Topic: VP2 Rain Gauge Disparity

[wmcatty]

I have read several of the past posts regarding the innaccuracy of the VP2 tipping gauge compared to a stand alone rain gauge like a CoCoRaHS.  My Davis system has usually under-reported rain by 4-10/100's per event since its installation last summer.  Over the past 4 days, my Davis reported a total of 1.27 inches while my CoCoRaHS indicated 1.74 inches.  A difference of .47 inches.  The rain was slow, steady, and without much wind.  My Davis is mounted at 8' and is located approx. 10 feet from my CoCoRaHS gauge. Would you all recommend a set screw adjustment on the Davis, and if so, what would be a safe setting on the set screws to close the disparity between the two readings?   

[George Richardson]

2 suggestions.

For proper installation of 4" manual gauge to compare to VP2, see http://tobaccovilleweather.com/about.htm , third picture down.

When adjusting the screws under the tipping buckets, only apply 1/2 the correction calculated. I way overcorrected and it took me nearly 2 years to finally get things within specs.

George

[miraculon]

I went through this last Fall with the 4" CoCoRaHS style gauge (actually an old Weather Channel one) and the Rain Collector II. My discrepancy wasn't nearly this large, just a few 1/100ths. I bought a cheap oral syringe at Wally World (10mL) and put in the requisite 5.4mL per tip. I made excruciatingly minute adjustments, but at the end of the day I decided that since it was tipping on the correct volume I gave up.

I put this into the CWOP (wxqc) discussion board and it generated a ton of controversy about even trying to make a comparison.

I would check to see if you get 5.4mL per tip before making any adjustments.

You can see the spirited discussions at http://server.gladstonefamily.net/pipermail/wxqc/2011-September/014414.html and http://server.gladstonefamily.net/pipermail/wxqc/2011-September/014376.html

Greg

[dalecoy]

Can we presume that you have checked your Davis rain gauge inside and out for cleanliness, spider webs, etc.?

[wmcatty]

Dalecoy, I check the rain gauge every couple of weeks and wax the collection funnel as well.  A week ago, all was fine but I have not checked it since the most recent rain event.  I will check it out again this week and try the 5.4ml of water test that Greg suggested.  Thanks for the suggestions.

[Beaudog]

Before I tweaked my rain gauge i would switch to Cumulus or WUHU which both have tweaks where you can tweak the rain gauge readings via the program to account for errors from the gauge.

Your signature says VWS   I don't use it but I think it also has tweaks.

Some trivia about your signature.   You have spaces in your CWOP number and your WU ID which should not be there which kinda makes it hard to just cut and paste to see your sites.

Your WU ID is currently showing no data.

[SLOweather]

I would check to see if you get 5.4mL per tip before making any adjustments.

Probably better to try for 10 tips with 54.4 ml of water. My recent experiments noted elsewhere here indicate that the tipper is asymmetrical, (probably on purpose), requiring differing volumes of water on each side.

[Stevea]

From the time I got my VP2 to when I installed DFARS (about 4 months) it typically read about 5 to 10% more than the 4” TFA rain gauge I have mounted next to it (on other side of 4x4 post in the garden).   When I tested the TFA and the VP2 with a slow drip of a known volume they both give the right answer so I am assuming wind conditions on the day were the main reason for the differences between them. 
After adding DFARS the VP2 started reading up to 25% less than the TFA.
It took several weeks to get the VP2 back in line.  I am guessing the bolts for the DFARS distorted the base slightly when I tightened them.  I would concur with the comment above on the need to make small adjustments as mine got worse before it got better and this was mainly due to my making too large changes when I first tried to correct the under reading.
Overall, while they still rarely agree on a specific rain event, they are now averaging within less than a 4% difference over a 10 event sample size.


Steve

[wmcatty]

Chris, what would be a good rate-of-pour for testing the unit?  And thanks to all the other commentators for your input.

[dalecoy]

Of course, Segal's Law applies.  (It even applies to lawyers).   

[SLOweather]

Chris, what would be a good rate-of-pour for testing the unit?  And thanks to all the other commentators for your input.

1) As slow as you can.... My NovaLynx calibrator with the smallest orifice is about 3"/hour, or, better...

2) Test/calibrate at the rate at which you want the most accuracy. For storms around here, I'd shoot for 1"/hour. That would be about 1.67 tips per minute, one tip every 36 seconds, or, for 10 tips/54.4 ml,  10 tips in 6 minutes.  

[wmcatty]

Thanks Chris and Dale.  I will let you all know this weekend how it tests out.

[wmcatty]

Just found out I have to work this weekend and my child bride does not understand what I need her to do for the calibration...my fault for marrying a woman a couple of years younger than I am.  Anyway, I will let you all know next week how the calibration turns out.

And keep the lawyer jokes coming Dalecoy...it makes my day getting ball bashed by anyone from Lee's Summit, MO.!!! Have a good weekend and stay out of trouble!  Wayne

[dalecoy]

In case it wasn't clear, I have much respect for lawyers.  In my opinion, it's a very honorable profession.

