Solar readings wrong?

[Adrian Hudson]

Its been lousy weather recently. Dull, wet and today, windy. Cumulus thinks its sunny and has logged many hours of sun over the last few days.

My Vantage Pro2 (in a period of real sunshine this afternoon, no less!!), was showing values in the mid 400s (W/M^2) but Cumulus was calculating the theoretical max in the low 300s (see screen shot of data and graphs).

Other times, its been quite dull and the actuals have been approaching or slightly above the theoreticals and Cumulus says "Its sunny!!". For example at 3:30 today, it was as dull as dishwater outside and yet the sun "LED" was on.

Now, in the past I have seen regular spikes in solar, way above theoretical max, but only for a few minutes as high white clouds reflect sunlight. adding to the direct light from the sun, but I don't remember ever seeing protracted raised values.

The instantaneous readings showing on Cumulus' console agree with the Davis Console, so its not a problem with Cumulus "adjusting" the actual readings. (As I side issue I note there are no calibration settings for Solar, is there any way of calibrating the solar data should one want to?).

My settings for solar are threshold 75%, Transmission factor 0.8 and I have put a setting of 75 W/M^2 for "Sun Min)

My immediate thought is Cumulus is calculating the theoreticals too low but I can't really believe that as others would have noticed by now.

Has anyone got any ideas please?

(Edit: Forgot to say, I have checked the station location settings)

Davis VP2 with Solar sensor (no UV).
Cumulus 1.9.4

[steve]

My immediate thought is Cumulus is calculating the theoreticals too low but I can't really believe that as others would have noticed by now.

This is actually quite likely. You will need to experiment with the settings until you get something that works for your location. I've found that a transmission factor of 0.9 gives me a theoretical max that matches my sensor reasonably well on a clear day.

There's no solar calibration setting, but it's on the list of things to do.

[steve]

Note also that the reading from the Davis can be exaggerated by things like reflections from clouds, so even with the theoretical max calculation calibrated correctly, the readings can often be above the max.

[Adrian Hudson]

Oh, right. Thanks. I will fiddle with the transmission factor and see what we get, and yes, I knew about reflectivity, I have seen it a lot in the past.

Thanks.

[Adrian Hudson]

I just thought I would mention that 0.9 in the transmission factor makes a much better match in between theoretical and actual solar. It was a very clear, bright, mostly cloudless day today and all morning actual tracked just under theoretical.
Adrian