[zipidy66]

Hi,

The UV Index is a linear scale, so you should use a multipler to correct for any attenuation that you introduce, or to correct for calibration errors (except for any zero/bias offsets).

One problem is identifying a material which is (semi-) transparent to UV. I suggest a sheet about half a mm thick of PTFE (Teflon). See post #98 here.

Cheers, Alan.

Thanks,
I happen to have a sheet 0.5mm teflon, I shall give it a go and work out the appropriate multiplier.
Cheers

[Wizza]

Hi Guys, Its been about 5 months since the last post in this thread. I'm looking to start my UV/Solar sensor shortly. Does anyone have an update to this topic, and any feedback or more experiences in what is a good material to use to cover the UV sensor and solar panel..???

Many thanks,

Mark R

[AllyCat]

Hi Mark,

No, I haven't done any more research/measurements (it's been winter here) but I believe that most commercial sensors use PTFE (Teflon). Having just examined the apalling state of some of the plastic fittings on my conservatory roof, I'm satisfied that something "special" like Teflon is necessary for resistance to weather/sun.

So my recommendation (still) is to use Teflon sheet as both protection and as a diffusing window (for the correct cosine response). To determine the effective attenuation/transmission of the Teflon, just point the surface of the PV panel/sensor directly at the sun, with and without the sheet inbetween and compare the reported values.

Cheers, Alan.

[Wizza]

Thanks Allan, what sort of thickness PTFE Teflon sheet are we talking..? And have you found PTFE Teflon in any everyday product, that is common/ accessible..? Where would one find PTFE Teflon sheet..???

Thanks Mark...

---

Allan, I've found some 1mm PTFE Teflon plastic sheet on eBay and have some on the way. Do you cover both the UV sensor and the PV cell with the PTFE Teflon plastic sheet, in a suitable enclosure..?
Thanks
Mark R

[werk_ag]

Hi Mark

Regarding the PV cell, and just to protect it from environmental elements, use only a thin glass.
The teflon may be useful for the UV sensor (enhance the cosine response), but I think that it will gonna change the output voltage of the UVM-30A sensor, dramatically.

[Wizza]

Ok... Thanks Werk...

Mark R

[Wizza]

Ok, I've got a thought that I want to run past you guys. What if I use a small IP-rated weather proof enclosure, cut out a section about the size of the PV cell and then silicon in a piece of normal 4mm or 5mm glass, with the pv cell mounted below it. And then make a second smaller cutout, about 20 or 30mm round, and silicon in a piece of the PTFE White opaque plastic, with the uv sensor below that. Does that sound like plan, and can you see any possible issues here with this design..? Many thanks.....
Mark R

[AllyCat]

Hi Mark,

Thanks Allan, what sort of thickness PTFE Teflon sheet are we talking..? And have you found PTFE Teflon in any everyday product, that is common/ accessible..? Where would one find PTFE Teflon sheet..???

PTFE (Teflon is a brandname) sheet is available from several sellers on ebay. One problem is that they specify the mechanical properties but not the optical properties. So I bought a small sheet to test. This is the seller I chose for two reasons: They are local (to me) and they specified the material grade (G400). I don't know what "G400" actually means, but it may help to get the same next time!

When the (1mm) sheet arrived I was pleased to discover that its light tranmission is almost exactly 50% (i.e. half the light gets through to the sensor. For my particular application I (might) want to reduce the light to 1/4 or 1/8, so can use 2 or 3 sheets. Note that if the (cut) "sheet" is small, then some light may be received through the edges, which gives an opportunity to modify the reception angle to a true "cosine" response. I believe that's one of the reasons why it's an excellent material to use in "professional" applications.

The seller above doesn't appear to export to Australia (or anywhere), but you may find another one in Australia or China. However, whilst checking, I noticed that the most commoin "filler" of PTFE is glass, so maybe that's what the "G" stands for, and explain why the optical properties are (or may be) very good.

On a more general point concerning "windows": It's important to understand that some light is reflected at every interface between air and glass (or any material with a refractive index more than unity) and thus does not get through to the far side. Furthermore, the amount reflected depends on the angle of incidence, which is why so much attention is paid to achieving the correct "Cosine Response". I believe that Werk suggests the window should be as close as possible to the PV panel. But strictly any air gap should be filled with a liquid/glue that has the same refractive index as the glass (of the window and/or the PV panel) to avoid "unnecessary" reflections at any surfaces. However, as I understand it, PTFE largely overcomes this problem because its diffusing action gives a cosine response and thus the directionality of the sensor becomes much less important.

Cheers, Alan.

[Wizza]

Thanks for the reply and the info Allan. All seems to make sense. I guess from here all I can do is construct the idea, make sure its weather tight, and then run tests and calibrate.

Do you think heat buildup inside the enclosure will effect the sensors and the readings..??? I was thinking about the possible heat issue last night, and if that will be a problem with this design/idea..???

Cheers for now..
Mark R

[AllyCat]

Hi Mark,

My prototype experimental solar sensor happens to also measure its own internal temperature, so I could write quite a lot, but I'll try to stay on-topic: Even in moderate UK sunshine its internal temperature rises by about 30 degrees C. It's basically a black "box" with a PV panel on most of its top face (used only to charge the battery, the sensor is a small BPW34). However, it heats up significantly more (in sunlight) than a white-boxed temerature-sender which I used for calibration. Therefore, a white box might be preferable; but black plastics are said to be more resistant to UV !

I was interested to discover that another "application" of PTFE is for light reflectors. Maybe that's for its high heat tolerance (e.g. from filiament lamps), but even a few mm thickness of the sheet that I tested will reflect much more light than it transmits. So perhaps a PTFE "wndow" will also help to keep the sensor/electronics cool?

But back to your question: It's well-known that the output power from all PV panels falls considerably when they heat up. But I believe this is almost entrely because the (maximum) output voltage falls; the current stays almost the same. That is one of the reasons why solar light sensors are designed to measure the (short-circuit) current from the sensor/panel.

So, in summary, I don't believe that you need to worry too much about errors due to overheating. But most electronic components "dislike" heat (above a moderate temperature), so it would be wise to keep the solar heating as low as practical. It is of course much less of an issue in the UK, compared with Australia (or even Portugal).

Cheers, Alan.