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Offline StrathspeyWeather  
#1 Posted : 08 August 2019 16:15:33(UTC)
StrathspeyWeather

Rank: Advanced Member

Joined: 12/09/2012(UTC)
Posts: 494
Location: Cromdale, Highland

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?

Live Weather from Cromdale, in the Cairngorms National Park of Highland Scotland.

http://strathspeyweather.co.uk (Met Office Climate Station)

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Offline four  
#2 Posted : 08 August 2019 16:30:32(UTC)
four

Rank: Advanced Member

Joined: 07/04/2006(UTC)
Posts: 19,291
Location: N.Y.Moors

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..

Edited by user 08 August 2019 16:32:13(UTC)  | Reason: Not specified

Offline four  
#3 Posted : 08 August 2019 16:37:22(UTC)
four

Rank: Advanced Member

Joined: 07/04/2006(UTC)
Posts: 19,291
Location: N.Y.Moors

https://www.scaledinstru...llector-assembly-inches/

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

Offline Chunky Pea  
#4 Posted : 08 August 2019 16:47:02(UTC)
Chunky Pea

Rank: Advanced Member

Joined: 28/10/2013(UTC)
Posts: 13,490
Man
Ireland
Location: East Galway, Ireland 35 m asl

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

"There are nights when the wolves fall silent and only the moon howls"

--George Carlin.

Offline Chunky Pea  
#5 Posted : 08 August 2019 16:49:44(UTC)
Chunky Pea

Rank: Advanced Member

Joined: 28/10/2013(UTC)
Posts: 13,490
Man
Ireland
Location: East Galway, Ireland 35 m asl

Originally Posted by: four Go to Quoted Post

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. 

"There are nights when the wolves fall silent and only the moon howls"

--George Carlin.

Offline Devonian  
#6 Posted : 08 August 2019 18:13:36(UTC)
Devonian

Rank: Advanced Member

Joined: 02/05/2006(UTC)
Posts: 27,142
Man
Location: East Dartmoor

Originally Posted by: StrathspeyWeather Go to Quoted Post

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.

"In a 52-48 referendum this would be unfinished business by a long way"

Nigel Farage, Daily Mirror, 16/5/2016

"I think the mistake the government made - led by Theresa May - from the start was to try and claim that a country that had voted 17 million to leave the EU, 16 million to stay, wanted a 100% Brexit"

Osborne, 22/12/18.

Offline StrathspeyWeather  
#7 Posted : 08 August 2019 18:27:45(UTC)
StrathspeyWeather

Rank: Advanced Member

Joined: 12/09/2012(UTC)
Posts: 494
Location: Cromdale, Highland

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.
Live Weather from Cromdale, in the Cairngorms National Park of Highland Scotland.

http://strathspeyweather.co.uk (Met Office Climate Station)

Offline Chunky Pea  
#8 Posted : 08 August 2019 18:33:47(UTC)
Chunky Pea

Rank: Advanced Member

Joined: 28/10/2013(UTC)
Posts: 13,490
Man
Ireland
Location: East Galway, Ireland 35 m asl

Originally Posted by: Devonian Go to Quoted Post

 

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. 

"There are nights when the wolves fall silent and only the moon howls"

--George Carlin.

Offline Global Warming  
#9 Posted : 08 August 2019 18:49:12(UTC)
Global Warming

Rank: Advanced Member

Joined: 07/10/2006(UTC)
Posts: 6,122

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.

Chineham, Basingstoke, Hampshire, 86m asl
Offline Devonian  
#10 Posted : 08 August 2019 19:20:36(UTC)
Devonian

Rank: Advanced Member

Joined: 02/05/2006(UTC)
Posts: 27,142
Man
Location: East Dartmoor

Originally Posted by: StrathspeyWeather Go to Quoted Post
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...

"In a 52-48 referendum this would be unfinished business by a long way"

Nigel Farage, Daily Mirror, 16/5/2016

"I think the mistake the government made - led by Theresa May - from the start was to try and claim that a country that had voted 17 million to leave the EU, 16 million to stay, wanted a 100% Brexit"

Osborne, 22/12/18.

Offline richardabdn  
#11 Posted : 09 August 2019 17:10:07(UTC)
richardabdn

Rank: Advanced Member

Joined: 10/04/2006(UTC)
Posts: 6,500
Location: Aberdeen

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.

 

Edited by user 09 August 2019 17:11:59(UTC)  | Reason: Not specified

2010s - The Worst Decade for Warmth (or any other type of extreme)

Decadal High Temperatures at Dyce:

1950s: 28.9C, 7th June 1950

1960s: 27.4C, 14th July 1969

1970s: 28.4C, 25th August 1976

1980s: 28.0C, 14th July 1986

1990s: 29.7C, 21st August 1995

2000s: 29.8C, 17th July 2006

2010s: 27.2C, 22nd July 2018

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