I had another exact match between the CoCoRaHS gauge and the Davis. The common thread has been that these are rather heavy, straight-down rainfall events with little or no wind. (very light wind from the SW)
I do have a problem with a neighbor's tall shrub to the SE, that can have a rain shadow effect, but I have had lower catch on the Davis from other wind directions when it is windy.
I am becoming more convinced that the problem with the Davis rain collector is the tapered shape, rather than calibration or tipping mechanism issues in general. I know that spider webs, dirty "buckets", friction in the pivot, etc. have an effect, but we are talking about an inherent issue here.
Greg H.
Significant rainfall overnight. Total for CoCoRaHS was exactly an inch. (this doesn't happen very often...)
Wind was from the SE when rain was at the heaviest.
CoCoRaHS: 1.00"
VP2: 0.86"
NovaLynx 8" Tipping Bucket: 1.07"
The NovaLynx was 7% high and the VP2 was -14%.
Is this the VP2 cone shape, or the rain shadow? Which predominates? The local Marina VP2 recorded 0.84", which is roof-mounted in the clear of any obstructions.
One recent day with little/no wind not out of SE rain shadow matched CoCoRaHS, last night with SE wind negative discrepancy.
Greg H.
My Davis during heavy events is typically 15 to as high as 20% low compared to the manual CoCoRaHS gauge. Less so during nice steady rain with minimal wind. I'm surprised to see that your NovaLynx was higher, albeit slightly. I may have you confused with another forum member but at one time were you making comparisons with a RainWise TB? I might try adding one in the future.
The Rainwise was probably ValentineWeather, I would guess.
There are two calibration targets for the NovaLynx gauge. Assuming a 946mL amount of water (measured on the approved CoCoRaHS Escali scale), these targets are either 1.15" or 1.19". I used a #72 gas orifice on my homebrew calibrator and got a consistent 1.19" over three runs. With the 1/16" orifice, I got 1.09" which is low against the 1.15" target.
Since this is so rate dependent, and I do get agreement within 0.01" of the CoCoRaHS gauge from time to time, I don't plan on changing the calibration again. I think that there are so many variables with rain rate, wind, geometry, etc. that it is impossible to get an exact match anyways.
Greg H.