On 22/04/2018 12:45, Keith Harris wrote:
I'm closely comparing my InstroMet and Blake-Larsen sun recorders, especially through this period of long days of sunshine, both of which work in different ways.

Keith, I am finding that the BL under-records by about 15%. It seems to
have difficulty registering sunshine if conditions are slightly hazy as
the thresholds appears to be set too high even though there are
well-defined shadows. I have contacted Ole about this but he seems to
think the recorder is working fine.

A classic example was the other day when there were thin contrails and
these were enough to cause the reading to drop below the threshold even
though the sun was still shining strongly with very distinct shadows.

My R&D recorder, like yours turns on about 40 minutes or so before the
BL at dawn. At dusk the BL turns off about 20 minutes before the R&D.
The only way the two recorders nearly record the same is when the sky is
exceptionally pristine with no haze at all, i.e., extremely deep blue
all the way down to the horizon - and that doesn't happen that often.

Here are my sunshine data for the last few days noting that the skies
were cloudless on the 18th, 19th & 20th:
R&D BL
18th 13.1 11.3
19th 13.2 11.6
20th 13.1 10.9
21st 7.4 5.4

Another issue that I have with the BL recorder is that it is totally
reliant on the program running all the time. There have been occasions
when the program crashes and doesn't restart. All the data during the
downtime does not get recorded. It really could do with a data logger
and until it does I don't see it as being a serious contender to the
present sunshine recorder line-up.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalleyweather.me.uk