Philip Eden wrote:

"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%?

Sorry to come to this thread so late ... and following all
the other excellent advice all I am going to do is repeat
the Eden mantra which applies to anyone and everyone
with an AWS ... however fantastic your AWS is,
please add a traditional manual rain-gauge to your site;
it is the only way you will achieve a rainfall record
under standard conditions, comparable with all official and
historical data (with the proviso that your site characteristics
should also be as close to the official standard as you can
manage).

Philip

The amounts measured by the raingauge on my Davis VP average about
10-15 percent lower than what the manual gauge measures but there is
great variability. Sometimes the measurements are almost identical and
sometimes the AWS is as much as 30 percent lower. The variability seems
to be related both to rainfall intensity and to wind direction. In
prolonged slight to moderate rain the AWS compares very well to the
manual gauge but in heavy rain it under-reads quite markedly.

The AWS rainfall measurements are interesting but I agree that a manual
gauge is essential for reasonably accurate measurements.

Norman.

--
Norman Lynagh
Chalfont St Giles, Buckinghamshire
85m a.s.l.
(remove "thisbit" twice to e-mail)