Author Topic: VP2 tipping spoon raingauge?  (Read 14544 times)

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Offline lightmaster

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Re: VP2 tipping spoon raingauge?
« Reply #325 on: June 03, 2019, 08:50:59 PM »
If a perfect calibration is .544ml per tip, and these are all seemingly calibrated to .500ml per tip, then these will top more often than they should and have a positive bias, right? Could Davis have done this intentionally? Maybe in their tests they saw that due to stations having to be placed higher than recommended for rain alone, and missed raindrops due to wind and having a smaller funnel than the more accurate 8" gauges, they could have decided to create the positive bias to account for those issues.
No idea where you got 500ml from, regardless, to do this would be technical suicide. There are just waaaaay to many variables to assume a particular bias. I'm not saying they're all golden from the factory, but no way are they intentionally dialing in a bias.

There a decimal point in front of 500, as in 0.500ml. Bad habit of mine to leave off the 0 in front of a decimal. Even so, I royally screwed up those numbers and will correct them, mean 5.4ml and 5.0 ml.

I don't have a decent syringe at the house, just a baby medicine syringe. I might still have some insulin syringes around the house that should be more accurate.

Offline WheatonRon

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Re: VP2 tipping spoon raingauge?
« Reply #326 on: June 03, 2019, 09:11:47 PM »
Well seems like a simple enough test. Those interested in sharing results should get a small syringe and see at what amount their gauge is set to tip.
Insanely small sample size. IMHO, if it's to be done right, send an inch through it, and not just once.

I agree with CW2274. Relying on a small sample when calibrating a rain gauge is not the best answer—the more water you can put through the gauge before calibrating the better—AND be sure your tipper is clean. I was surprised this weekend when opening my VP2 that has the new spoon tipper how much dirt was in the tipper-something that would contribute to positive bias. Granted, my Chicago suburb location received over 10 inches of rain in May, and that probably contributed to the increase in dirt in the tipper, although it was not in service the entire month of May.
« Last Edit: June 03, 2019, 09:19:49 PM by WheatonRon »
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Online galfert

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Re: VP2 tipping spoon raingauge?
« Reply #327 on: June 03, 2019, 10:02:55 PM »
Yep okay forget the syringe. Use a graduated cylinder and use a larger amount of water.

There are plenty of old threads like this one that have great information on how it has been done before.
https://www.wxforum.net/index.php?topic=11373.0
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Offline drew1021

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Re: VP2 tipping spoon raingauge?
« Reply #328 on: June 03, 2019, 10:36:32 PM »
With the new design of the tipping spoon you should have excellent repeatability. With that in mind using the syringe to fine tune the calibration of the tipping spoon should be sufficient.
Time will tell though, Lots of days ahead with decent shower chances [tup]
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Offline mcrossley

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Re: VP2 tipping spoon raingauge?
« Reply #329 on: June 04, 2019, 04:20:14 AM »
For those of us in the rest of the world, a tip is 4.22 cm3  ;)
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Offline CW2274

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Re: VP2 tipping spoon raingauge?
« Reply #330 on: June 04, 2019, 04:49:00 AM »
For those of us in the rest of the world, a tip is 4.22 cm3  ;)
Touché.

Offline johnd

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Re: VP2 tipping spoon raingauge?
« Reply #331 on: June 04, 2019, 05:23:02 AM »
For those of us in the rest of the world, a tip is 4.22 cm3  ;)

Personally, I'd have said 4.28ml (for a 0.2mm spoon @ (0.200/0.254)*5.44). This is not to nitpick over decimals but more to suggest one shouldn't be too optimistic about the degree of accuracy that's achievable with this sort of calibration. For instance, I'm not completely convinced about the accuracy of the 544ml baseline figure - it does depend on exactly how you measure the funnel rim diameter, what happens when raindrops hit the rim etc.

And I bet a cheap syringe is not accurate to better than 1-2%, if that. If I was going to use the syringe approach then I'd probably weigh the syringe  on a sensitive 100g digital balance (which are available quite cheaply, and even with calibration weights, and sold for jewellery and other 8-) uses) before and after achieving a (pre-wetted) tip. I suspect that's going to give a more accurate weight/volume.

