For a long time I have been questioning the accuracy of the Davis Vantage Pro2 when it comes to measuring rainfall. For the last 10 years I have been using a standard 5 inch rain-gauge alongside the VP2 set-up. Since then I have found that the VP2 regularly under-reads the rain-gauge in the region of 10-20%. I suspect that during periods of severe/heavy rain however this inaccuracy rises even higher. One particular wet spell earlier this week tends to support this: The 24 hour period ending 10 am Wednesday 7th the VP2 showed 50.0 mm of rainfall, whilst the rain-gauge showed 68.8 - a difference of around 37%. The rainfall during that period was particularly heavy, and was sustained. I suspect that the tipping bucket system is losing rain during its operation by being overwhelmed in effect by the severity of the rain. Has anybody else using the VP2 had cause to question the system's accuracy?

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I thought the same and played with the adjustment screws but the change was minimal - so I added an offset of about 15%.
There's an upgraded rain collector which does away with the tipping bucket just come out.
This is it I think, not outrageous money
https://www.weatherspares.co.uk/davis-aerocone-rain-collector-for-pro2-pro-and-wizard--6461-513-p.asp

Or is that just the bucket .. anyway there is a new option which just plugs in..

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https://www.scaledinstruments.com/shop/davis-instruments/parts/vantage-pro2-parts/rain-collector-parts/davis-7345-425-single-spoon-tipping-bucket-rain-collector-assembly-inches/ 

This is the new assembly you can just swap it out I think.

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Have noticed this myself. Davis V2 is not the most accurate regarding rainfall and mine does under record by about 10% thereabouts. 

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I thought the same and played with the adjustment screws but the change was minimal - so I added an offset of about 15%.
There's an upgraded rain collector which does away with the tipping bucket just come out.
This is it I think, not outrageous money
https://www.weatherspares.co.uk/davis-aerocone-rain-collector-for-pro2-pro-and-wizard--6461-513-p.asp

Got one of these last year Four, hasn't really made much of a difference to be honest. 

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For a long time I have been questioning the accuracy of the Davis Vantage Pro2 when it comes to measuring rainfall. For the last 10 years I have been using a standard 5 inch rain-gauge alongside the VP2 set-up. Since then I have found that the VP2 regularly under-reads the rain-gauge in the region of 10-20%. I suspect that during periods of severe/heavy rain however this inaccuracy rises even higher. One particular wet spell earlier this week tends to support this: The 24 hour period ending 10 am Wednesday 7th the VP2 showed 50.0 mm of rainfall, whilst the rain-gauge showed 68.8 - a difference of around 37%. The rainfall during that period was particularly heavy, and was sustained. I suspect that the tipping bucket system is losing rain during its operation by being overwhelmed in effect by the severity of the rain. Has anybody else using the VP2 had cause to question the system's accuracy?

Agreed, and pretty much my experience. Having recorded weather for many year I sort of know what a inch of rain is like. I don't claim to be able to know that to 10% but last year the gauge was so inaccurate (but still factory settings) it simply was obvious it was wrong.

I did the test where you 'pie'r*2 it (aka drip ~23cc (something like that...)  through the funnel and you should get five reed switch clicks, which equals 1mm). I did that and it clicked the right number of times (indeed, I poured more through and it was still counting right), but it still under records by a large amount, 20% or so - which of course is a whopping one click in five.

I'm still unsure why it counts clicks right but under records by 20% or so . And i don't think it's to do with rain rate, it has to rain very heavily for the gauge to be swamped. Nor am i convinced by the argument about wind or exposure, for the money Davis really ought to provide kit that gets closer to single figure inaccuracy imo - if it's 20% out you might as well just eyeball a bucket each morning.

Anyway, I fiddled around with the adjusters and I get readings closer to the check gauge than I did.

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Thanks guys for each of your replies, and it's interesting to hear that others too experience the same problem. It appears that Davis are at least aware of the problem, which given the age of the VP2 one would expect. The single bucket tipper looks better, along with the updated cone but begs the question as to whether they can consistently provide acceptable levels of accuracy.

