Author Topic: Davis rain gauge overwhelmed last night  (Read 11782 times)

Felix

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Davis rain gauge overwhelmed last night
« on: June 21, 2015, 11:55:02 AM »
We had a major storm front pass through yesterday evening with three very intense rainfall peaks occurring at roughly one-hour intervals between 7 and 9:30 p.m. WeatherCat calculated rain/hour intensity levels exceeding 10 inches/hour for periods ranging from two to seven minutes at various times. Apparently the Davis tipping-bucket rain gauge design is unable to keep up rainfalls of that intensity because the total amount recorded was 0.22 inch lower than what was observed in the manual CoCoRaHS gauge. OTOH, a Vaisala piezoelectric rain gauge was within 0.04 inch of what was observed in the manual gauge....out of a total rainfall amount just slightly under 2? inches.

Blicj11

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Re: Davis rain gauge overwhelmed last night
« Reply #1 on: June 21, 2015, 03:21:35 PM »
Wow. That is a major storm! Most of of us with the 4 inch CoCoRaHS manual gauge have observed that the manual gauge is slightly more accurate than the Davis tipping-bucket.
Blick


Felix

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Re: Davis rain gauge overwhelmed last night
« Reply #2 on: June 21, 2015, 07:03:02 PM »
Blick, I've noted in those previous threads that my CoCoRaHS gauge usually reads a couple hundredths more than the Davis rain gauge...a hundredth to fill up the bucket the first time and a hundredth left in the bucket when the rain stops. In this case, the point I was making was that the tipping bucket apparently just couldn't keep up with the volume of water. We're talking a lot more difference than a couple hundredths.

LesCimes

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Re: Davis rain gauge overwhelmed last night
« Reply #3 on: June 21, 2015, 09:27:25 PM »
I don't think I have ever had it rain that hard here; so haven't pushed the Davis rain gauge to its limits. I wonder if Davis Instruments has any tech info on the upper limits of the rain gauge's ability to accurately record rainfall. Ten inches in an hour is torrential! Neat that WeatherCat allows rain data to be entered manually so corrections can be made at times like this.

elagache

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Are you sure? (Re: Davis rain gauge overwhelmed)
« Reply #4 on: June 21, 2015, 10:18:18 PM »
Dear Felix, Blick, and WeatherCat station caregivers,

We had a major storm front pass through yesterday evening with three very intense rainfall peaks occurring at roughly one-hour intervals between 7 and 9:30 p.m. WeatherCat calculated rain/hour intensity levels exceeding 10 inches/hour for periods ranging from two to seven minutes at various times. Apparently the Davis tipping-bucket rain gauge design is unable to keep up rainfalls of that intensity because the total amount recorded was 0.22 inch lower than what was observed in the manual CoCoRaHS gauge.

I just checked the Davis console manual and it claims that the maximum rate that a Davis VP-2 rain gauge can measure is 100" of rain an hour.  Do you think you were getting that much rain?

If not, perhaps your rain gauge needs some servicing.  Maybe there is some junk that is interfering with the bucket tip mechanism - spider webs maybe?

FYI . . . . . . . Edouard

Felix

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Re: Davis rain gauge overwhelmed last night
« Reply #5 on: June 21, 2015, 10:46:50 PM »
Well, based on what you found in the manual Edouard, I suppose I could have something stuck in the tipping-bucket mechanism. I'll check it tomorrow. We had periods of really hard rain but it certainly didn't come anywhere near 100 inches/hour! Thanks for the info.

xairbusdriver

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Re: Davis rain gauge overwhelmed last night
« Reply #6 on: June 22, 2015, 02:03:20 AM »
After discovering how much stuff was in my rain funnel in the first month, I've started inspecting it weekly. Last week I also discovered that the treated post had really shrunk! That made it lose enough that it could move from the vertical. I think being non-vertical can also affect the tip bucket; increased friction, or over/under filling the buckets.
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LesCimes

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Re: Davis rain gauge overwhelmed last night
« Reply #7 on: June 22, 2015, 11:30:46 PM »
Quote
I've started inspecting it weekly

Good advice for all of us! 100" an hour - that is quite a threshold. Noah's flood perhaps?  [thunder]

Felix

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Re: Davis rain gauge overwhelmed last night
« Reply #8 on: June 23, 2015, 06:05:06 PM »

Good advice for all of us! 100" an hour - that is quite a threshold. Noah's flood perhaps?  [thunder]

That's nearly three tips of the 0.01 inch bucket per second! If that's even possible, I'd think the accuracy would decrease significantly as the rate goes up...which may explain why I had such a large error in the three 10 inch/hour downpours. Looks like I may have to dig out the book Edouard referenced to see if it gives any accuracy guarantees at the higher rainfall rates.

