I have both a 5" Met Office Standard copper rain collector together with a
glass Met office pattern measuring cylinder, as well as a Vantage Pro AWS,
which is set at around 2 metres above ground level. The two collectors are
about 4 metres apart.

I noticed that the AWS readings were consistently about 10-15% lower than
than that measured in the 5" collector.

Consequently, and with hopefully some careful logging of actions, I rotated
the two hexagonal stop screws anticlockwise (using a very small adjustable
spanner), so that they were raised a small amount. At present, these
amounts a South stop 1 turn, N stop 1/2 turn.

The tipping points can be checked using a syringe to add a measured amount
to make one tip directly into the bucket (about 4.25 ml). Or a measured
amount from a 5" measuring cylinder (if you have one) into the funnel. This
latter is good for checking larger amounts, but must be done slowly, of
course.

I now find that the readings match those in the 5" fairly consistently, with
a trend towards slightly over-reading by about 2%.

All this done after the rainfall counter has been unplugged on the ISS and
the collector base levelled, of course!

I wonder if anyone else has tried this?

Roy Avis