Author Topic: Davis bucket calibration  (Read 4536 times)

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Offline Bashy

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Davis bucket calibration
« on: July 06, 2017, 02:51:56 PM »
Hi folks, it. Looks like my bucket is under reading, I have a climet 100mm manual gauge and after the storm today the climet was about 2mm more than the Davis, I also have another Davis bucket that is modified to work with my OS  station and that too is under reading compared to the climet.

Where do I go from here, I read somewhere about turning those screws but I can't find anything at the minute and there's nothing in the manual..... 
« Last Edit: July 06, 2017, 02:54:24 PM by Bashy »
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Online WheatonRon

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Re: Davis bucket calibration
« Reply #1 on: July 06, 2017, 03:06:20 PM »
Hi folks, it. Looks like my bucket is under reading, I have a climet 100mm manual gauge and after the storm today the climet was about 2mm more than the Davis, I also have another Davis bucket that is modified to work with my OS  station and that too is under reading compared to the climet.

Where do I go from here, I read somewhere about turning those screws but I can't find anything at the minute and there's nothing in the manual.....

Per Davis manual below:

The non-metric version of the rain collector is calibrated at the factory so the spoons tip (and records rainfall) for each 0.01" of rain. The metric version is calibrated so the spoons tip for each 0.2 mm. To adjust the calibration slightly, use a 3/16” (or 5 mm) wrench to rotate the adjustment screws which are located underneath the tipping spoons. (See “Rain Collector Internal Components” on page 2.) The adjustment guide embossed in the platform shows how far you must rotate both screws in turn to effect a 1% and a 2% change. Moving the screws in the positive (+) direction causes the spoons to tip more times (i.e. give a larger count) for a given amount of water. Note: Modify both adjustment screws by the same amount. To check the accuracy of the rain collector, compare the Davis rain collector with a tube type rain gauge. Use a rain gauge with an aperture of at least 4 inches. Any smaller and the readings obtained may not be accurate. Place the tube type rain gauge directly next to the Davis rain collector. Compare the totals on three storms to determine whether your rain collector needs calibration and by how much. Adjust the screws to fine-tune the reading for the next three storms if necessary. Note: Avoid comparison to rainfall readings obtained from television, radio, newspapers, or neighbors’ readings. Such readings are not an accurate measurement of the weather conditions in your specific location. The rain collector is carefully tested at the factory to conform to the specifications listed in the back of this manual.

© Davis Instruments Corp. 2015, Product Number 7857, “Rain Collector”

WheatonRon editorial comment. Before calibrating, make sure the gauges are level, approximately the same height from the ground, and are as close as possible to each other and don't calibrate based on a single rainfall! Use at least three storms and then average them. All this being said, getting all three of your gauges to measure rainfall in a single storm exactly the same is highly unlikely as a result of wind, nearby trees, etc., and maybe most importantly, automated "tippers" like the VP2, normally underreport rain during heavy rainfalls as they have trouble keeping up with the volume of rain falling. I have 4 gauges and they rarely read the same--it is the nature of the beast. However, if a large rainfall occurs and Station A reads .96", Station B .94", Station C .97" and Station D .93", I would be very happy and view the differences as insignificant. But when I get results .96" at Station A, .68" at Station B, .54" at Station C, and 1.06" at Station D--those are unacceptable differences requiring calibration! One final word of caution--once you decide to begin the calibration process, make sure both screws get turned in the same direction using the same number of turns or you will never fix the issue and will likely be worse off. In other words, if you turn the first screw clockwise one complete turn--make sure the other screw is turned clockwise one complete turn--use a marker, if necessary, to keep track of your process!

« Last Edit: July 06, 2017, 08:36:14 PM by WheatonRon »
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Offline dalecoy

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Re: Davis bucket calibration
« Reply #2 on: July 06, 2017, 03:47:29 PM »
All precipitation-measurement devices have errors - and that includes your Climet.

The inherent errors are always in the "under-measurement" direction, and mechanical devices (example: tipping buckets) generally have larger errors than capture devices, but they all have errors.

And most of the errors are not constant - they depend on factors like the size of drops, rain rate, wind velocity, turbulence, quantization, temperature, etc. etc.

So, feel free to adjust the screws if it makes you feel better.  Just realize that you aren't making it "better".

Offline Bashy

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Re: Davis bucket calibration
« Reply #3 on: July 06, 2017, 04:02:15 PM »
Thank you for the informative replies, I suppose you could say 2mm is insignificant, the climet always reads the most out of all 3, should I be more inclined to go with the climet and end of month adjust wd rainfall to suit or just leave the amounts alone? Nearing in mind that all three are on the same fence spaced about 2ft apart. Nothing over hanging them and nearest fence is about 10ft away, not ideal granted, thankfully there's no such thing as the "sensor siting police" :)

I spose the last paragraph of your post dalecoy pretty much sums up what I was wanting to hear... I would only either a) break it or b) make it worse....
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Bashy

Offline dalecoy

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Re: Davis bucket calibration
« Reply #4 on: July 06, 2017, 04:26:42 PM »
The inherent errors are always in the "under-measurement" direction, and mechanical devices (example: tipping buckets) generally have larger errors than capture devices (i.e., your Climet).

