Cumulus Support

On MX Dashboard and also on website - mouse-over dial produces the following message in the balloon..

Solar Radiation 93W/m2 - 194% of maximum.
Current theoretical max reading 48W/m2

Had to double check, but yes lat/long correct in the Settings

I'd need to see the graph, but readings above the theoretical value are to be expected with broken cloud, especially with low sun angles.

OK - It didn't seem logical to be experiencing almost twice the theoretical maximum.

These are global irradiation sensors. If they were direct irradiation sensors (= v. expensive) then you would not over read like that. Plus an expected value of 48 is quite low, so the errors are bigger.

But as I said you need to look at the graph to make sense of the numbers.

You also need to calibrate the theoretical value to your sensor an location. You need a clear cloudless day to do that, preferably near mid-summer. Though you may find your air transparency differs from winter to summer.

mcrossley wrote: ↑Fri 10 Dec 2021 4:38 pm These are global irradiation sensors. If they were direct irradiation sensors (= v. expensive) then you would not over read like that. Plus an expected value of 48 is quite low, so the errors are bigger.

But as I said you need to look at the graph to make sense of the numbers.

You also need to calibrate the theoretical value to your sensor an location. You need a clear cloudless day to do that, preferably near mid-summer. Though you may find your air transparency differs from winter to summer.

Graph shows a standard bell shape under a clear sky by day. Up to around 180% above the second bell, which I'd guess is the theoretical.

If your latitude and longitude and correct then it sounds like your sensor is over reading by quite a lot.

Really you want clear sky curves where the sensor reading is about 90%+ of theoretical for the sunshine calculation to work properly.

You will have to do some calibration of the solar calculation via the settings, but I'm not even sure if it will adjust that much.

Again ideally you will want to do that near the summer solstice as well

Remind me what make is your sensor?

Do you have an official station nearby that provides solar readings to compare against?

My Davis solar sensor compares very well with a calibrated university station less than 10 miles away. It also only requires minor tweaks to the CMX theoretical calculation to get a good match.

Hi Mark,

No local site that I'm aware of and Google isn't helping identify any.

The sensor is a recent addition and is the standard Davis accessory for the VP2.

It's showing 16 at the minute under an overcast sky. I'll have to have another look at the leaflet that came with it, though I remember it being rather vague on calibration. On face value it is over-reading.

I wonder if there's a way to roughly approximate from a light meter....

Geoff

It's just about the worst time of the year to do any calibration because the sun is so low and the weather bad.

If you look at my graphs you can see on 10th December around 13:00 there is a brief spell of what looks like a section of "normal" curve. It is slightly above the theoretical value, but I expect there was also some cloud around at the time.

Indeed.

On a cloudy day I've managed to record 1.2 hours of sunshine!

Back to the leaflet that accompanied the sensor. Keep calibrated, at least yearly ( as Davis' experience is a drift of 2% annually) is the advice without further explanation...

One hour after sunrise and I've maxed at 18W/m2 and recorded 0.6 hrs of sunshine under a relatively cloudy sky - certainly 6+ octas.

How are others finding the sunshine hours recording?

I think this is the same Geoff who emailed me! He wanted to know the url of my Expected Solar Value Calculator which is now at https://www.stella-maris.org.uk/maxsolar1.phtml in case anyone else needs to access it.

Stuart

Chuckles.

Indeed!

Cheers Stuart.