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Agreed, and pretty much my experience. Having recorded weather for many year I sort of know what a inch of rain is like. I don't claim to be able to know that to 10% but last year the gauge was so inaccurate (but still factory settings) it simply was obvious it was wrong.

I did the test where you 'pie'r*2 it (aka drip ~23cc (something like that...)  through the funnel and you should get five reed switch clicks, which equals 1mm). I did that and it clicked the right number of times (indeed, I poured more through and it was still counting right), but it still under records by a large amount, 20% or so - which of course is a whopping one click in five.

I'm still unsure why it counts clicks right but under records by 20% or so . And i don't think it's to do with rain rate, it has to rain very heavily for the gauge to be swamped. Nor am i convinced by the argument about wind or exposure, for the money Davis really ought to provide kit that gets closer to single figure inaccuracy imo - if it's 20% out you might as well just eyeball a bucket each morning.

Anyway, I fiddled around with the adjusters and I get readings closer to the check gauge than I did.

There is an option on the Davis to record in 0.1mm steps. I tried this a few years back and it was a disaster. Under recorded by about 50%. 

But even on factory settings, it isn't great. A couple of weeks ago it recorded a daily total of 39mm, yet my trusty mug rain gauge (equal in circumference top to bottom, measured nearly 45mm. That is a huge and unacceptable difference to be honest. 

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I have had the same experience. The Davis VP2 under records and my traditional gauge over records.

I didn’t have the time or wherewithal to do the official calibration check. So instead I played around with the adjustment screws until I was getting readings more consistent on a regular basis with the closest Met Office official station (Odiham) and high graded local stations in the COL network (including A graded Stratfield Mortimer).

I am now much more confident my readings are reasonably accurate. By no means perfect but a lot more accurate than they were. The readings tend to be least accurate in high winds because the VP2 gauge is relatively high off the ground in the default setup and hence subject to swirling and eddying winds.

Thanks guys for each of your replies, and it's interesting to hear that others too experience the same problem. It appears that Davis are at least aware of the problem, which given the age of the VP2 one would expect. The single bucket tipper looks better, along with the updated cone but begs the question as to whether they can consistently provide acceptable levels of accuracy.

I dunno, it ought not to be beyond the wit of man (Davis) to make a rain gauge for a several hundred pound weather station and make it acceptably (+-10%?) accurate. The VP2 is a great bit of kit (mine has otherwise been faultlessly and accurately running since 12/16) but its rain gauge clearly isn't. How it is that the VP2 gauge looks just like the VP1 one but (to my mind) isn't as accurate is odd - which brings me back to how it counts clicks...

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I have a VP2 tipping rain bucket and think it is dreadful for the money. Very inaccurate and I'm constantly having to manually adjust rain totals.

Today it says we have had 11mm which seems low to me. Compare that with Westhill to the west which had 22mm and Nigg to the east which has recorded 10mm http://apps.sepa.org.uk/rainfall

I am about equidistant between the two so would expect to have rainfall of perhaps nearer 15mm. Obviously this will not always work e.g. in the July 2018 thunderstorms I watched all the heavy rain go to my east then later it went to my west so in that instance the total was indeed much lower. However that is not the case today with more persistent rain which will be heavier with increasing altitude and distance from the sea hence the difference between Nigg and Westhill.

A cheap garden centre rain gauge which I keep for a check, and cost about £5, has recorded 16mm. I am not saying this is accurate but it seems nearer the mark to me. Dyce has logged 18mm but it always seems wetter there for some reason.

One other thing is that it also seems to get blocked up with debris very easily. This occurred the other night when it logged 2.6mm when the real total should have been about 8mm.

 

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I'm checking mine out right now. During March it appeared to underreport by a huge margin. 

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I'm checking mine out right now. During March it appeared to underreport by a huge margin. 

I certainly have found mine way better since decoupling it from the ISS and having it at ground level, I bought the separate rain gauge for this and have it a few metres away.

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I certainly have found mine way better since decoupling it from the ISS and having it at ground level, I bought the separate rain gauge for this and have it a few metres away.

When you say "the separate rain gauge" do you mean it is a standalone piece of kit? If so can you post a link to it?