Or, as you said:  "the climet always reads the most out of all 3,"

It's up to you whether you "end of month adjust wd rainfall to suit".  Of course, that would assume you (when necessary because it's raining at midnight) read your Climet at midnight.  [But then, of course, your Climet would miss some precipitation while you were emptying it]

Personally, I just post what the Davis says - and know that my 4-inch capture gauge will show a bit more when I check it after the storm is over.

Online Mattk

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Re: Davis bucket calibration
« Reply #5 on: July 06, 2017, 04:38:40 PM »
.... after the storm today the climet was about 2mm more than the Davis, I also have another Davis bucket that is modified to work with my OS  station and that too is under reading compared to the climet. 

2mm means nothing by itself, I don't see where you have mentioned the total amount of rain, the % of the total is the thing to go by regardless of what the difference is   

Offline Bashy

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Re: Davis bucket calibration
« Reply #6 on: July 07, 2017, 12:50:36 AM »
15.4mm Climet and 13mm (Davis) it fell in the space of about an hour
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Offline DaleReid

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Re: Davis bucket calibration
« Reply #7 on: July 07, 2017, 12:47:50 PM »
Bashy:  Had the Davis not been full enough to get one more tip in?  I know last night I was fiddling with a couple different ones that I was replacing reed switches in, and when I went up on the roof to swap one out, I pulled the collecting funnel and just the sligtest touch made the tipper tip.  So it was close to being another count on the indicator.  The Davis is 'behind' until it tips, so being a wee bit less than the CliMet makes sense.

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Offline Bashy

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Re: Davis bucket calibration
« Reply #8 on: July 07, 2017, 01:19:42 PM »
Hi when I took off the cone to make sure there was nothing in there insect wise, or sommat needed cleaning  from the tipper, the tipper was bone dry,
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Offline dalecoy

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Re: Davis bucket calibration
« Reply #9 on: July 07, 2017, 01:25:13 PM »
...... the tipper was bone dry,

How long after the rain?  In any case, if it was bone dry, there had obviously been some evaporation of remaining precipitate.

Offline Bashy

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Re: Davis bucket calibration
« Reply #10 on: July 07, 2017, 01:29:18 PM »
I checked it probably about 2 hours later
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Offline Bashy

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Re: Davis bucket calibration
« Reply #11 on: July 19, 2017, 03:15:53 AM »
Hi folks, back to this if i may please, i have just done this

Quote
1. Don't even try and calibrate using single tips - it's almost guaranteed to be inaccurate. Drip eg 544ml water so that it registers 1" or 25.4mm rain as 100 tips (0.01") or ~127 tips (0.2mm) through the complete gauge, ie into the funnel.

2, Water must be dripped through slowly, for example over an hour.

Thats a quote From Johnd

When i did it, it only registered 17.4mm, thats a big drop....Advice please?
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Bashy

Online Mattk

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Re: Davis bucket calibration
« Reply #12 on: July 19, 2017, 03:21:24 AM »
Hi folks, back to this if i may please, i have just done this

Quote
1. Don't even try and calibrate using single tips - it's almost guaranteed to be inaccurate. Drip eg 544ml water so that it registers 1" or 25.4mm rain as 100 tips (0.01") or ~127 tips (0.2mm) through the complete gauge, ie into the funnel.

2, Water must be dripped through slowly, for example over an hour.

Thats a quote From Johnd

When i did it, it only registered 17.4mm, thats a big drop....Advice please?

If you are satisfied all your numbers are correct v the type of tipper you have (Imperial v metric etc) then you need to do the test again and maybe even a third time to confirm making sure the test is over a suitable period of time. Once confirmed then you start adjusting the tipper screws.

BTW if this test is ok then it really doesn't surprise me 20+% is not abnormal out of the box and it is extremely rare to be over, always generally under
« Last Edit: July 19, 2017, 03:24:39 AM by Mattk »

Offline Bashy

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Re: Davis bucket calibration
« Reply #13 on: July 19, 2017, 03:22:57 AM »
Thanks Matt, i will run it twice more over the course of today and see whats what :)
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Offline Bashy

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Re: Davis bucket calibration
« Reply #14 on: July 19, 2017, 09:18:17 AM »
I have done 4 over the course of the day, taking in to account slight miscalculations when filling jug, the scores from all 4 are
17.4
18.0
18.6
17.4

So, not quite the amount we are looking for, any thoughts please?
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Bashy

Online Mattk

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Re: Davis bucket calibration
« Reply #15 on: July 19, 2017, 05:45:06 PM »
Then if you are absolutely happy with the numbers then do an initial adjustment, both screws out maybe 2 full turns (12%) and test again. Don't try and adjust the full % difference in one go, it doesn't work like that.

