Last winter I bought a 4" manual gauge and mounted it on the other side of the same pole my VP2 ISS is mounted. Mainly I bought it for snow measurements. This spring I decided to compare results to check the calibration of my VP2 rain gauge. Today we had the first good rain since I decided to monitor it. Previously we had some small rains (under .10"), and I had noticed it was under reading, but there wasn’t enough rain to make a proper comparison. Today we had a storm that came in two phases, with a break between them. The rain was fairly steady, with the highest rainfall rate well under 1" per hour (so the under read wasn’t because of a very heavy rain which tipping buckets are know to not measure accurately).
The first batch of rain was:
VP2=.63" 4"gauge=.86"
second batch was:
VP2=.11" 4"gauge=.16"
total for storm:
VP2=.74" 4"gauge=1.02"
This is a surprisingly big under read. Tomorrow I will check it for level, and remove the bucket to insure no obstructions, webs, etc. But I don’t expect to find anything as I know the funnel orifice is clear, and I had the bucket off within the past week. So barring any obvious cause, I’d like to get some opinions if you all think I should just replace the tipping mechanism or calibrate using the calibration screws or maybe some other suggestion.
Don’t touch the calibration screws until you’ve exhausted other possible explanations - it’s difficult to get them exactly back to the factory settings, which may well be OK.
Is the funnel-top of your 4" gauge at the same height as the funnel-top of your VP2 gauge? (I couldn’t be exactly sure from your description and height above ground level can have a very significant effect on recorded rainfall.)
An obvious point and one which you very probably haven’t changed but well worth a double-check: Presumably your VP2 gauge is set to measure in 0.01" increments? In the console set-up is the increment definitely still at its default setting of 0.01", ie it hasn’t been inadvertantly changed to 0.2mm?
There are some instructions for checking the calibration of the VP2 gauge by dripping through a known volume of water on my website at http://www.weatherstations.co.uk/vp_faqt2.stm (towards the bottom of the page under ‘Calibration by simulating rainfall’)
when you say you purchased a 4 inch guage… what kind are you talking about. is it 4 inches long, 4 across. perhaps you could post an url to show a picture refererence. I am curious .
If the guage is one you bought at the hardware store, they are not calibrated like the vp2. I purchased this one from ambient http://www.ambientweather.com/stprraga.html
and my vp unit hit right on the mark in measurement. I tested mine 6 years after service after a conversation on another board. I wanted to see for my self the accuracy.
Finally just one other thing… real simple. when you tip the pan one time does it read on the console 0.01 ?
I’d be careful on the calibration. Did you set the calibration within weatherlink to your console? you can calibrate it there and it is a heck of a lot easier to change the calibration number that way than screw with the screws. Prodata is correct here.
The gauge image that you posted is also known as a 4 inch rain gauge as the orifice is 4 inches in diameter (102 mm). The official NWS cooperative observer network uses the larger 8 inch standard gauge. Both compare favorably for accuracy.
Davis rain gauge isn’t particularly accurate. I always find it always under estimates the rainfall by default from the factory and needs a good adjustment.
Mine works well steady and light rain but when you get a storm it tends to over do it. If I adjust that out it’ll register to low for normal rain.
I think it’s mainly design restraints of the tipping bucket system that prevents accurate readings.
The 4" rain gauge is a standard 4" opening gauge. The top of the 4" gauge is within an inch of the height of the VP’s gauge top. Here is a picture (click on the picture to enlarge it - the 4" gauge is hard to see). As you can see in the picture, I’m using a 4x4 post. I will also be cutting the top of the post at an angle to reduce the chance of splash. However, I don’t think that would explain the difference. The top of the 4" gauge is still several inches above the top of the post, and most of the rain was a gentle, light rain. One other note, in the picture I have the funnel and inner tube out because it was winter. The gauge looks just like the one in the picture posted by aardvark.
Remember too that the larger the diameter of the gauge the more accurate it will be. To do a fair comparison the additional gauge could have a 6.5 inch diameter the same as the Davis.
There have been posts on the Davis rain gauge under reading but i would not say it isn’t accurate. I suppose every so often one may come out that hasn’t been adjusted to within fraction of the norm but as far as tipping buckets are concerned you wont get much better.
As had been said before don’t be tempted to turn the calibration screws. You may get into a right bunch of sausages. I think someone on here tried & had to take it back to Davis.
Adjusting the calibration screws is no big deal if you pay attention to what you’re doing and make small adjustments. I’d suggest no more than a half-turn for each screw at a time.
Use a marker on one edge or corner of each screw before you do anything, so you can tell how much you’re adjusting them. Once you figure out which way you turn the screw to raise it, you can draw a small arrow on it to remind yourself.
Keep a record of how much and in what direction you’ve turned each screw. Adjusting them both the same amount is easiest unless for some reason one screw is a lot lower than the other.
