I installed my Fine Offset WH1081 about 6 weeks ago -- unfortunately the anemometer supplied did not work and while waiting for a replacement I have had a temporary installation for testing which favours easy access rather than a well-performing WS .
While I have been cooling my heels waiting for a replacement, I have been lurking and learning on this forum and planning the installation and mods.
My primary concern has been my windvane which is very sensitive to low speed wind but appears to be too sensitive, with frequent windmilling, in gusty weather. From other entries in the forum this seems to be a common problem.
For some time I have had a bee in my bonnet about using eddy-current damping to solve the wind-vane instability problem as I consider its characteristics to be ideal for this application. I have now completed a proof of concept prototype that I can share.
How does eddy current damping work? The theory is quite simple, a changing magnetic field creates eddy currents in conductive materials, think of them as vortexes. These eddy currents create forces which oppose the change in the magnetic field. In application of the theory, I rotate a magnet over an aluminium plate. The movement of the magnet creates a changing magnetic field in the aluminium plate. The eddy currents induced in the plate create a reverse polarity magnetic dipole which attracts and slows the magnet. The faster the magnet moves the greater the retardation force. Thus when stationary there is absolutely no attraction so the sensitivity of the wind vane to low air movement is not affected. This is the ideal characteristic I refered to.
Mindful of my warranty, the prototype for the PoC had to be low-impact without permanent change.
Step 1. I attached a powerful magnet to the vane rudder. [I chose the rudder to keep the new magnet as far as possible from the existing one]. The magnet chosen was a neodymium button 9.5 mm diam x 6.4mm thick with a pull of 9.2lbs. This was purchased over the Internet [USD8 for 10 (AUD20 including p&h)]. I considered using a fridge magnet, but these are not very strong, necessitating very close separation of the magnet and the plate to get equivalent drag. The magnet is fixed to the rudder using the cap of a bic-click ballpoint pen (I have a lot of these for some reason) which has a thin cut vertically to slide onto the rudder. At the moment it is a push fit only but I will silicon seal it soon. In the tip of the cap there is a 6G self-tapping screw to which the magnet self-adheres. The screw gives a crude adjustment capability to control the separation of the magnet and plate.
For the plate I used a 20cm fryingpan, with handle removed, and drilled a 15mm hole through the centre of the base. To the north and south sides of this hole there are joined two 6 mm holes to accomodate the wires. The base is about 2mm thick and quite rigid. The plate is pushed up against the skirt of the vane and is held in place with a thick rubber washer (rescued from a Nylex Gardena hose fitting). Only slight compression of the washer was needed to insert the mounting bolt and a snug fit was achieved.
The magnet is tracking about 1mm above the plate. The gap seems to be quite uniform over the locus of the magnet. If it had been variable I would have levelled the plate using shims between the skirt and the plate.
I considered removing the curved sides of the fryingpan but elected not to as I don't have a lathe and thought I might warp the plate if I cut it by hand. The plate is not attached to the frame but if necessary could be supported using the handle mounting hardware.
Testing has been very rudimentary. I gave the rudder a tap and counted the revolutions -- without damping I counted four revolutions for a moderate tap; with damping I could only get a quarter revolution for even quite hard taps. Light breezes, which is all I have had in the past 24hrs are still moving the vane. That is sufficient for me to assume the mod is performing as designed. Here is a short movie of the tests. [It's pretty ragged as I took it wth my phone, but you will get the idea].
For ligth wids it's performing quit well. It remains to be seem whether it will significantly affect the recordings over the full range of wind speeds!
You can see the published results for my temporary installation at
- I won't have a problem with snow but the plate would be a good place for it to lodge. perhaps a vertical plate would be better. Could keep it in line with the skirt. A coke can is about the right size
- Don't know if 1 mm clearance will be enough to avoid problems with lodged rain water
Additional ideas and comments would be welcome.