Online WheatonRon

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Re: Davis bucket calibration
« Reply #16 on: July 19, 2017, 07:57:12 PM »
Then if you are absolutely happy with the numbers then do an initial adjustment, both screws out maybe 2 full turns (12%) and test again. Don't try and adjust the full % difference in one go, it doesn't work like that.

The Davis calibrating instructions, see my prior post (reply 1), state that turning each screw one complete turn will effect a 2% change, not 12%. But I agree, do at most a couple of turns and test--don't complete the change all at once.
« Last Edit: July 19, 2017, 08:00:20 PM by WheatonRon »
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Online Mattk

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Re: Davis bucket calibration
« Reply #17 on: July 19, 2017, 08:57:00 PM »
Then if you are absolutely happy with the numbers then do an initial adjustment, both screws out maybe 2 full turns (12%) and test again. Don't try and adjust the full % difference in one go, it doesn't work like that.

The Davis calibrating instructions, see my prior post (reply 1), state that turning each screw one complete turn will effect a 2% change, not 12%. But I agree, do at most a couple of turns and test--don't complete the change all at once.

You need to read the instructions again as 1 full turn of the adjustment equates to 6%, the adjustment guide these days is embossed shows as +/- 1% (2 or 10 o'clock), 2% (4 or 8 o'clock) & 3% (6 o'clock) which makes 6% for 1 full turn. Being probably over 20%+ low in this instance I suggested an initial 2 complete turns which is 12%

Online WheatonRon

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Re: Davis bucket calibration
« Reply #18 on: July 19, 2017, 10:02:46 PM »
Then if you are absolutely happy with the numbers then do an initial adjustment, both screws out maybe 2 full turns (12%) and test again. Don't try and adjust the full % difference in one go, it doesn't work like that.

The Davis calibrating instructions, see my prior post (reply 1), state that turning each screw one complete turn will effect a 2% change, not 12%. But I agree, do at most a couple of turns and test--don't complete the change all at once.

You need to read the instructions again as 1 full turn of the adjustment equates to 6%, the adjustment guide these days is embossed shows as +/- 1% (2 or 10 o'clock), 2% (4 or 8 o'clock) & 3% (6 o'clock) which makes 6% for 1 full turn. Being probably over 20%+ low in this instance I suggested an initial 2 complete turns which is 12%

I stand corrected. Sorry. I am standing in the corner for 10 minutes in punishment.
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline Bashy

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Re: Davis bucket calibration
« Reply #19 on: July 20, 2017, 12:44:20 AM »
Thanks guys, i appreciate the replies, once i have another coke bottle (binned tother one) i will adjust and test straight away
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Bashy

Offline dalecoy

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Re: Davis bucket calibration
« Reply #20 on: July 20, 2017, 10:13:21 AM »
...... once i have another coke bottle .......

Huh??????????

Offline Bashy

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Re: Davis bucket calibration
« Reply #21 on: July 20, 2017, 10:30:29 AM »
My method for the delivery of water
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Offline dalecoy

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Re: Davis bucket calibration
« Reply #22 on: July 20, 2017, 11:33:43 AM »
My method for the delivery of water

Oh, so how do you regulate the rate of delivery? 

And over what period of time do you deliver the measured quantity of water?

Offline Bashy

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Re: Davis bucket calibration
« Reply #23 on: July 20, 2017, 11:39:33 AM »
tiny slit in the bottom, so tiny that when the lid is tight, no leakage,  well, perhaps 1 drop an hour, see where im going now?
Correct, i use the lid as the regulator, you can set the tightness or looseness (same thing in this method) of the lid, this then
regulates the flow, the tighter the lid the slower the flow :)

PS, bottle is stood upright
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Offline dalecoy

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Re: Davis bucket calibration
« Reply #24 on: July 20, 2017, 11:47:28 AM »
tiny slit in the bottom, so tiny that when the lid is tight, no leakage,  well, perhaps 1 drop an hour, see where im going now?
Correct, i use the lid as the regulator, you can set the tightness or looseness (same thing in this method) of the lid, this then
regulates the flow, the tighter the lid the slower the flow :)

PS, bottle is stood upright

So, the flow rate varies (faster when there is more liquid in the bottle, slower as there is less liquid).  And there is (probably) some liquid left in the bottle at the end of the period (which is how long?)

Is the bottle suspended above the cone?  Or resting in the cone?  And in either case, is the "drip" in the center of the cone or is it off-center?

 

anything