I went out and serviced the ISS this morning. I took everything off the post, cut the top of the post at an angle, then reattached the ISS and the 4" gauge. This time I mounted them a little lower (I had them a little high), so the temperature sensor is now around 5’ and the top of the rain gauge is a little under 6’. That also makes it so I can now look down into the bucket without a ladder. The bucket orifice was clear, and there were no obstructions around the tipping mechanism. They were also clear of settled dust in the little tipping buckets (though I would think that dirt in there would make it read high). The post was out of level about a bubble, so that would be a problem. I fixed that. The console was set to .01", so that’s ok. But thanks anyway to the person that suggested that. It was a good thing to check, and one I would have missed. I remember there was an extra piece for the tipping mechanism when I first got the VP2. It changes the bucket from metric tips to .01" tips. I still need to dig that extra piece up so I can ensure I actually do have the .01" piece in the mechanism and not the metric piece. I doubt that’s it though since I left it the way it came from the factory, and I would expect the default for US VP2s is the .01" piece.
Given the advice, I will hold off adjusting the calibration screws until I get more data from the next rain event. That may be a while since we only get about 10"/year. Not many storms drop a 1/4 inch or more. And I’ll double check the ISS for level right before the next storm so I can know whether or not that’s the culprit. If I still have the same major under read, I don’t know what else I can do besides work with the calibration screws. I’d rather not use the WD % adjustment because I have other weather programs also getting data from the console, and I’d like them all to agree.
I calibrated my VP2 last year with the screws. The first thing is to check the the screw settings are equal. I used a large syringe to measure the amount of water it takes to tip the gauge. Adjust screws so that same amount of water is used with each tip. There are little arrows cast into the plastic rain gauge base by the screws with a + & - for directions to turn. (My tips were not equal) I made some adjustments to may then equal. I have the same 4" gauge as pictured above. They are on same post and higher than top of post and back to back. I then use the same adjustments as Kray1000, and made small adjustments after each rain storm. I now get the same readings every time. Joe
I still need to dig that extra piece up so I can ensure I actually do have the .01" piece in the mechanism and not the metric piece. I doubt that's it though since I left it the way it came from the factory, and I would expect the default for US VP2s is the .01" piece.
The manual states you have to change to the metric adapter and the default is .01".
I went through the same thing with mine. Same manual gauge and mounting. Mine read about 9% low when I checked it during 7 different rain events. Davis did give me the directions to adjust the screws but strongly warned against it. I ended up changing the off-set in WD. But other programs and the console don’t show the true total.
I’ve dug up a precise dropper with 1cc increments. I think I will use it to see if the two tipping buckets tip with the same amount, and also use it to drip .10" of water (54 ml, according to niko’s post in another thread) directly onto the tipping mechanism. If that shows the same approx. error that the 4" gauge comparison did, I think I’m just going to go ahead and adjust the calibration screws. Otherwise, I’ll just wait and see what the next rain event shows, now that I’ve balanced up the ISS, and cut the post to reduce the chance of splashing into the 4" gauge.
there has been other threads in the past, and most people tended to agree that the vp rain gauge tended to under read by about 10%,…but your results may vary
54ml isn’t really enough for a decent calibration check. That only equates to 10 tips. If the last drip doesn’t quite tip the buckets for a final time then that would show up as a 10% under-recording. I’d rather see at least 250ml used, if not 544ml (the exactly correct figure according to my calculations). If you haven’t got an accurate measure for these larger volumes but do have some decent-quality digital kitchen scales then 544g won’t be too far out (there are some exact figures for water density at Mass, Weight, Density or Specific Gravity of Water at Various Temperatures).
I know people have used a clean plastic drinks bottle with one or more pinholes made in the bottom as a water container for this drip test. It gets a bit slow towards the end and you may have trouble persuading the last few drops to drip out but may well be the simplest readily-available apparatus.
Steve, one other point: Again this probably doesn’t apply to your particular VP2 set-up, but intermittent wireless reception (eg significant periods of reception dropout) can also be a common cause of apparent rainfall under-recording.
I was planning on taking the funnel off and dripping directly onto the tipping buckets so I could count the tips and put in enough to complete the last tip. I was thinking that this would give me confirmation (or not) of the comparison with the 4" gauge during the last storm. If the test showed the same under read, then I figured that was enough (two separate sources) to just go ahead and start working with the calibration screws. I agree that .10" is a small sample, and I wouldn’t use that by itself to calibrate the Davis gauge. I understand what you’re saying though, and may bump that to .20". It’s just that I’ll be standing there with an eye dropper, doing it by hand, and I’m not sure I’m that patient.
Since I would be watching the bucket tips while doing this test, it might bring to light any physical problems in tipping mechanism. If the mini-drip test doesn’t confirm the 4" gauge comparison, then I will look into a tip counting problem (uncounted tips from either a tipping assembly malfunction or a transmission problem), or a possible over read by the 4" gauge. I don’t think it’s a reception problem as my reception was above 95% during each archive interval of that storm, and was 100% for most intervals. My overall reception has been in the 97%-99% range lately. For example, the reception since midnight right now is 98.9%.
I was planning on taking the funnel off and dripping directly onto the tipping buckets
I don’t think doing it that way will correctly simulate the effect of rain running down the funnel and tipping the tipper. Unless maybe you set up some kind of fixture to hold the dripper in a fixed position. Remember you are going to be doing this s l o w l y.
What numbers are you using John? I measure my VP2 gauge diameter as 164 mm, which gets me to 53.6 (54) ml for one tenth and 536.5513527 ml for one inch.
I completely agree that a significant number of tips are necessary to have a valid result with this test.
