Can I add just a small word of caution here. First, I wouldn't
recommend anyone altering the screw settings on their Davis rain gauge
unless they're really convinced that it is misreading significantly
and have completely ruled out other possible explanations. Often an
apparent miscalibration has another explanation, eg the gauge
increment is not correctly set on console or software or the reference
rainfall readings are not properly comparable, which can be for a
variety of reasons.And in practice it's pretty difficult to return the
screw heights exactly to their factory settings once altered if you
subsequently decide that the error wasn't as great as you initially
thought and you didn't keep an accurate record of any changes that
were made.

In general the VP2 gauges do record reasonably accurately though like
all less costly TBRs will inherently tend to under-record slightly and
will typically be mounted at eg 4ft height rather than at ground
level, so exaggerating this tendency a little. That said, I suspect
that there was a particular batch of VP1 gauges that were
mis-calibrated at the factory and this is primarily where the reports
of 20% under-recording come from.

Finally, I agree - the drip method is a good way of checking the
calibration - volume figures for the Davis gauges can be found on our
website towards the bottom of the page at:

http://www.weatherstations.co.uk/vp_faqt2.stm

(NB Apologies - this page is now a few years old and really needs
updating soon but the figures are still all correct AFAIK)

Don't underestimate the care and patience that's needed to run the
drip method reliably - although it's reasonably straightforward to do,
things like measuring (or weighing) the total volume of water
accurately, not exceeding the suggested drip rate etc all need
attention. Unless you're meticulous with the method, it's still not
easy to get a precision on repeat calibration runs of better than
4-5%.

John Dann
www.weatherstations.co.uk