Overall, I think nothing beats field comparison over time against a good reference gauge like Stratus/CoCoRaHS or better.
« Last Edit: June 04, 2019, 05:26:21 AM by johnd »
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Offline mcrossley

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Re: VP2 tipping spoon raingauge?
« Reply #332 on: June 04, 2019, 05:42:19 AM »
For those of us in the rest of the world, a tip is 4.22 cm3  ;)

Personally, I'd have said 4.28ml (for a 0.2mm spoon @ (0.200/0.254)*5.44). This is not to nitpick over decimals but more to suggest one shouldn't be too optimistic about the degree of accuracy that's achievable with this sort of calibration. For instance, I'm not completely convinced about the accuracy of the 544ml baseline figure - it does depend on exactly how you measure the funnel rim diameter, what happens when raindrops hit the rim etc.

Agreed, I base my 4.22 cm3 on measuring my collector bucket using the inside edge of the rim. If you add an extra 0.5 mm to the radius  (1 mm diameter) you get 4.28 cm3, so fine margins in an imprecise world of rain drops ;)
Mark

Offline Jester

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Re: VP2 tipping spoon raingauge?
« Reply #333 on: June 04, 2019, 10:08:47 AM »
Okay, results after calibrating spoon assy are as follows:
Moderate to heavy showers came through last night with light winds,

Stratus.        1.02
VP2.      1.02


Ok Drew, how did you calibrate?

Looking underneath the spoon, locate the stainless steel screw. You'll notice that there isn't a notch to fit a flat blade screwdriver, and it will have locktite on it. This is where it gets a little tricky as you will have to get needle nose pliers and grasp the threaded portion of the screw firmly and turn clockwise to increase the amount of tips and counter clockwise to decrease. So in my case I was experiencing a positive bias so I turned it counter clockwise slightly. I tested after calibrating using a 10cc medical grade syringe to tip at around 5.4cc. If someone knows of a tool to use to turn the screw let me know as I dont like using needle nose pliers to do the job. As far as I can see it's the easiest way without disassembling the entire spoon assy. Hope this helps.


Awesome! Thanks Drew.
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Offline drew1021

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Re: VP2 tipping spoon raingauge?
« Reply #334 on: June 04, 2019, 03:04:50 PM »
You're welcome!
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Offline DRoberts

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Re: VP2 tipping spoon raingauge?
« Reply #335 on: June 06, 2019, 09:07:52 PM »
Moderate rain, wind less than 10 mph, about 15 minutes

VP2 spoon  .14"
CoCoRaHS  .13"

(Corrected)

« Last Edit: June 06, 2019, 09:54:51 PM by DRoberts »

Offline miraculon

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Re: VP2 tipping spoon raingauge?
« Reply #336 on: June 14, 2019, 08:24:52 AM »
Some more results from the past several days:
(VP2 has tipping spoon)

6/10 VP2: 1.74"   NovaLynx: 1.72"   CoCoRaHS: 1.61"
6/11 VP2: 0.12"   NovaLynx: 0.12"   CoCoRaHS: 0.09"
6/12 VP2: T        NovaLynx: 0        CoCoRaHS: 0
6/13 VP2: 0.59"   NovaLynx: 0.60"   CoCoRaHS: 0.56"
6/14 VP2: 0.44"   NovaLynx" 0.42"   CoCoRaHS: 0.40"

Given the proximity of the gauges to the garage and house these seem to be good results. My site doesn't comply with the standard rain gauge siting guidelines because my lot is too small.

Greg H.





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Offline drew1021

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Re: VP2 tipping spoon raingauge?
« Reply #337 on: June 14, 2019, 09:46:13 AM »
Some more results from the past several days:
(VP2 has tipping spoon)

6/10 VP2: 1.74"   NovaLynx: 1.72"   CoCoRaHS: 1.61"
6/11 VP2: 0.12"   NovaLynx: 0.12"   CoCoRaHS: 0.09"
6/12 VP2: T        NovaLynx: 0        CoCoRaHS: 0
6/13 VP2: 0.59"   NovaLynx: 0.60"   CoCoRaHS: 0.56"
6/14 VP2: 0.44"   NovaLynx" 0.42"   CoCoRaHS: 0.40"

Given the proximity of the gauges to the garage and house these seem to be good results. My site doesn't comply with the standard rain gauge siting guidelines because my lot is too small.

