My R&D sunshine recorder actually under-recorded by about 10% when I first
got it. This wasn't brought to my attention until a few cloudless days had
happened and I also compared my records with a couple of reasonably close
stations which showed significantly higher sunshine levels.

There is a way of adjusting the clock counter on the circuit board of the
unit, and by taking 1 hour slots of unbroken sunshine and a stopwatch you
can actually get the adjustment pretty much spot on.

I now use a datalogger which eradicates any timing issues but at the end of
a sunny day with say, 10 hours of sunshine, the R&D unit display is usually
within a minute or two at the most of the datalogger.
________________
Nick.
Otter Valley, Devon
83 m amsl
http://www.ottervalley.co.uk