And my lawyer friends have much better lawyer jokes than I have.   

[wmcatty]

It was always clear to me Dalecoy...just tugging at those Missouri Mule halter straps a bit.

[wmcatty]

I really do not know what to think about my Davis system.  Just when I make a disparaging comment about the rain gauge, we happened to have one hell of a weather event here in Texas this week.  Lots of rain, lightning, thunder, flash flooding and of course...tornado's.  Throughout the two day event, my Stratus 4" collected 4.16 inches of rain and my Davis VP2 collected 4.15 inches!  I do not know what to think now!  I started a written log which I intend to keep updated and will just monitor the two for inconsistencies and try to differentiate between a slow rain, medium rain and a downpour like we had this week. 

[d_l]

I would think that the biggest difference between the two would be during the windiest storms and that the Davis would have the higher accumulations of the two if they are mounted close together at the same height.  My reasoning is that some of the wind hitting the sides of the collectors is diverted upwards and causes upward lifting eddies above the collecting cones.   These eddies carry some rain drops that otherwise would have been collected in the cones over to the far side of the cone where they miss.  The greater diameter cone has less chance of the eddied drops clearing the entire cone area.

[wmcatty]

My 4" is mounted on a pole with the collection funnel at 36" and the Davis is at 8'.  We did have some hellacious winds (45-70 mph) with this past storm, as I had several shingles blown off the roof that I found in one of my fields this afternoon approx. 100 yards from the house.  Maybe my Davis unit compensated for the wind eddies, who knows.

[wmcatty]

Dalecoy, I also checked out my ISS today and it was as clean as could be.  There was absolutely nothing within the rain gauge nor near the reed switch, other than a few minute flakes of dust (which I blew out).  The cone was clear and there was no obstructions within the wire screen I put inside the cone itself.  

[chief-david]

My 4" is mounted on a pole with the collection funnel at 36" and the Davis is at 8'.  We did have some hellacious winds (45-70 mph) with this past storm, as I had several shingles blown off the roof that I found in one of my fields this afternoon approx. 100 yards from the house.  Maybe my Davis unit compensated for the wind eddies, who knows.

Caclulate the wind run on a shingle

[dalecoy]

Dalecoy, I also checked out my ISS today and it was as clean as could be.  There was absolutely nothing within the rain gauge nor near the reed switch, other than a few minute flakes of dust (which I blew out).  The cone was clear and there was not obstructions within the wire screen I put inside the cone itself. 

It was worth verifying, at least.  And by the way, I also use a wire screen - but I know that anything in the cone can cause differences in water flow (and the differences vary with wind and rain rate).  And water flow variations actually cause variations in tipping-bucket measurements.

You're getting some interesting results.  Note that it's possible to calibrate the two gauges to agree, given no-wind conditions and the same flow rate.  The CoCoRahs (and Stratus) isn't much affected by flow rate, but wind has different effects because of the differing geometry.  You won't get the same measurement from 4" and 8" gauges mounted next to each other, if there's a breeze.  And of course, wind (and the effects thereof) varies with height above terrain.  

It's not unusual to see variations of a few percent in precipitation measurements taken by 4" gauges from different manufacturers mounted adjacent to each other - nor to have that variation change depending upon the exact storm conditions.

I'll follow your adventures with great interest.  

[wmcatty]

I hope this is an adventure that will proffer some identifiable results.  BTW Chief-David, what do you mean by Caclulate the wind run on a shingle?  You lost me on this particular recommendation.  Wayne

[tomcj2]

My CoCoRaHS gauge and VP were fairly close until recently, and then my VP2 started to give me higher readings.  I immediately thought that it was dirty, or out of calibration.  That was not the problem.
I had initially installed my CoCoRaHS gauge on a short piece of 2x4, and attached it to a metal T-Bar fence post.  This was intended as a temporary installation until I was certain that I had it placed in a good location. So much for temporary,3 years later it remained unchanged.  When the VP2 seemed accurate after I dribbled a test volume of water through it I looked at the CoCoRaHS more carefully, and found that the post had tipped over a little and the gauge was no longer perfectly level.  After re- installing the gauge on new wooden post set in concrete the 2 gauges are once again in sync.

[ocala]

That's great you have them both synced.
Just wondering if they are even during different rain events. Light, moderate and heavy.
Also find it odd the Cocorahs gauge was off from not being completely level. I could see the VP2 for sure but not the plastic gauge.

[tomcj2]

We seldom get heavy downpours here, but when we do the VP2 gives a slightly higher reading; my theory is more rain is bouncing out of the CoCoRaHS gauge.

Also find it odd the Cocorahs gauge was off from not being completely level.

When you look at a circle, and rotate it , you will initially see an ellipse, and eventually a straight line.  The surface area decreases to zero as the circle is rotated through 90°