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A Better Way to Calibrate a Davis Instruments Anemometer
11/9/03

A few days ago I tried to calibrate the wind direction using the Davis Instrument's method, page 11 of the Weather Monitor II Owner's Manual. I climbed up on the garage roof where it is mounted on the top of a TV antenna mast (no antenna), loosened the set screw securing the weather vane to its shaft, and turned the wind direction shaft while my Wife ran in out of the door below yelling the Weatherlink direction to me. The problems with that procedure, other than the obvious, is that the wind direction takes noticeable time to settle-out, and being perched on the ridge of the roof while trying to sight the weather vane along the mounting arm, which is little above eye level, is quite awkward. After I spent a half an hour or so on the roof, we gave up. The Davis procedure might be fine if the anemometer is mounted on a tripod on terra firma, but it is a terrible away to do it on a roof.

My method: after unplugging the Anemometer cable from a coupler in the attic, cutting-off the plug on the end of the cable, and feeding it out the hole that brings into the attic above the garage, I removed the mast and brought the whole thing into my shop/office where the Weather Monitor II and my computer are located. I used a cable tie to temporally secure the mast in a vertical position to some shelves in my shop. I then installed a new RJ-11 plug using the one I cut-off as a guide (the black wire goes furthest right when the plug is up and clip is away), plugged the cable into the Davis junction box on the wall near the Weather Monitor II, and adjusted the weathervane to read exactly north when aligned with the mounting arm and pointing away from the mast. I then climbed up on the roof, remount the mast, use a magnetic compass to align the arm to true--which is the direction I want it pointed anyway (because sometimes I'll look at it from the sunroom at the back of the house to get an idea of the general wind direction)--by twisting the mast before tightening. One could also loosen the U-bolts holding the Anemometer to the mast and adjust it to true north.

The present magnetic declination (the angle between magnetic north and true north at a particular location) at my latitude and longitude in Sanford, Maine, U.S.A. is -16 degrees, 15 minutes. That means that the direction to geographic north pole will be 16 1/4 degrees to the East of the direction the needle in the magnetic compass points.

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