Topic: Inaccurate Rain Readings From VP2

[Bohaiboy]

I have my VP2 rain catcher mounted at approx 8 ft above ground with nothing around it and about 50 feet from the house.  I had a feeling my readings are on the low side, so I bought a basic site glass rain gauge that is about 15 ft away.  Last night we had a hard rain event, and my VP 2 read 0.84" while my site glass had approx 1.75".  A neighbor about 1/4 mile away had had 2.4".  I know it can vary but mine and his are consistently diff with his being higher.  Not sure what type of weather station he has but it does use the see-saw type of measuring mechanism.  What could be wrong with mine and how would I go about calibrating it?  TIA.

[Mattk]

0.84" and 1.75" is certainly not a physical gauge calibration issue.

This would appear to be an issue between a metric or imperial tipping bucket and incorrect application of rain calibration for the appropriate bucket 0.01" or 0.2mm (a 0.1" tipper is metric equiv to 0.254mm)     

[Bohaiboy]

Its the tipping bucket Davis supplied with the unit and it is set as 0.1" and measurements are set to inches.   Sorry 0.01"

[doubleohwhatever]

Have you taken your bucket off to see if your tipping mechanism is dirty? Also check for pests.

[NK7Z]

I just had an almost exact copy of your problem.  There was "stuff" in the collector, and a spider had built a web across the tipping bucket...  Cleaned both, and all is well again...

[WheatonRon]

Is your tipper the see-saw version or the spoon type? The former can be calibrated, the latter no. If you are within warranty, return it after checking for debris, etc.

[Bohaiboy]

It resembles two gravy boats on a see-saw.   Will try to get a pic.

[WheatonRon]

It resembles two gravy boats on a see-saw.   Will try to get a pic.

No need for a picture. What you described is the old see-saw version which you can calibrate. How old is your system? Still under warranty? However, to be honest, the large error you described likely isn’t a calibration issue. Is the mechanism level?

[Mattk]

Yes tipping buckets can be calibrated and typically they read under but have never seen one this amount under, there wouldn't be enough length in the screws to account for a 50% adjustment.

There is something else amiss with this one.   

[Bohaiboy]

I will try to climb up there this week and check level.  Of course the measurements could be correct and the others wrong.  I'll report back once I make the ascent.

[ldelatorre]

Is your tipper the see-saw version or the spoon type? The former can be calibrated, the latter no. If you are within warranty, return it after checking for debris, etc.

I have the new tipping spoon type and there is a calibration screw over the magnet.  The closer the screw is to the magnet, the more water it will take to tip the spoon.

[WheatonRon]

Is your tipper the see-saw version or the spoon type? The former can be calibrated, the latter no. If you are within warranty, return it after checking for debris, etc.

I have the new tipping spoon type and there is a calibration screw over the magnet.  The closer the screw is to the magnet, the more water it will take to tip the spoon.

Davis support told me you can’t calibrate the spoon tipper.

[Mattk]

Is your tipper the see-saw version or the spoon type? The former can be calibrated, the latter no. If you are within warranty, return it after checking for debris, etc.

I have the new tipping spoon type and there is a calibration screw over the magnet.  The closer the screw is to the magnet, the more water it will take to tip the spoon.

Having to dismantle the complete spoon assembly to access that screw makes the whole concept of field calibration impractical 

[johnd]

Davis support told me you can’t calibrate the spoon tipper.

The official position is that it's strongly recommended not to try and calibrate the tipping spoon, not that it's impossible to do so. The problem is that unlike the older buckets, the new adjustment is very non-linear, which has two consequences. One is that any adjustment needs a lot of trial and error and tedious checking of rainfall amounts over time. And the second that there's no scale against which you can make incremental adjustments or easily return the unit to its factory setting (which was probably good anyway).

[Bohaiboy]

Hi all, I was finally able to check and have determined I am not level soI have to adjust that.  Will report in a few weeks when I have time to get to it.

[davidmc36]

I just recently put up a new Tipper. I have a few of them and overhaul for ready replacement. Never noticed when first put up the screws were spun way up.

I have been monitoring over that last several weeks with different types of rain events.

The screws are pretty close now. I monitor over 5 to 10 rain events to see if it stays plus or minus. I find there are variances depending on type of event.

Probably no single event should be taken as the only calibration point, rather, an average performance over several storms.

While true some single days may show skewed data, the history will be true over time.

[davidmc36]

A few light rain events lately gave me data points.

The next 24 hrs or so will give me some great data also.

https://weather.gc.ca/warnings/report_e.html?on6

 

[davidmc36]

After tracking for quite some time and adjusting the screws, I have an observation that I believe is echoed in some reports from others.

It seems the large size of the Davis rain cone can cause the first bit of a rain event to wet the inside and not really register until it is soaked enough to drip into the see-saw. It seems a black cone in the sun would lose a lot of short light rain to evaporation too.

I have seen often where the ISS units are all identical and 0.01 behind the Stratus collector. And it stayed that way all through a rain event that dropped 0.85" in 24 hrs. Stations were one tick behind on average.

I see other rain collectors have smaller and/or shallower cones.

Can I make the inside more slippery?

[BigOkie]

After tracking for quite some time and adjusting the screws, I have an observation that I believe is echoed in some reports from others.

It seems the large size of the Davis rain cone can cause the first bit of a rain event to wet the inside and not really register until it is soaked enough to drip into the see-saw. It seems a black cone in the sun would lose a lot of short light rain to evaporation too.

I have seen often where the ISS units are all identical and 0.01 behind the Stratus collector. And it stayed that way all through a rain event that dropped 0.85" in 24 hrs. Stations were one tick behind on average.

I see other rain collectors have smaller and/or shallower cones.

Can I make the inside more slippery?

I've put Rain-x on the inside of mine but can't attest to how much more accurate it is.  I just don't give too much thought to accuracy, rain amounts can vary based on location.  Hell, even calibrated gauges 20 feet away in the same yard can have variance.  If it's within a tolerable variance, I don't worry much.  Living in Oklahoma, if I worried about the difference in rain amounts between my station and the official reporting station at Tulsa Int'l Airport, I'd have more grey hair than I currently do.

[davidmc36]

I may try something like Rain X.

All my final checks are done with manual and tipper two feet apart. The tippers run within 5% or less so I am pretty happy overall. But it takes several different events to get there.

[johnd]

All my final checks are done with manual and tipper two feet apart. The tippers run within 5% or less so I am pretty happy overall. But it takes several different events to get there.

+1 . The VP2 gauge and CoCoRaHS  do need to be very close together and with the funnel openings at the same height if at all possible (and with no nearby trees, bushes, buildings etc potentially casting more of a rain shadow over one than the other). And yes you do need several rain events to get any reliable comparison and with very light or very intense events often not providing the best data. Steady rainfall events are best but Nature doesn't necessarily oblige of course. Something like monthly rainfall amounts can give a decent comparison, provided your location gets say 2-3" or more in that time (of rain not snow or hail).