On Wed, 17 Oct 2007 06:55:21 +0100, Zak wrote:

After carefully comparing rainfall using both the Davis VP and a
standard manual gauge it seems, in calm weather, my VP is under-reading
by 20%. The tipping mechanism is clean and moving freely.

I know the two screws under the tipping buckets need to be turned
anti-clockwise in order to lessen the amount of rain required to tip,
but can anyone suggest the amount they need to be turned in order to
achieve the 20%?

If you're sure that there's no other explanation (eg rain gauge step
incorrectly set on console or software, VP and reference gauges not at
same heights/locations etc) then you should find a calibration legend
embossed on the plastic under each tipping bucket. From memory,
turning the screw by one flat makes a 1% difference in calibration.
(Direction of turn as shown on the legend.) Obviously both screws need
to be adjusted by the same amount.

John Dann
www.weatherstations.co.uk