|WMR-918/WMR-968 Rain Gage Calibration|
Checking and Adjusting WMR918/WMR968 Rain Gage Calibration Copyright (C) 2001-2004 James S. Seymour
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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.
Checking and Adjusting WMR918/WMR968 Rain Gage Calibration by Jim Seymour A number of folks have reported inaccurate readings (sometimes wildly inaccurate!) yielded by their Oregon Scientific WMR-918 or WMR-968 rain gages. This FAQ puts together information, gleaned from several sources, on how to check and, if necessary, adjust your rain gage. How It Works The way the OS WMR-918/-968 rain gage works is by funneling collected rain to land on the pivot-point of a two-sided "bucket." When one side fills, it pivots down--causing the "bucket-tip" to be recorded, emptying the bucket and bringing the empty side under the collection point. Each bucket-tip is *supposed* to occur when 0.04 inches (1 mm) of rain has been collected. Problem #1: Unequal "bucket-tips." One problem is that, as shipped from Oregon Scientific, some rain gages have been found not to tip equally on each bucket. The problem with this may not be apparent when there's heavy rainfall--the unequal tips may average each other out. But light or slow rainfall may result in inaccurate rate and total readings. Solution #1: Equalize the "bucket-tips" Posted to CAIS' WMR-918 Wireless Weather Station Bulletin Board (http://www.weatherwatchers.org/bbs/wmr-918board/) by Robert Nicholas on June 07, 2000: When I measured the water required to tip the buckets, one took quite a few more drops than the other, even though the gauge was level. By adjusting the screw under the buckets you can calibrate the gauge. raise the screws to make the gauge read higher: lower them to reduce the readings. Note that the bucket that is up is adjusted by the screw under the one that is down. It's also important that it takes the same amount of water to tip each bucket. I used a turkey baster and counted the drops for this. A dab of silicone rubber is a good way to fix the screws and easily removable if future adjustment is required. Problem #2: Inaccurate Measurement Even if the bucket-tips are equal: if they're tipping too soon or too late, the gage will measure too high or too low (respectively). Solution #2: Calibrate the gage Posted to CAIS' WMR-918 Wireless Weather Station Bulletin Board (http://www.weatherwatchers.org/bbs/wmr-918board/) by "Jeff" in North Central Ohio on June 08, 2000: I calibrated my unit by computing the area of the collection funnel (78.54 sq. cm.); therefore 1cm. of rainfall would equal a total volume of water collected of 78.54 cc. Since each tip of the bucket is equal to 1mm of rainfall, each bucketful should be 7.854 cc. Conversions: Not many of us will have the tools on-hand to measure-out 7.854cc of water :-). U.S. Americans are likely to have measurement devices in cups and multiples-of-a-cup handy. Europeans and others: liters and milliliters. So... 7.854cc (ml) == 1mm rain == .004in rain 250ml == 31.83mm rain == 1.25in rain 500ml == 63.66mm rain == 2.51in rain 1l == 127.32mm rain == 5.01in rain 1 cup == 236.588 ml 1 qt == 946.353 l 1 cup == 1.19in rain == 30.12mm rain 2 cups == 2.37in rain == 60.25mm rain 1 qt == 4.74in rain == 120.49mm rain Robert Nicholas submitted an easy-to-remember set of numbers: 1 inch rainfall == 199.5 ml - darn close to 200 ml. Procedure: Here's how *I* would go about it: 1. Find out if the gage is accurate in the aggregate--i.e.: regardless of whether the bucket-tips are equal or not. Do this by feeding it enough "rain" so that any unequal bucket tips average out. At least the equivalent of 1 inch. Two would be better. ---------------------------------------------------- Note: When you're doing this step, and step 3, don't pour the water in too rapidly! You're trying to simulate rainfall, not a tidal wave :-). If you pour too quickly, you may loose accuracy due to water over-flowing the bucket divider as it tips back-and-forth. (Don't know what this says about the unit's accuracy in a for-real deluge.) ---------------------------------------------------- 2a. If the aggregate is accurate, but the bucket-tips are uneven: equalize the bucket-tips by adjusting one up and the other down--in tandem. Go to step 3. 2b. If the aggregate is high or low: first see if one of the buckets is tipping sooner or later, respectively, than the other and first adjust *that* bucket to be even with the other. Go to step 3. 2c. If the aggregate is inaccurate, but the bucket-tips are equal, adjust the buckets to tip sooner or later--each by the same amount. 3. After the bucket-tips are equal: check for accuracy (as in step #1) and re-calibrate if necessary. (As in step # 2c.) Perform these steps with the funnel in place. You'll hear the bucket-tips. And, as Robert notes, this will lead to a more accurate simulation of the unit in rainfall. Tips, Tricks and Other Miscellania From Robert Nicholas: I would like point out a real accurate way to equalize the buckets. You can count drops of water from an eyedropper or a turkey baster, but this is very tedious as it takes many drops. I recommend you find a small container or hollow object (I used a very small test tube) who's volume will fill the buckets to the threshold of tipping, then counting the drops needed to complete the tipping. It's a real time saver. [For the "high-volume" accuracy tests] I measured the water out, then used a squeeze-type squirt bottle (like an empty gator-aid one) to slowly add it: Make sure you don't squirt directly into the collection hole, or the force of the stream may contribute to the bucket tip! I also would like to point out that I think the water drop leveling gauge built in the the gauge is cumbersome to use: Just uses a small spirit level on the top of the cover. Much easier!!! I noticed that the rain total shown in the main display does not always increment in steps of .04. It probably uses metric units internally and converts to inches, hence the occasional step variation due to rounding. For example, my unit will read 1.02 inches, which isn't even a multiple of .04. Real perfectionists may also want to remove the cover and make sure that [very little or no] amount of water remains in the bucket that is the last one up!! Also, when adjusting the screws I found it was easier to carefully remove the factory plastic thread lock completely (with a small screwdriver) than try to force them to turn. [Note from Jim Seymour: I removed the stuff they used to lock the screws on my rain gauge--which appeared to be clear RTV "caulking" material--by grabbing it with a pair of hemostats and pulling it off.] Acknowledgments: This FAQ is little more than consolidating the work and ideas of others. So thanks and a tip o' the hat to: Robert Nicholas for most of the material! "Jeff" in North Central Ohio for the calibration volume work. And, of course, Jessie Ferrell of CAIS, who so kindly provides the bulletin board system upon which I found posted the articles that comprise the core of this FAQ. I followed a procedure similar to that detailed under "Procedure", above. It seemed to work out well. Jim Seymour Mar. 10, 2001
|Comments or Questions?||Created: 13 Jan, 2001 / Last updated: 29 June, 2008|