Topic: VP2 tipping spoon raingauge?

[drew1021]

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.

Just to be sure, you are referring to the new single spoon assembly? And the stainless screw with no notch is the one that looks like it is the bottom of a screw sticking up from the plastic?

Yes and Yes

Excellent, thank you for sharing! Mine has a major negative bias, but I was still afraid to mess with that screw.

I'm surprised you have negative bias. I would recommend that you remove the spoon assy from the base, bring it indoors, and put it on a level surface. Then use a syringe and slowly inject 5.4cc of water into the spoon to verify that you indeed have a negative bias. I have two brand new spoon assy's and the both have positive bias out of the box. They both tipped at 5.0cc initially.

[CW2274]

For those unaware or have forgotten and don't want to screw with the tipper itself, software such as WL (can't speak to the others but imagine they do) will allow you to calibrate + or - to a single percent.

[twcmaster]

I'm surprised you have negative bias. I would recommend that you remove the spoon assy from the base, bring it indoors, and put it on a level surface. Then use a syringe and slowly inject 5.4cc of water into the spoon to verify that you indeed have a negative bias. I have two brand new spoon assy's and the both have positive bias out of the box. They both tipped at 5.0cc initially.

Yeah, it is a big bias too. I have been noticing up to a tenth of an inch shortage for every half inch that falls in my NWS 8" gauge and stratus. I belive I was one of the first to receive my tipper, so I am not sure if that has anything to do with it or not. I have a syringe from calibrating my Rainwise, so I will definitely give that a try. Thanks again!

[galfert]

Can someone explain how 5.4cc = 1 tip? Because it just doesn't add up for me.

[CW2274]

Can someone explain how 5.4cc = 1 tip? Because it just doesn't add up for me.

Assuming it's the same capacity as the old design, 544ml equals 1" of rain from a 6 1/2" diameter bucket.

[galfert]

Can someone explain how 5.4cc = 1 tip? Because it just doesn't add up for me.

Assuming it's the same capacity as the old design, 544ml equals 1" of rain from a 6 1/2" diameter bucket.

Ah the diameter of the bucket, yes. 
Not having a Davis myself I made the mistake of thinking it was 4 inch bucket in my calculation.

[CW2274]

Just as an FYI, the 8" RW bucket is 800ml.

[lightmaster]

If a perfect calibration is 5.44ml per tip, and these are all seemingly calibrated to 5.00ml 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. Seems that as I read all these comments about comparing the new tipper to the higher quality and larger rain specific gauges, most people that had both units in essential the same spot had essentially identical readings between the different units. If all those units are calibrated to 0.500ml per tip (I don't know that they are, just assuming) doesn't that mean they are reading as accurately as they can?

[CW2274]

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.

[galfert]

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.

Just like all of you are sharing your comparison to the Stratus I think it would be informative to see if there is a specific dialed in calibration by Davis or if they are sloppy and there are various results.

Anyone have a good Amazon link to a syringe to recommend?

[CW2274]

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.

[lightmaster]

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.

[WheatonRon]

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.

[galfert]

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

[drew1021]

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

[mcrossley]

For those of us in the rest of the world, a tip is 4.22 cm³ 

[CW2274]

For those of us in the rest of the world, a tip is 4.22 cm³ 

Touché.

[johnd]

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 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.

[mcrossley]

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

[Jester]

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.

[drew1021]

You're welcome!

[DRoberts]

Moderate rain, wind less than 10 mph, about 15 minutes

VP2 spoon  .14"
CoCoRaHS  .13"

(Corrected)

[miraculon]

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.

[drew1021]

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?

[graculus]

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 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