Author Topic: Frost crystals on the solar lenses?  (Read 277 times)

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Offline worachj

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Frost crystals on the solar lenses?
« on: January 30, 2019, 10:41:56 AM »
Iíve notice on some days when temperatures are below 32 degrees that I will get early morning and late afternoon UV reading spikes. Today at 8:00 AM I had a UV reading of 7, when it should be zero. Iím speculating that frost crystals on the solar lenses are affecting my UV readings. As the sun continues to rise and the frost evaporates the UV readings will began dropping. They begin rising again when the sun starts to fall in the afternoon. I havenít noticed any problems on the other mornings when temperatures are above freezing, just when temperatures stay below freezing. Pretty sure itís not a hardware problem, but something else.

Is this happening to anyone else?




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Offline worachj

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Re: Frost crystals on the solar lenses?
« Reply #1 on: January 30, 2019, 02:27:38 PM »
Back to normal and tracking the same as other nearby stations in my area now that the frost has burned off by the sun.


https://ibb.co/bssDNgG


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Offline worachj

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Re: Frost crystals on the solar lenses?
« Reply #2 on: January 31, 2019, 08:21:12 AM »
The UV index spiked again at the end of the day. Strange that the spikes happen at the beginning and ending of daylight when the sun is at its lowest. If itís not frosts crystals causing the problem then I have a crack or something which is allowing stray light to the sensor. I donít have problems when the temps are above freezing. Too cold to check the unit, I'm waiting until it gets warmer outside.

https://ibb.co/4KjrDqX


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Offline John Z

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Re: Frost crystals on the solar lenses?
« Reply #3 on: January 31, 2019, 10:18:16 AM »
Worachj,

I just had some physics fun playing with an online Snell's Law refraction calculator. Your notion that ice is bending low angle light into the sensor seems plausible.
« Last Edit: January 31, 2019, 10:20:02 AM by John Z »

Offline worachj

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Re: Frost crystals on the solar lenses?
« Reply #4 on: January 31, 2019, 10:32:50 AM »
Worachj,

I just had some physics fun playing with an online Snell's Law refraction calculator. Your notion that ice is bending low angle light into the sensor seems plausible.
Thanks for doing that calculation. Its happening again today. I looked at the last 31 days of graphs and the problem seems to only occur when the temp stays below 32 degrees. Its not a big thing, but more of a curiosity of what may be happening.

My UV indexes for last 31 days.
  https://ibb.co/z50D4JY


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Offline John Z

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Re: Frost crystals on the solar lenses?
« Reply #5 on: January 31, 2019, 11:12:36 AM »
The online calculator only handles the case of two mediums with different refraction indicies. The situation on the sensor is more complex, as a light ray would pass through air, water ice, plastic, and then into air again on the chip side of the lens. What I think that means here is that there could be a good deal of refraction going on, with an unknown final result.

Ninchelser's teardown of the sensor showed a dark plastic foam ring around the chip. He believes it's there to prevent unwanted low angle light from getting in, and I think that is probably correct. Ice may be partially thwarting that function, or maybe the ring in your sensor is somehow not doing it's job.

Offline worachj

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Re: Frost crystals on the solar lenses?
« Reply #6 on: January 31, 2019, 11:21:53 AM »
Thanks, it nice to get someoneís insight and knowledge. Its too cold to bring my unit down and take a close look at it. If the spikes continue when the temp get above freezing, Iíll take it down and look closer for cracks and check that seal around the sensor.


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Offline worachj

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Re: Frost crystals on the solar lenses?
« Reply #7 on: January 31, 2019, 11:54:06 AM »
Unrelated to my problemÖ

Iím wondering if AcuRite has a rounding problem with whatís reported for the UV index. The report UV index seems to be close but higher than other stations in my area. It wouldnít surprise me if the sensor returns a value in tenths and it always gets rounded up.

I wonder how sensitive the UV sensor is, down to the tenths or just a whole number.


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Offline John Z

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Re: Frost crystals on the solar lenses?
« Reply #8 on: January 31, 2019, 01:01:41 PM »
This Si1133 sensor is quite a piece of work. I'm looking over the datasheet and application notes, but it will take a while to really understand them.

A couple of things jump out, though, to address your questions.

The chip includes two 23-bit analog-to-digital converters. So it's clear that raw measurements from the photodiodes are taken with much, much finer grain than the UV index that is finally reported.

The chip includes separate pairs of photodiodes, one set for UVA, one set for UVB. The measurements from each are weighted and combined according to an algorithm.

How the final determination of UV index is calculated is not yet clear to me. More reading required.

Finally, I looked at Nincehelser's tear down photos again. I'm uncertain that the foam ring has any significant optical function. It may be mostly a bug blocker. The application note recommends limiting the chips viewing angle with a donut ring of black ink on the lens. I'm not sure exactly what we have in these sensors.
« Last Edit: January 31, 2019, 01:04:43 PM by John Z »

Offline worachj

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Re: Frost crystals on the solar lenses?
« Reply #9 on: January 31, 2019, 03:32:18 PM »
I didnít think it would be easy to determine, its above my ability. Thatís why I love these types of forums, the ability to learn and get information from others.

I apricate anything you can gleam and share!


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