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When you say "the separate rain gauge" do you mean it is a standalone piece of kit? If so can you post a link to it?

Yes exactly Brian, I just unplugged the built in rain gauge (and left it there collecting spider's webs), and plugged this one in instead.

https://www.weathershop.co.uk/davis-6464m-aerocone-rain-collector 

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Yes exactly Brian, I just unplugged the built in rain gauge (and left it there collecting spider's webs), and plugged this one in instead.

https://www.weathershop.co.uk/davis-6464m-aerocone-rain-collector 

Thanks, I may go down that route. I wonder if animals are a potential problem? Fox roams our garden, we get regular visits from badgers and not so long ago a deer found its way in. 

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Thanks, I may go down that route. I wonder if animals are a potential problem? Fox roams our garden, we get regular visitors from badgers and not so long ago a deer found its way in. 

I would have thought the bird spikes would put pay to most of those, but who knows, they can all be persistent buggers. We are very rural but probably because of that hardly ever see a fox (there are one family in the woods), the badgers similarly are in the woods but do terrorise some gardens nearby. We did have a problem with muntjac, but every time they come in I just light the bbq and they soon fear the worst 😋

I was a bit more wary when one of the wolves escaped a few years back, the roads were closed and there were armed police and a chopper around, but other than that it's just rabbits mostly. 

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I was a bit more wary when one of the wolves escaped a few years back, the roads were closed and there were armed police and a chopper around, but other than that it's just rabbits mostly. 

That was Torak, who was let out by some animal rights activists one night.

You were right to be wary, he would have been a severe risk to your hotdog sausage supplies!

(In reality, he was a lovely old wolf, I hand-reared him back in 2006 and was one of his handlers right 'til the end of his life many years later. When he escaped, after running around for a bit, his owner caught up with him, called him over and put a collar and lead on him.)

Incidentally, with the topic in question, I have had doubts about my own VP2 rain guage, but whenever there's been an issue it's been down to gunk collecting inside, down the very small hole at the bottom of the cone.

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Still looking into this and it seems my VP2 is consistently recording about 50% of total rain. I've tried tipping the rain collecting mechanism each way and it registers 0.2mm in each direction. 

Any ideas?

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Still looking into this and it seems my VP2 is consistently recording about 50% of total rain. I've tried tipping the rain collecting mechanism each way and it registers 0.2mm in each direction. 

Any ideas?

There's obviously a number of factors Brian, many of which are well-documented so apologies if any or all have been done or considered, I'm sure they will have been. Other than that it's pretty well documented it's not the greatest rain collector, but then what is in the amateur to semi-pro range, and how many gardens have the ideal locating site possible.

1. Have you done a pipette test or similar to see if it is actually recording the right amount of rain for a 0.2mm tip?

2. It would be interesting to know against what you are comparing it to, presumably a manual, but if it's another similar to the VP2 (let's call it mechanical) some are known to actually record more than they should, from the browsing I did ages ago.

3. Dirty tipping buckets are always a problem with dust and dirt build up meaning that it takes less than 0.2mm to tip. Obviously that is unlikely to be your issue as it would tip more..

4. Spiders webs can always cause an issue with sticking

5. Other than that I would say location is the main issue. I have certainly seen an improvement compared to my manual reader by decoupling and moving the VP2 to ground level, but it's still not perfect. Height is always a killer for rainfall with any wind. Obviously rain shadow is another big influence. In reality the height has an easy fix (decoupling) but for most people that usually increases the impact of rain shadow, so it's swings and roundabouts. Mine is at ground level so good, but is actually at the end of and in between two veg cages, so not ideal. The incoming rain from the South or SW is fine but a NW direction would be impacted... but bar putting it in the middle of the lawn, there's not much other option.

I just took at a look at the WeatherLink site, as although comparisons from one location even a mile away are impossible, it shows a picture. Within 5 miles of me - Most of the sites in a town, with presumably less option to site well due to likely smaller gardens are averaging 3mm today, most in a rural location are averaging 5mm. Mine is at 6mm, with only one higher at 7.4mm. A pretty big variation considering the radar looks reasonably similar in the locale.

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