Greg H.

It's interesting that the CoCoRaHS is reading lower than both the VP2 and Novalynx. Are the gauges close to each other?
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Offline graculus

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Re: VP2 tipping spoon raingauge?
« Reply #338 on: June 14, 2019, 10:17:11 AM »
Personally, I'd have said 4.28ml (for a 0.2mm spoon @ (0.200/0.254)*5.44). This is not to nitpick over decimals but more to suggest one shouldn't be too optimistic about the degree of accuracy that's achievable with this sort of calibration. For instance, I'm not completely convinced about the accuracy of the 544ml baseline figure - it does depend on exactly how you measure the funnel rim diameter, what happens when raindrops hit the rim etc.

And I bet a cheap syringe is not accurate to better than 1-2%, if that. If I was going to use the syringe approach then I'd probably weigh the syringe  on a sensitive 100g digital balance (which are available quite cheaply, and even with calibration weights, and sold for jewellery and other 8-) uses) before and after achieving a (pre-wetted) tip. I suspect that's going to give a more accurate weight/volume.

Overall, I think nothing beats field comparison over time against a good reference gauge like Stratus/CoCoRaHS or better.

You're correct, even med syringes are not super accurate. https://chemistry.stackexchange.com/questions/41119/what-is-the-accuracy-of-medical-syringes

Offline miraculon

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Re: VP2 tipping spoon raingauge?
« Reply #339 on: June 14, 2019, 10:25:06 AM »
The NovaLynx and CoCoRaHS are only about 3 ft apart. Just enough to get the mower in between them.



The ISS is on the other side of the yard, maybe 20' away.

I have seen good agreement on all three when winds are calm and the rain was "straight down". The wind currents around the house and garage wreak havoc with snow accumulation in the winter.

I used the NovaLynx calibrator for the NovaLynx gauge. I checked the VP2 back when it had the original rain cone and tipping-buckets, but I have not repeated it since. I made multiple back-to-back runs on the NovaLynx gauge when I calibrated it to ensure that it was repeatable. There was some discussion in the CoCoRaHS thread about under catch in that gauge a while back.

Because they have agreed from time-to-time, I attribute the variation to wind currents and my substandard siting.

Greg H.



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Offline drew1021

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Re: VP2 tipping spoon raingauge?
« Reply #340 on: June 14, 2019, 10:39:41 AM »
Definitely check the accuracy of the VP2 when you get a chance and post the results. I can see how the gauges proximity to the house would wreck havoc on totals during windy rain or snow events.
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Offline DRoberts

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Re: VP2 tipping spoon raingauge?
« Reply #341 on: June 14, 2019, 11:10:11 PM »
Thunderstorm with little wind early Friday morning. Results:

VP2 (tipping spoon)  .13
CoCoRaHS Stratus    .12


Offline Mattk

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Re: VP2 tipping spoon raingauge?
« Reply #342 on: June 14, 2019, 11:34:26 PM »
Personally, I'd have said 4.28ml (for a 0.2mm spoon @ (0.200/0.254)*5.44). This is not to nitpick over decimals but more to suggest one shouldn't be too optimistic about the degree of accuracy that's achievable with this sort of calibration. For instance, I'm not completely convinced about the accuracy of the 544ml baseline figure - it does depend on exactly how you measure the funnel rim diameter, what happens when raindrops hit the rim etc.

And I bet a cheap syringe is not accurate to better than 1-2%, if that. If I was going to use the syringe approach then I'd probably weigh the syringe  on a sensitive 100g digital balance (which are available quite cheaply, and even with calibration weights, and sold for jewellery and other 8-) uses) before and after achieving a (pre-wetted) tip. I suspect that's going to give a more accurate weight/volume.

Overall, I think nothing beats field comparison over time against a good reference gauge like Stratus/CoCoRaHS or better.

You're correct, even med syringes are not super accurate. https://chemistry.stackexchange.com/questions/41119/what-is-the-accuracy-of-medical-syringes

Typically a 1mm syringe is only used to do/check a ball park adjustment, not a calibration as such and this only takes a few minutes initially as opposed to a full type calibration where you make take an hour or so using 425mm (which is the amount I use), top off the last full tip with a syringe then calculate the % error, adjust accordingly and re-run the whole process again, sometimes at a faster rate, sometimes a slower rate.     