BTW, pulled the collector off and took a look, no leaves nor critters were in the tipping mechanism.

xairbusdriver

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Re: Davis rain gauge overwhelmed last night
« Reply #9 on: June 23, 2015, 08:38:30 PM »
Quote
pulled the collector off and took a look, no leaves nor critters were in the tipping mechanism.
Do you have the upside down plastic cone/screen thingy? I'm not sure they've always had that. I see they have a different one that's more like a slotted disk, I think he cone-shaped one has smaller slots and traps stuff better, but I've not run any scientific tests...

Part #: 7345.530, $8.00
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elagache

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Davis backs off - a bit. (Re: Davis rain gauge overwhelmed)
« Reply #10 on: June 24, 2015, 12:15:27 AM »
Dear Felix, X-Air, and WeatherCat Davis station owners,

Looks like I may have to dig out the book Edouard referenced to see if it gives any accuracy guarantees at the higher rainfall rates.

In my almost 6 year old copy of the Davis console manual, you'll find the number on page 49.

Looking at the current manual which is available online:

http://www.davisnet.com/product_documents/weather/manuals/07395-234_IM_06312.pdf

You'll find the specifications on page 52 instead.  Also, they have backed off the peak rate just a bit - to quote:

Quote
Rain rate: to 96''/hr.; 2438 mm/hr

Do you have the upside down plastic cone/screen thingy? I'm not sure they've always had that. I see they have a different one that's more like a slotted disk, I think he cone-shaped one has smaller slots and traps stuff better, but I've not run any scientific tests...

Most of us have this simpler design (photo from Scaled Instruments:)



Scaled Instruments does have the upgraded debris screen available for those who want it:

https://www.scaledinstruments.com/product/davis-7345-530-pro2-bird-spikedebris-screen-kit/

Cheers, Edouard  [cheers1]

Felix

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Re: Davis rain gauge overwhelmed last night
« Reply #11 on: June 24, 2015, 12:45:02 AM »
Found this link, Edouard and they make no accuracy guarantee above a rainfall rate of four inches/hour.

http://www.davisnet.com/product_documents/weather/manuals/07395-275_IM_07852.pdf

 And my collector screen is the same as yours.

xairbusdriver

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Re: Davis rain gauge overwhelmed last night
« Reply #12 on: June 24, 2015, 01:25:00 AM »
Highly recommend the new and improved model. You might ask for "upgrade pricing" and a loyalty discount! I'm bettin' you can arrange a "deal" for less than $50! :o
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elagache

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Odd, isn't it! (Re: Davis rain gauge overwhelmed last night)
« Reply #13 on: June 24, 2015, 11:23:21 PM »
Dear Felix, X-Air, and

Found this link, Edouard and they make no accuracy guarantee above a rainfall rate of four inches/hour.

Hmm, interesting!  The same document gives the same absolute limits.  Sure looks to me like Davis is trying to have it both ways.

What sort of rainfall rates was your station reporting at the time?  Were you significantly above 4"/hr?

Highly recommend the new and improved model. You might ask for "upgrade pricing" and a loyalty discount! I'm bettin' you can arrange a "deal" for less than $50! :o

It does look nice.  However, my rain gauge has relatively few things getting into it because it is on a second story with not much in the way of trees nearby.  However, if I have to buy some other part for my station, I'll probably buy it then.  I do like the improved collector cone design, but that would prevent me putting a cap on the rain gauge during the summer.  Having a cap on it seems to solve most of my bird problems since they have nothing to land on.

Cheers, Edouard  [cheers1]

xairbusdriver

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Re: Davis rain gauge overwhelmed last night
« Reply #14 on: June 25, 2015, 02:23:21 AM »
Actually, the cone/top is well below the top/rim of the gauge. Whatever you were using for a cap should still work. I assume that you do not use/have the anti-bird "spikes" either. The newer collector has ting holes in the top of the little bump thingies around the rim of the collector. You are "given" a straight piece (about six inches long) of stainless (I hope) wire to stick into each hole. Supposedly helps prevent a bird for using the collector as a perch. Wouldn't mind them sitting there if they were facing the center of the collector, but facing outward buts the south end of a north facing bird over the collector. Not the end we enjoy learning about! [blush] Of course, even the other end could have some seed parts that they would drop into the collector. Apparently, having "Davis" plastered on two sides of the collector was not enough to discourage these aviators... [banghead] [rolleyes2]
THERE ARE TWO TYPES OF COUNTRIES
Those that use metric = #1 Measurement system
And the United States = The Banana system