Having just repositioned and re-calibrated my various weather sensors i have found that my 1 wire rainwise rain gauge is actually more accurate than my vp2. Overnight we had some light rain, the vp2 console says 1.8mm whilst my Rainwise says 2.1mm which is correct (comparing with the traditional rain gauge). To be honest i find that a little disappointing as i always thought that Davis where the best for mid value equipment in terms of accuracy, but here being outperformed by a sensor much much cheaper.
Is this a common error with the vp2 Gauge, looks to me like its always lower than my other correct readings. ?
You really can’t reach any useful conclusion about rainfall accuracy based on a single, relatively small, rain event like 2mm.
Any TBR (tipping bucket rain gauge) may read low by a single bucket tip (0.2-0.3mm for a VP2 gauge, depending on exactly how the gauge is configured) because in any individual test the gauge could have been about to tip if say one more drop of rain had fallen and thereby that single drop would have added 0.2-0.3mm to the total. The official figures for the VP2 gauge are
Its not just based on last night, i have noted its inaccuracy since i purchased my 1 wire gauge. An example, we had 101.6mm of heavy rainfall on the 20th, the Davis registered 99.0mm whilst the 1 wire 101.6mm which is exactly what was in my trad gauge. Now on singular rain events maybe only a couple of mm out, but when it comes down to yearly / Monthly totals it can be quite a way out.
All funnels are near to each other, and out in the open so i cannot see how it can be anything other than the tipping mechanism in the Davis.
OK, good - but that information wasn’t in your initial post! I think what you’ve confirmed is that both the Davis VP2 and the Rainwise gauges that you have give excellent accuracy. Let’s say that the true rainfall was indeed 101.6mm (it probably wasn’t precisely but we need to use some reference figure) then the Davis gauge was reading about 2.6% low, in other words comfortably within its specification of
This is something that I have been unable to gather any information on, or why it is so often quoted as far as accuracy is concerned. The UKMetO quotes requires the funnel opening to be at 300mm above ground level, however it doesn’t elaborate. I can understand a very small amount of rain bouncing back off the ground and entering the funnel under certain conditions, but not in sufficient quantities to alter the readings by small amount. That would seem to point to there being possible localised wind effects around the funnel itself, which could have serious implications. As it is unlikely that anyone would use a VP2 that is lying on the ground, has there ever been a study to indicate what a mounted VP2 (or any other rain gauge for that matter) at a height of say 4 to 5 ft above ground level would differ by (in readings) to a standard rain guage at ground level. As I doubt that the quoted +/-4% that you have alluded to (measuring a known amount of water), is the significant part, maybe there could be a far greater inaccuracy in readings (due to possible wind/vortex), as WorcesterWx has outlined with his his different setups…I’m sure that I have seen wind-breaks used in some circumstances on this forum for such conditions… 8O
I just wonder how much we all hang onto our rain gauge readings, without having any real comparison with a genuine manual rain gauge set at the correct height. :? :?
The influence of gauge height (above ground level) on rainfall measurements has been extensively investigated in the meteorological literature over many years. One reference from as long ago as 1769 suggested that a 20% low reading would be seen at 30ft and a 50% low value at 150ft (up a church tower!). This general principle - due apparently to wind effects - has been confirmed many times over. More recent and detailed measurements suggest something like a 5-10% under-reading at 4-5ft rising to 15% low at 20ft. One of the problems is defining just what is meant by ground-level measurements because partly burying the gauge in the ground - ie so that the gauge opening is closer to true ground level - can give up to a 5% increase in rainfall catch. I’d imagine that the ‘official’ 300mm (1ft) height is actually a standard-type gauge sitting on the ground, when the opening of the gauge would indeed be around 300mm above ground level.
This is the reason that many people go on (and on…) about height of the opening above the ground in any discussion of rainfall accuracy - it really does make a significant difference and - IMHO at least - it’s really pointless trying to dissect the origin of the odd percentage point or two in readings between two gauges unless you’re sure that height and exposure are identical.
Personally I think that the VP2 design would be improved by separating the rain gauge from the ISS so that it can be mounted closer to ground level. In the original design I’d guess that Davis were keen to maximise the convenience of mounting, but I know they are gradually becoming more conscious of the height effect. Having the rain gauge at 4-5ft height probably is the cause of a 5% or so under-recording compared to an official gauge truly at ground level.
My 3 gauges are all 1 meter above the ground (moved them recently before most of the rainfall) in the same position with an opening of the same height, this is how i know for sure that is a tipping under calculation rather than an environmental issue.
Thanks for that. My Googling had failed miserably!
I couldn’t agree more…There seem to be considerable efforts being made by various individulas on this forum to disect their VP2’s into sensor components in an effort to place them in the best locations. 8) 8)
If you have been following my discussion about siting in the siting portion of this forum, you will know that I indeed separated my rain bucket from the ISS/temp part. I find it curious (as mentioned elsewhere) that the UK standard is 1’ above the ground vs. 2’ here in the US? Anyway, since I am following US standards, I mounted the temp sensor at 5’ and the top edge of the rain bucket at 2’ above grass. It is obvious that Davis conceded some accuracy to provide easy installation. I noticed another manufacturer’s sample pictures showed the rain bucket lower on the pole but had it mounted on the same pole as the other stuff, therefore blocking the bucket in one direction which was worse than having the rain bucket higher but not blocked. In my install I put the rain bucket on an arm over 3’ out from the pole/mast and the pole/mast is on the side the least precipitation comes from so even though there is minimal rain “shadow”, it is from a direction that doesn’t matter.
Yes it’s certainly possible as you’ve discovered, but the great majority of users don’t do so.
I find it curious (as mentioned elsewhere) that the UK standard is 1' above the ground vs. 2' here in the US?
Yes, I agree it’s a bit odd if the US standard is as high as 2’. Presumably the aim is to make an estimation of the amount of rain that actually falls on the ground, in which case the funnel opening ought to be at ground level - or probably an inch or two above simply for the practical purpose of stopping soil etc from falling into the gauge. I think the UK standard is to get as close to that ideal as reasonably practicable but short of burying the gauge in the ground. For various mechanical reasons a manual gauge is typically10-12 inches high, so as per my post above I think the UK standard is set to place a typical gauge on the ground (ie rather than in the ground) and hence the opening height becomes around 300mm.