Topic: Hobby Boards UV sensor oddity

[KeithBC]

I finally got my UV sensor installed and working - today is the second day in operation.  Both yesterday and today, I noticed an anomaly.  Both in the morning and afternoon, at times that are symmetrical around solar noon, the UV level shows an odd fluctuation between 09:20-10:10 and between 15:30-17:10.  The shape and timing of the anomaly was identical both days.  Here is today's graph:

The brown bars are total solar radiation; the purple dots are the UV level.  Clearly, the sensor has some strange sensitivity to the sun angle.  I find it hard to believe that a natural fluctuation would occur with the exact same timing two days in a row.  Except for a small amount of cloud between 12:00 and 13:00, the sky was clear all day.

The sensor is mounted horizontally (i.e. looking straight up) with a clear sky view in all directions except north.  It is mounted in the Hobby Boards case, with the supplied UV-transparent window.

Has anyone else noticed anything similar?  Any explanation?

[Weather Display]

UV does reflect well
maybe its reflecting off things around the sensor when the sun is lower in the sky?

[DanS]

Going with GBW's thoughts that UV reflects well and since it appears to occur at symmetrical times around solar noon, could there be reflections inside the case, window edges, etc.? One side during the morning hour and the other side during the afternoon? You could rotate the sensor 90 degrees horizontally for a day to see if the results differ.

[KeithBC]

Thanks, guys.  We had our first partly cloudy day since I installed it, and it didn't do it today.  So, I think you are right - it's got to be reflections off the edges of the window.  I'll have to see if I can block the line of sight from the window edge to the sensor with tape or paint, hopefully without affecting the sensor's sky view.

[DanS]

I was thinking yesterday afternoon about this and what I'd do if I discovered it was a reflection of the edges. You could remove the case contents and window then lightly sand the edges with emery cloth to rough the surface and remove the glaze.

Dan

[KeithBC]

I thought about sanding the edges, but that's just about impossible now.  On a brand new case, you could almost do it, though you might have to break the window out of its glue to do a good job.  Now that I have the original case and window glued into a weatherproof box, it's pretty much out of the question - the edges are not accessible.

However, I should be able to identify the sensor's sight line on the window and paint everything outside of that sight line, which would mask out the edges.  Tape, I realized, wouldn't work, since the temperature inside the box in sunlight gets over 45ºC, and would unstick the tape pretty quickly.