"John Dann" wrote in message
...
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

The calibration legend on my VP1 shows that a one flat turn makes a 1%
difference confirming that John's memory is correct. It seems a general
experience that VP gauges under read compared to a standard manual gauge.
However the amount will depend on the intensity of the rainfall so there
will always be some difference.

I think my under recording is less than 10% and without a refernce gauge I
am reluctant to adjust the readings.

Alan