|WMR-918/WMR-968 Wind Vane Calibration|
Checking and Adjusting WMR918/WMR968 Wind Vane Calibration Copyright (C) 2001-2004 James S. Seymour
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I found the instructions included with my WMR-968 Wireless Weather Station for assembling and calibrating the wind vane... insufficient, to say the least. After another new user posted the same question that had entered my mind upon reading them, I decided maybe it was time for a FAQ.
Lest these pages be construed as an endorsement for these products, let me make one thing abundantly clear: I definitely do NOT recommend you buy the Oregon Scientific WMR-968. In my opinion the build quality is poor. I found the anemometer (wind speed gauge) to be worthless. Both my indoor and outdoor hygrometers (humidity gauges) were inaccurate and very slow to respond. I say "were" because, after only seven (7) years, the outdoor temperature & humidity sensors up & died. Replacing that sensor/transmitter group is too expensive to justify--particularly since, from all I've read, the manufacturer hasn't bothered to address a single one of its deficiencies in those seven years. The rain gauge transmitter was marginal at well under the claimed maximum transmitter distance. That, too, died about a year after the outdoor thermometer & hygrometer. If you're at all interested in a quality, dependable system that will give you something resembling useful data, I suggest you look elsewhere. But if all you want is a neat toy that'll last a few years, maybe this is for you.Here's what I did to calibrate the wind vane...
- I lined the "tail" of the wind vane up *precisely* with the center-line of the boom of the T-bar and set that to be 180° (south) exactly.
- Wind direction is usually reported relative to true north, as opposed to magnetic north. At least on land. So the next step is to determine the difference between the two at your location. This difference, known as magnetic declination is either subtracted from or added to the compass bearing in the next step.
- When I mounted the T-bar to the mast, I laid a compass on the boom of the T-bar, ensuring that the center-line of the north/south axis of the compass was *precisely* aligned with the center-line of the T-bar's boom, and adjusted the boom to point to true south. (180°, ±magnetic declination.)
When I first calibrated and installed my anemometer and wind vane, I had completely forgotten to take into account that there even was a difference between true north and magnetic north. (See: "Acknowledgments", below.) So the original version of this FAQ failed to account for this, as well. Now I know better :-).
So what exactly is this all about, and how do you determine what your magnetic declination is? The following list of URLs points to various explanations and tools to answer both of these questions.
ExplanationsMagnetic North VS True North
Magnetic Declination FAQ
Magnetic Deviation MapPay close attention to the legends on this map! Note that its purpose is to derive magnetic bearing from true bearing. So for converting from compass bearing to true bearing, you have to subtract where it says "add" and vice-versa.
What is the definition of latitude and longitude?How latitude and longitude are defined. Converting between "decimal degrees" notation and degrees:minutes:seconds notation.
ToolsEd Williams' Geomagnetic Field Calculator (on-line, web-based calculator)A couple of notes on using Ed's on-line calculator:USGS Geomagnetic Field Calculator (on-line, Java-based calculator)
Source code for Ed Williams' magnetic variation software is available. Check his home page for details.
- Note that there are "N/S" and "E/W" selectors. So don't enter signed values for these. I.e.: longitudes West of the prime meridian and latitudes South of the equator are usually noted as negative values. Don't put the minus sign in.
- In its output, "west" for "magnetic variation" means subtract the variation from the compass bearing to get the true heading. Conversely: an "east" value means you should add to the compass bearing.Some notes regarding Ed's downloadable code:
- The "magi" executable defaults to "West is positive." Also, the output yields magnetic variation as an offset from true north. To use conventional notation for longitude and get variation expressed as a correction to compass bearing, do this first:set plus NEEthen enter your lat, lon and (optional) elevation.
- The "magc" program, on the other hand, apparently uses conventional West/East longitude notation (i.e.: "West is negative") and outputs a result, for variation, as a correction to magnetic bearing.Note: By their own admission: doesn't display well with Netscape browsers. (They've got that right!) I'll withhold comment, other than to suggest not bothering with it. I was unimpressed. It does use conventional notation for longitude and outputs a magnetic declination value that you subtract from the compass bearing to get the true heading.For each of the above tools: select "IGRF2000" ("IGRF00" with Ed's magi calculator) for the most accurate results. Also, don't neglect to set the current date!
Finding YourselfFor any of the above tools, you'll first need to know your latitude and longitude. You may be able to find these (at least in the U.S.) with one of these resources:
Latitude and Longitude of World Cities (InfoPlease/Learning Network)
Latitude-Longitude of World Cities (www.realestate3d.com)
Suggestions and URLs for other resources (particularly those useful to for folks outside the U.S.!) are welcome.
How one might accomplish Part 3 (installation on the mast) of the above procedure if they can't safely get in such a position so as to sight over the top of the T-bar's boom is another question entirely. Suggestions for additions to the FAQ are welcome. I have some ideas, and I'll add them if they pan out. (I'll have to find an approach because, come spring, I'm going to elevate my anemometer/wind-direction cluster to well above the height over which I'll be able to safely execute the step!)
Thanks and a tip o' the hat to Tony K. for bringing the issue of true north vs. magnetic north to my attention!
Thanks also to Ed Williams for his magnetic variation software and the on-line calculator he so graciously provides. Ed was also kind enough to make time to answer some of the questions I had.
Feb. 5, 2001
|Comments or Questions?||Created: 28 Jan, 2001 / Last updated: 29 June, 2008|