WeatherDuino Forum

Hi team

I received an email from one of my website users today, pointing out something that I was monitoring and trying to get to the bottom of myself. It reads:

Quote:Hi Palmy Weather; I have noticed lately that the solar readings always have a large and seemingly inexplicable 'drop-off' about mid-morning, and return presumably to normal, late afternoon. This also seems to be reflected in the recorded daily sunshine hours. Thank You, Peter

I have taken some screen shots of this and have attached it to this post to show what happens during the day with different events. (Please excuse the different formatting between some images, the source of images has come from my website and Cumulus).

[attachment=551][attachment=553][attachment=552][attachment=554]

This only occurs on days where there is not a single cloud in the sky, or occurs when morning cloud burns off. You can see a significant drop during the middle of the day and a significant rise in the evening.

When there is cloud, the readings during the middle of the day show more like what what I would expect and much closer to the theoretical maximum also shown in these graphs.

All I can think of, based upon the time of day when the drop off occurs and returns to "normal" at the end of the day, this is when the sun is directly on the panel.

I have mounted the panel flat like mentioned in the forum to do, but it is under a cover.

Sadly there is no other weather station in my town which is broadcasting UV and solar information. I have however attempted to compare readings from my old Find Offset WH1083 with what the WeatherDuino Pro2 is displaying. When I have done this at different times during the day with different sky conditions, the results are very similar or more accurate on the WeatherDuino Pro2 system.

As a thought to solve this problem, I could try mounting the panel out of any protective clear housing, so that it is fully exposed to the sun. But how will the panel handle this over the long term?
Is it going to survive being rained on and in full sun light?
Do I maybe need to tweak the pre-set resistor on the board Solar Radiation board to match the harsh southern hemisphere sun?

Does anyone else have any ideas on how to solve this problem? Has anyone else noticed such things?

Thanks for your help!

Hi,

Such dramatic steps in the level strongly suggest an electrical/mechanical fault of some type. Have you considered/measured how hot it might be getting inside your enclosure? The first test I would do is to monitor the voltage directly across the PV panel with a multimeter, to check that there are no faults with the panel.

Personally, I avoid using "preset" pots whenever possible (because they are basically a "mechanical" component), and certainly for any "precision" measurement. It should be possible to calculate a suitable fixed value, or simply substitute a fixed resistor after test. Then "trim" or calibrate the final value in software (i.e. within Cumulus, or I guess the WeatherDuino itself ?).

Probably, not directly related to your "steps", but putting any "window" over the solar sensor potentially upsets the sensor's "Cosine Response". This is because the amount of light reflected off the surfaces of the window (and thus not measured by the sensor) increases as the angle of incidence reduces (i.e. as the sun gets closer to the horizon).

The PV panel itself is presumably designed for direct use outdoors, but if it is necessary to protect the rest of the electronics (and electrical connections, etc.) then I recommend a thin (< 1mm) sheet of PTFE ("Teflon") for the "window". PTFE acts as an excellent "diffuser" to achieve a good Cosine response, is "transparent" to UV, visible and near-IR and has good physical and chemical (unreactive) properties. Note the little white dome in the FO solar pod, although I've no idea whether that works properly, or is even made from Teflon.

Cheers, Alan.

(02-02-2016, 08:34)Palmyweather Wrote: [ -> ]I have mounted the panel flat like mentioned in the forum to do, but it is under a cover.

As AllyCat said, at first sign it seems an electrical failure, and it can be the PV cell itself, but it also can be due to other reason. You said that you have the PV cell inside a closure, which is good to protect the PV cell, but whatever the closure you are using, put the PV cell directly under the transparent side, with no space between them, not just near, or at the middle.
Don't use teflon or any diffuser to cover the PV cell, the calculations for the shunt resistor are made based on a fully exposed cell, however it can be good for the UV sensor (I haven't tested it covered ).
Could you please post a picture of the top side of the solar sensor?

(02-02-2016, 08:34)Palmyweather Wrote: [ -> ]As a thought to solve this problem, I could try mounting the panel out of any protective clear housing, so that it is fully exposed to the sun. But how will the panel handle this over the long term?
Is it going to survive being rained on and in full sun light?

From my experience, the epoxy that covers the light sensitive material is not very resistant to weather conditions, and after a year it starts to lose the glossy aspect and turns opaque.
The best material I found to cover the PV cell is a thin glass. After more than two years of continuous work , the surface of my PV cell still is glossy as new, and working perfectly.

Thanks for your replies

Please find attached some photos of my solar radiation enclosure. It will look familiar as I re-worked the Find Offset WH1083 solar panel enclosure to house the panel and UV sensor. I used sound dampening mat to support the panel and UV sensor up against the clear plastic of the original enclosure. The circuit board mounted tidily underneath this. The enclosure is vented as well to prevent heat build up as I did not seal the original holes left behind in the bottom of the enclosure.

