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Weather Station Hardware => Davis Instruments Weather Stations => Topic started by: ANPweather on August 11, 2018, 06:08:55 PM
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I have a new Vantage Vue. I know that tipping bucket gauges won't always match a manual gauge and can significantly under report during a heavy rain.
Until today, it has always been close to my 4 inch CoCoRAHS gauge, sometimes matching, sometimes plus or minus .01 inches. Today though, over a 2 hour rain event the Vue reported .92 inches versus my manual reading of .78 inches. I'm at a bit of a loss explaining this.
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I have a new Vantage Vue. I know that tipping bucket gauges won't always match a manual gauge and can significantly under report during a heavy rain.
Until today, it has always been close to my 4 inch CoCoRAHS gauge, sometimes matching, sometimes plus or minus .01 inches. Today though, over a 2 hour rain event the Vue reported .92 inches versus my manual reading of .78 inches. I'm at a bit of a loss explaining this.
How gusty was it during that down pour? Gusty winds *could* be causing the tipping-buckets to "tip" before they actually filled, resulting in too many tips.
That was my first thought...or...perhaps there is debris/dirt caked on one or both sides of your tipper. This would cause them to tip more frequently.
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Was it heavy rain and do you have the debris lug inserted?
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Rain in the middle of the night so I can't speak from personal experience. Data show it to be a fairly steady rain over an hour or so. No wind gust over 11.5 MPH. Yes the debris guard is in place. It's not on a long pole so shaking shouldn't be a problem.
https://www.dropbox.com/s/7id6ahulwevxclr/2018-08-12%2016.41.04.jpg (https://www.dropbox.com/s/7id6ahulwevxclr/2018-08-12%2016.41.04.jpg)
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Other thing I can think of is - is the station completely level? Did you check the bubble level?
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It's mounted high enough that I can't see the bubble level on top. I have verified that the mounting pole just underneath is plumb in all directions so it should be good.
Several smaller rains since then have matched my manual gauge exactly.
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I only see one of your two gauges in the photograph. I'll surmise that there is a noticeable difference in the positioning of your two gauges relative to structures, trees, or other things that can interact with the wind.
As it happens I operate three gauges and routinely compare the results after rainstorms. One is a Davis VP2, one is a Rainwise, and one is a 4 inch plastic analog gauge that appears to me to be exactly the physical structure of the CoCoRaHS. My long-term observation is that slow steady rain with negligible wind gives almost identical results among the three gauges. Extremely high rain rate has the two tipping bucket gauges lagging behind, with the (bigger) Rainwise gauge lagging a bit more. But even a moderate amount of wind can lead to noticeably different results among three gauges. None of the three is out in the middle of a field hundreds of feet away from trees or structures. Instead they are along a fence which is only 10 to 20 feet away from my house, and the Davis is well under 30 feet from a fairly tall tree. I am pretty sure that even moderate wind interacts with the nearby features to create local variation in rain rate which is faithfully captured by the gauges and turns into noticeable differences in total from the same storm.
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You're right, there is a bit of a difference between the two locations. That's probably the most likely explanation for the discrepancy. It's certainly the largest discrepancy I ever seen and, I think, the only time where my manual gauge recorded less than the Davis and Acurite stations I've had.
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Well, it happened again. We're dealing with the remnants of Hurricane Rosa here and have had significant (for here) rain. The first couple of days of light rain, both of the measurements were the same. But for the last 2 days the Vue significantly over reported what vs the manual gauge:
Man gauge: 2.68" over three days
Vue 3.41" over three days
Recorded sustained winds were mostly under 5 mph, some as high as 10 mph.
Each picture is taken from one gauge looking toward the other.
https://www.dropbox.com/s/7vaaeygqdkcklov/2018-10-03%2009.50.22.jpg?dl=0
https://www.dropbox.com/s/aroatus4w6hrcdy/2018-10-03%2009.50.39.jpg?dl=0
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If I'm seeing the location of your manual gauge correctly, it appears to be on the ground next to the big rock. If that's the case, I would expect measurements in that gauge to be close to those in your Vue ONLY during a rain event where NO wind occurs. I suspect some of your rainfall is being blocked by the rock as well as the surrounding fences. You really need to get that gauge to height of 5-6 feet so that wind-driven rain won't be impacted by obstructions.
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Yes, next to the rock. I'll work on moving it higher, probably mounted to and just above the fence. Unfortunately it'll probably be several months before I get rain (significant or otherwise) again. :-(
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Had another significant rain event today after moving the manual gauge to the fence. 0.73 inches in the manual gauge and 0.94 inches as measured by the Vue. Since the Vue is new, could the calibration be off by that much?
(https://www.dropbox.com/s/et18xglxjruesab/2018-10-13%2016.58.43.jpg?raw=1)
(https://www.dropbox.com/s/re9a4d1vgt6szmb/2018-10-13%2017.09.18.jpg?raw=1)
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Hard to say, quite a few things can come into this type of comparison. The only real way to confirm this is to mount a manual gauge as close to and at the same height as the Vue.
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One thing I forgot to mention is that the Vue is a replacement / upgrade to an Acurite station that was mounted in the same location for 4 years or so. The rainfall measured by the Acurite was much closer to the manual gauge. Either matching or under-measuring as compared to the manual gauge.
That said, I moving the manual gauge to the top of the pergola which gets it much closer to the Vue. I think I'll also contact Davis and see if they have any suggestions.