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SoMDWx
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« on: March 14, 2010, 05:38:54 PM » |
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Hi All,
I am wondering if anyone here has ever tested / calibrated their rain gauge and looked at the accuracy of the tipping buckets? I am thinking about taking a measured amount of water and checking to see what the true amount each bucket holds before tipping......
Jim
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wxtech
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« Reply #1 on: March 14, 2010, 06:53:33 PM » |
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I think most of us has calibrated their tipping bucket rain gauges. Search this and other forums for the subject. There are lots of opinions on their accuracy.
My opinion: you can calibrate the gauge to be accurate at a rain rate but they aren't accurate at all rain rates. Some gauges have 15% under reporting error. Dr. Nolan Doesken of CoCoRaHS has done decades of testing on rain gauges.
Some rain observers have used a hypodermic syringe and counted the drops until the bucket tips.
A problem occurs when the bucket tips. It can be in the motion of tipping as water continues to enter the bucket. This causes under reporting. Also at high rain rates, some splatter may occur and water doesn't stay in the bucket until it tips.
I have personally experimented with many brands of tipping bucket gauges. I also have siphon adapted tipping bucket gauges.
I also have weighing rain gauges, R M Young Siphon gauge, and the Peet Ultimeter drop counting rain gauge.
Try this: measure how much water it takes to make the buckets tip 10 times at a very slow rate. Then use a larger nozzle and cause that same amount of water to flow at a higher rate and see if the buckets tip the same number of times.
You want your rain gauge to be level. Calibrate both buckets to tip evenly.
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« Last Edit: March 14, 2010, 07:04:17 PM by wxtech »
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Al Washington, Lexington, Ga., Davis VP2+ WLIP 5.9.2 w/soil temp, VP(original) serial. Acu-Rite 1015/1010/639/1055 5-n-1/3-n-1, bridge beta test group,
NWS Coop station=LXTG1, Fischer Porter, SRG, MMTS.
CoCoRaHS=GA-OG-1 manual & electronic ET gauges. CWOP=CW2074. XP with serial port, VWS v14.01p0, laptop with Win7 and USB ports.
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ocala
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« Reply #2 on: March 14, 2010, 07:21:49 PM » |
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Jim I tried replying yesterday but the forum would not let me. Said I was unauthorized.  That's a first Most of us have tried to calibrate are gauges. I did mine about 2 years ago. About that time I joined CoCoraHs so I had something to check it against. Slowly but surely it started to read high. I adjusted it again and got it as close as possible to the other gauge. I think mine may be "tipped" out. I have had it since 05 and with the amount of rain we get in Fla it has tipped a lot. The wire that holds up the tipper has elongated one of the holes on the the tipping bucket. May be time for a new one. |
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port1
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« Reply #3 on: March 14, 2010, 07:59:50 PM » |
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 A "known" amount of water is an accurate way of checking, but you could also get a 4 inch CoCoRaHS style rain gauge, which is accurate in it's own right, and check your tipping bucket to that. Mine has been accurate so far, for several years. No problems. I've learned from experience that the tipping bucket gauge you have must be dead level and accurately positioned. That's primary.  Henry
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KNYFLORA5 WMR968 VWS v14.00 p73 CoCoRaHS NY-NS-7 CWOP DW1891 SKYWARN 09-148  |
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DanS
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« Reply #4 on: March 14, 2010, 09:07:23 PM » |
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Hi All,
I am wondering if anyone here has ever tested / calibrated their rain gauge and looked at the accuracy of the tipping buckets? I am thinking about taking a measured amount of water and checking to see what the true amount each bucket holds before tipping......
Jim
Jim, a good and easy way to check out your gauge's accuracy is by using http://www.wxforum.net/index.php?topic=6764.0 . I believe that the OS gauge is 4.47 in diameter which measures 257.2 ml of water to show an inch of rain fall. You could put in half of that to read .5. |
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stevebrtx
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« Reply #5 on: March 15, 2010, 08:56:05 AM » |
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Calibration is fine, but consider the larger scenario. As the rain stops a bucket is collecting and then it's just a "droplet" short of being full and tipping, so it doesn't register that last collected amount. The remainder of the water evaporates and is never registered, so even though it's accurately registering a bucket full, the overall amount may not (in fact, probably is not) accurate.
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SoMDWx
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« Reply #6 on: March 15, 2010, 11:43:09 AM » |
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By 1mm..........  |
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tomcj2
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« Reply #7 on: March 16, 2010, 08:57:10 PM » |
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A properly calibrated tipping bucket should never read more than an accurate funnel type collector.
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 Davis VP2 (6163), WL 5.9.0.. VWS 14.01 p25, Panasonic HM371A camera. WU & W4U KORCANBY3, CoCoRaHS OR-CC-27 |
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SlowModem
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« Reply #8 on: March 17, 2010, 10:45:28 AM » |
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A "known" amount of water is an accurate way of checking, but you could also get a 4 inch CoCoRaHS style rain gauge, which is accurate in it's own right, and check your tipping bucket to that. Mine has been accurate so far, for several years. No problems. I've learned from experience that the tipping bucket gauge you have must be dead level and accurately positioned. That's primary. Henry That's what I did. Both my WS-2315 and WMR968 are very close now to my 4-inch gauge after I tweaked the bias on the software just a little bit. The accuracy varies somewhat with the rainfall rate. But I'm very satisfied, considering the small investment.  |
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SoMDWx
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« Reply #9 on: March 17, 2010, 08:29:23 PM » |
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Greg,
How much of a bias did you put in for your results?
Jim
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SlowModem
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« Reply #10 on: March 18, 2010, 04:02:56 AM » |
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Greg,
How much of a bias did you put in for your results?
Jim
In WHUH, 0 bias setting is 1. My setting is 1.15, so it's not much. |
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