I do notice in taking photos tonight, the washer spacers I used to assist with supporting the UV sensor under the enclosure has moved over the corner or the panel.

I am unsure what this clear plastic is made of and do not have my vernier callipers with me to measure the thickness at this stage.

I think maybe a redesign is necessary and I will try to hunt down something glass like that I can adapt for this project. Just find the readings odd with the difference between a clouded day and a clear sky day.

Thanks for your help and I will let you know how I get on when I find something more suable as a housing.

Hi, thanks for the photos.
Looking at them, it seems that the PV cell is not completely facing the sky in an horizontal position. This is particularly important, and should be made as much as possible with a maximum of 1 degree error. A bull's eye level can help in this task. This adjustment is so important, that, and despite they don't use PV cells, every commercial pyranometer includes a bull's eye level.

[Image: s-l225.jpg] [Image: s-l225.jpg]

More info about this subject along this thread:

http://www.meteocercal.info/forum/Thread...taic-Cells

Before trying anything else, please just try to place the PV cell horizontally. Let us know if doing this solves the problem, I'm sure that at least will dramatically improve the readings


If I correctly remember uncle bob was also tried to use the same WH1083 solar panel enclosure. Later he changed it's assembly to something like this:

The PV cell used is not the original that comes with this solar led device, but the one supplied with the solar / UV kit.

Thank you for the advice

I got back up on the roof this evening with a bulls eye level and checked the top of the clear plastic lid. Sadly it was spot on level with the air bubble sitting perfectly in the middle.

So I guess, like Uncle Bob, this type of enclosure is not the best thing to use.

I was scratching my head struggling to think about what I could use that would offer me a glass lid and I think I might have an idea. What about mounting the panel underneath the glass of an old fire alarm switch box like this?

[attachment=560] (Photo is an example off the internet, not something I own but could have access to)

Not sure how thick the glass would be, but it should not be too thick nor have any protective coatings over it and it should be easy to seal it up to protect it from water. Might give this idea a go?

Hi,

IMHO the "window" (and panel levelling) is nothing to do with the "steps" which I believe is your fundamental issue? Except perhaps indirectly, that heating / thermal expansion is causing some type of "mechanical" problem such as a bad electrical contact (or an "intermittent" electrical component).

Beware that glass (and some "clear" plastics) is NOT "transparent" to UVB rays, which are supposed to be measured by the "UV Index". Empirical "Evidence" of this is that lorry drivers are said to tan on only one side of their face (the one beside an open window).

However, as I've said before, I don't believe that particular UV sensor actually measures the UV Index "correctly", so it still may give a similar output (as does the FO sensor). At the moment your "UV" sensor seems to be correlating much better with the Visible light than is the Visible/IR sensor!

You haven't commented that your solar graphs are "skewed" sideways relative to the Cumulus "Maximum Solar Rad" curve (which could indeed be caused by a tilted panel). But I presume you understand the other reasons why this can a happen, and that it will actually change during the year?

Cheers, Alan.

(04-02-2016, 11:09)AllyCat Wrote: [ -> ]However, as I've said before, I don't believe that particular UV sensor actually measures the UV Index "correctly", so it still may give a similar output (as does the FO sensor). At the moment your "UV" sensor seems to be correlating much better with the Visible light than is the Visible/IR sensor!

Regarding this matter, the following text in from the manufacturer of the UV sensor used in the UVM-30A module.

GaN is a new type of semiconductor materials suitable for making high performance UV sensors, which can totally reject visible light due to its wide bandgap. GaN-based UV sensors have the advantages such as small size, high sensitivity, visible blind or even solar blind, low power consumption and long lifetime. Earlier UV sensors are traditionally based on Si. However, since Si photodiode has a strong response to visible light, unwanted output signals will be generated, causing measurement errors. As a result, expensive filter has to be installed in front of the Si photodiode. GaN-based UV sensors can overcome this inherent limitation of Si photodiodes by providing much higher measurement accuracy for UV light.

Source: http://www.gano-uv.com/en/products.asp?b...AsA28P8HAQ

What about something like this? So long as the lense is clear (some are smoked).
Http://www.ebay.co.uk/itm/3-8-Dia-Plasti...SwvUlWrHjU

Hi JT118

(05-02-2016, 00:50)JT118 Wrote: [ -> ]What about something like this? So long as the lense is clear (some are smoked).

I thought about something like that myself as I have an abondance of them at work, both glass and plastic options. I was concerned about the curvature of the dome however and how this might reflect / refract or even focus the sun light. Also, for the solar panel at least, would be very difficult to moiunt it directly under the dome with no air gap.

Love the idea though and this would be a great solution. Has anyone else tried a security camera dome before?