Offline drew1021

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Re: VP2 tipping spoon raingauge?
« Reply #343 on: June 18, 2019, 11:54:03 AM »
We had a real frog strangler here last night with the following results

Stratus.       2.88
Vp2.          2.97

Assuming the new spoon assy's accuracy is the same as the old tipper then this result is within spec at 3%
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Offline DRoberts

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Re: VP2 tipping spoon raingauge?
« Reply #344 on: June 18, 2019, 07:40:39 PM »

 One of our more decent soaking rains here.

 VP2 (spoon)  early 6:45 a.m. .14"     6:35 p.m.  1.00"   (total 1.14")
 CoCoRaHS (stratus)   "          .15"            "          .93"   (total 1.08")

Wind: Average less than 10 mph entire period

Gauges are located about 30 feet apart.

I cannot think why the disparity in the totals developed during course of 12 hours.

Ideas anyone?




Offline Old Tele man

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Re: VP2 tipping spoon raingauge?
« Reply #345 on: June 18, 2019, 07:49:35 PM »

 One of our more decent soaking rains here.

 VP2 (spoon)  early 6:45 a.m. .14"     6:35 p.m.  1.00"   (total 1.14")
 CoCoRaHS (stratus)   "          .15"            "          .93"   (total 1.08")

Wind: Average less than 10 mph entire period

Gauges are located about 30 feet apart.

I cannot think why the disparity in the totals developed during course of 12 hours.

Ideas anyone?

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Offline WheatonRon

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Re: VP2 tipping spoon raingauge?
« Reply #346 on: June 18, 2019, 08:06:02 PM »

 One of our more decent soaking rains here.

 VP2 (spoon)  early 6:45 a.m. .14"     6:35 p.m.  1.00"   (total 1.14")
 CoCoRaHS (stratus)   "          .15"            "          .93"   (total 1.08")

Wind: Average less than 10 mph entire period

Gauges are located about 30 feet apart.

I cannot think why the disparity in the totals developed during course of 12 hours.

Ideas anyone?

Not too surprised. I have concluded the spoon tipper has a mild wet bias, which in my view, is a positive. After several readings over the weeks testing the old tipper with the new tipper both only inches apart from my CoCo gauge is the basis for my conclusion. I think 30 feet apart could cause the abnormalities you had but all in all not bad. Tear down the goal posts and wait for the VP3! Your life expectancy is 110 years old, right?
« Last Edit: June 18, 2019, 08:26:09 PM by WheatonRon »
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Offline CW2274

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Re: VP2 tipping spoon raingauge?
« Reply #347 on: June 18, 2019, 08:07:24 PM »
Without a doubt, I think rainfall is the most difficult to truly measure accurately with present technology (comparatively speaking with other paramaters), just too many variables. Very high and low humidities are next in line.

Offline lightmaster

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Re: VP2 tipping spoon raingauge?
« Reply #348 on: June 18, 2019, 09:19:21 PM »

 One of our more decent soaking rains here.

 VP2 (spoon)  early 6:45 a.m. .14"     6:35 p.m.  1.00"   (total 1.14")
 CoCoRaHS (stratus)   "          .15"            "          .93"   (total 1.08")

Wind: Average less than 10 mph entire period

Gauges are located about 30 feet apart.

I cannot think why the disparity in the totals developed during course of 12 hours.

Ideas anyone?

I have seen it where it was an outright downpour in the field on the other side of my road, and no rain at all in my yard, and I could easily place a rain gauge in the field getting 1+ inches of rain, and a rain gauge in my yard getting no rain at all, but they'd still be less than 30 feet apart. If the distance is more than a few feet, there's no guarantee the 2 gauges will actually get the same amount of rain, much less record the same.

Offline lightmaster

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Re: VP2 tipping spoon raingauge?
« Reply #349 on: June 18, 2019, 09:28:42 PM »
Completely contrary to what I just said, my VP2 and 4" Stratus are about 150ft apart (cuz I'm not climbing a barbwire fence every day to go empty the Stratus, lol). Just had a very quick downpour (4.07in/hr for like 5 mins) and here are the comparisons:

VP2:       0.25"
Stratus: 0.22"

 

anything