 2003; Wilson, in press, J. Applied Meteorol). Cups also have a "w-error," a departure from true cosine response (Hyson 1972; J App. Met. 11, p843), perhaps as large as a 6% error at heights of around 4 m. etc...

I don't really want to spend too much time testing this out, but from what I had read on this forum, it seemed everyone was saying the FO wind speeds were low. I'm situated on top of a hill, more or less, wind has been pretty strong today, but it's only registered 10mph (24mph peak gust). I'd expected much more.

At the moment, the unit is located in an easily accessable position, I'll relocate it some time, but even in this location, there certainly 'appears' to be more wind than it is recording.

Best wishes,

Ray

[AllyCat]

Hi Ray,

Bear in mind that FO only claim (and FO specs are usually rather optimistic) 10% windspeed accuracy above 20 mph and a worse percentage at lower speeds. However, your figures do sound rather low; even here in London I've recorded a 33.6 mph gust and 18.3 mph high average here in London today. Have you checked that the anemometer is actually giving two pulses per revolution (perhaps using a multimeter, or simply listening for the clicks from the reed switch)?

Cheers, Alan.

[raym]

Hi Allan,

I've not checked pulses. If it is only one pulse, and should be two/rev, I wonder if they have changed the design, and got the parts mixed up? Come to think of it, a week or two ago, I had to replace the bearing, and I noticed that there appeared to be two magnet recesses (both on same side of bearing housing). Maybe the magnet is in wrong hole, or there needs to be two. I can check it tomorrow, and maybe move it to a more exposed location. wrt location, I want it measuring where I am, sort of. For example, I see no point in measuring air temperature ten foot up in the air, since I'm not ten foot tall .

Best wishes,

Ray

[AllyCat]

Hi Ray,

It's unlikely that FO have changed the design (the resolution would be worse with only one pulse/revolution) but the Quality Control of FO products is rather poor. I wasn't aware that the anemometer has locations for two magnets (the vane does), is it not as shown in the Photos of the insides of FO sensors thread?

Much has been said on this forum about locating the sensors, but probably the most important factors are mounting the wind sensors (significantly) higher than any nearby obstructions and siting the transmitter (i.e the temperature sensor) out of direct sunshine (also NOT mounting the rain sensor on a wobbly mast). Extending the cables is possible, but with some risks.

Cheers, Alan.

[Charlie]

I have 2 anemometers and both started putting out only 1 pulse after 6 months to a year. My theory is that a poorly positioned reed switch suffers mechanical fatigue or starts to become magnetized causing this. In any case, the sensor is in a difficult spot to get to, especially this time of year, so I simply put a multiplier of 2 in Cumulus. My readings now closely match those of other stations in my area. I've lost a little resolution, but nothing significant. I'll look to repairing it in the spring when the snow and ice is gone from my roof.

[raym]

I've just checked it out - my mistake, only one position for magnet as in https://cumulus.hosiene.co.uk/viewtopic.p ... 747#p33747. I confused it with the wind direction vane, I guess. It gives two pulses per rev, at least at slow 'hand turning' speeds. I think I realise what the problem is, it is probably mounted too low, and the convex shape of the hill protects it from the wind. I'll mount it at head height see if that gives a result that suits me better. I'll let you know, it seems the unit is working as it should, for the time being, so thanks for the info.

Best wishes,

Ray

[raym]

Well, I've moved it, raised it. It feels (wind on face' about 15mph or so), but it indicates 7. Local weather stations around here (based on xcweather.co.uk) indicate 22mph, gusting to about 45. It's dustbin day on Thursday

[Charlie]

Time then to change, or at least lubricate, the bearings.

[AllyCat]

Hi,

I believe that's already been done (see post #1), the issue is very probably that it's not mounted high enough.

I'll mount it at head height see if that gives a result that suits me better.

If you want to measure the windspeed that flattened your roses (other plants are available) then you might put the anemometer in your garden, but for any sensible comparison with other stations you MUST locate it at least as high as any nearby obstructions. Otherwise, whatever "compensation factor" you use, it's just a case of "garbage in garbage out".

Note that the "official" height of an anemometer is about 30 feet above a flat field (correction factors can be applied for somewhat shorter masts) and any "obstructions" should be at least ten times further away than their height. Difficult to achieve, of course, with most amateur stations. Similarly, the transmitter (temperature sensor) must be in a shady location or within a proper "Stevenson Screen" (not just FO's attempt at a cheap one).

Cheers, Alan.

[raym]

Hi,

Thanks for your replies. Of course, at the moment my testing has been very subjective, but I am not convinced that the wind-speed is recording as it should be. It appears that the maximum speed is being limited. I will need to carry out some more 'scientific testing', of course.
It is not calm enough to mount it to a vehicle and take speed readings.

I could try to borrow a hand held anemometer from some organisation/user of such a device, and do a side-by-side check, but I do not know anyone with such a device. I could buy a cheap anemometer from ebay/wherever, but I've no idea if that would be any better. I'm not looking for extreme accuracy, but I have no confidence at the moment in this device.

One aspect that has crossed my mind - a magnet moving over metal generates a current - eddy current brake. The copper pcb for the reed switch is quite substantial, and may be slowing the rotation at higher speeds. The reed switch may be faulty, in as much it operates OK at low speed, but not high speed. I would hope that the firmware has some contact bounce suppression, but that may be too high, but then that would effect other units. It used to be that for switching sensors, about 20mA was switched, in order to keep contacts clean, I doubt if that is the case here (but then, that was for relays/switches in air, not enclosed reeds). These units are built down to a price, and there will be no quality control on components.

Without being able to get some actual wind-speeds, I have really little idea of where the fault, if any, may lie. I think I need to get another anemometer to test it against. In the meantime, I'll mount it a bit higher, see if I can get the speed up - currently, it should be about 20 to 30mph, it's indicating 5 to 10mph.

Best wishes,

Ray

[AllyCat]

Hi Ray,

A search of this forum's posts might be useful (including "contact bounce" with me as author ). If any contact bounce protection is fitted, it didn't prevent my test FO Console recording a (simulated) gust of almost 200 mph! Several people (particularly Gina) have measured the FO anemometer but none seems to have found it to read as low as yours appears to.

The FO uses about 20uA (microamps) current through the anemometer reed, but it is encapsulated in glass (unlike some very cheap Chinese "window alarms" which apparently don't bother with the glass tube ). That low current does make it rather sensitive to any leakage current in the cable/connectors (due to mositure) and indeed my (greatly extended) wind cable does currently have a buffer transistor and lowered resistance value until I can be bothered to pull down the mast.

Gina has used eddy current braking to damp her (home made) wind vane, but it needs a very substantial magnet and metal disc (a saucepan lid I believe) to produce a significant effect.

As for linearity (pulses/sec v windspeed), the "classic" cup anemometer is generally considered to be remarkably linear and FO don't appear to have done anything fundamentally "wrong" (with the anemometer hardware design). But Steve (who certainly knows more about high wind speeds than the rest of us) has insisted that his FO is more accurate using the "gust" value to calculate an average (i.e. effectively using a multiplier factor) when compared against his Davis.

Cheers, Alan.