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Old 06-14-2012, 03:12 AM   #16
DarkCobra
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Curing cyanoacrylate emits fumes which can deposit a white substance on nearby objects, especially if they have any oil on them. It can and has been used by law enforcement to reveal fingerprints. But it's not corrosive.

Most hardware-store silicone produces acetic acid fumes as it cures. Which can be corrosive, like any acid; especially if trapped in an electronic enclosure. I know there's specialized silicones that don't release acetic acid, which can even be used in direct contact with electronics to totally encapsulate them; but I haven't used them and can't make a recommendation.

However, since you have an outer housing which screws on before any of these substances are applied, I'd think you can use anything that provides an adequate seal.

And as for the sensor cap, I think it's just a dust cap. I've never heard mention of any need to keep a photodiode in the dark most of the time.

Oh, and Mistergreen came up with an idea in the DIY PAR meter thread that adding a purple filter gel to reject some of the green, while passing a higher proportion of photosynthetic red/blue, might give results closer to that of a real PAR meter. The filter I see on your LUX meter appears green, doing the exact opposite of what you want. I doubt it's a UV filter, as photodiodes have poor response to that. They are however very sensitive to IR, and if it does remove any of the non-visible spectrum; that would be it.
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Old 06-14-2012, 03:37 AM   #17
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I have two sealing needs. One is around the joint between the cap and the box, or jar, both of which are acrylic plastic. The other is around the guts of the lux meter to hold it in place, and also to seal the pass thru hole for the electric cable. I was considering silicone for the second , and acrylic cement for the first (not methacrylic super glue).

My present plan is to leave the lid off the box until the silicone cures around the luxmeter guts, which would also reduce the volume of air in the box, but not in contact with any of the electric elements. Also a fillet of silicone outside around the electric cable. After the silicone cures, probably 2 days later, screw on the lid and apply high viscosity acrylic cement around the joint to seal it permanently. Would that adequately protect the diode, etc. from any fumes? And, do you think the limited adhesion of silicone to acrylic would be enough to seal up the electric cable hole - the cable fits pretty tight in the hole, so it is just the tiny surrounding gap the would be sealed.

Before I consider playing around with optical filters or other methods for changing the response of the diode, I want to try this as is, relying on calibrations using a PAR meter, for each type of lighting I will use it for. If that doesn't work well enough, then I can buy another lux meter and play with it further.
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Old 06-14-2012, 04:07 AM   #18
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Sounds like a good plan to me.

Out of curiousity, I fully disassembled the sensor on my Extech LUX meter. There are no identifying marks whatsoever on the actual photodiode. But here's the filter:



Close to what I'd expect from a LUX meter that's intended to mimic the human sensitivity to green. A bit more blue than expected, but that may be to compensate for photodiode response; which is probably biased towards red.

I also tested it for UV/IR pass. Only slight UV reduction. But it's a very good IR filter, I can't see the otherwise bright flashes from a remote control with my camera at all.

May come in handy later should you ever decide to play with alternate filters.
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Old 06-14-2012, 05:01 AM   #19
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Quote:
Originally Posted by DarkCobra View Post
Curing cyanoacrylate emits fumes which can deposit a white substance on nearby objects, especially if they have any oil on them. It can and has been used by law enforcement to reveal fingerprints. But it's not corrosive.
Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.

Quote:
Oh, and Mistergreen came up with an idea in the DIY PAR meter thread that adding a purple filter gel to reject some of the green, while passing a higher proportion of photosynthetic red/blue, might give results closer to that of a real PAR meter.
Shouldn't it be a green gel to deflect green out? Like chlorophyll although magenta makes sense.
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Old 06-14-2012, 05:10 AM   #20
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Quote:
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Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.



Shouldn't it be a green gel to deflect green out? Like chlorophyll.
The color of a filter is deceptive. The color can be the transmitted color reflected back from outside the filter, or it can be light reflected off the face of the filter. But, it is the color coming through the filter that is the important one. This filter definitely looks green, and I think it is light that passed through the filter twice that gives it that color. Another guess: it is to adjust the peak transmission into the green band, the peak at 550 nm shown on that little chart. It might even come closer to a PAR characteristic with no filter at all.
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Old 06-14-2012, 05:14 AM   #21
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Yeah, my photodiode peakes at 550 too but with some software & hardware fine tuning, it's pretty darn close to the real thing.
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Old 06-14-2012, 05:27 AM   #22
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Quote:
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Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.
Ooo, that's weird. The active elements of the photodiode are usually well encapsulated. Are you sure it was that? Is it possible it had a thin-film external IR filter layer which was reacting with the glue?

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Shouldn't it be a green gel to deflect green out? Like chlorophyll.
You specified a "green light gel". Filter gels work by absorbing certain colors rather than by deflection, so a gel that's green in appearance is absorbing red and blue; which would be counterproductive. So I assumed you really meant "a gel that filters out green"; which by absorbing green and transmitting red/blue would appear purple. I may have assumed too much.

Now if you'd specified a dichroic filter, which does work by deflecting certain colors instead of absorption, then the correct filter would appear green.
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Old 06-14-2012, 04:22 PM   #23
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I "potted" the sensor into the case this morning. It was difficult to get it to flow into all of the voids, so I gave up on trying to do that. There is enough there to hold the sensor in place, to seal the hole for the electric cable, and to greatly reduce the air volume, reducing "oil canning" due to temperature and barometric pressure changes. Now, a couple of days drying and I can cement the lid on.

The silicone added $4 to the total cost, now at about $28.
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Old 06-15-2012, 03:16 AM   #24
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I studied the "manual" that came with the lux meter. It says that these need periodic recalibrating, and that is because the photo diode sensitivity constantly drops, proportional to the product of the time the diode is illuminated and the intensity of illumination. So, that answers my question about why they always have caps over the sensor - to keep the sensor dark to slow down the deterioration.

After 11 hours I can still smell the ammonia given off by the curing silicone. This is the first time I ever cared about how long it takes, so it is probably typical. I decided to do the final cementing of the cap when the smell is gone.
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Old 06-15-2012, 03:48 PM   #25
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This morning the odor from the silicone cure is gone, so I screwed on the lid, after cleaning it, and put a heavy bead of Weldon #16 acrylic cement along the joint to seal it permanently. Of course, first I verified that it still works ok.

Tomorrow I hope to borrow our club PAR meter and calibrate it.
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Old 06-15-2012, 05:48 PM   #26
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Hoppy I am way excited to see what you come up with on this. Fingers crossed
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Old 06-15-2012, 06:57 PM   #27
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Old 06-16-2012, 03:59 PM   #28
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I think the only real unknown at this point is how well it is sealed from the water. That will be the last test, because if it fails the whole sensor is junk. Right now I have to wait to get the PAR meter so I can calibrate it.

If I ever do this again I think I will completely disassemble the sensor and fit the cosine correction dome and the sensor alone in a much smaller acrylic container. Sealing it remains a question.
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Old 06-17-2012, 08:04 PM   #29
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While I am stalled, waiting for access to a PAR meter, I've been thinking about the sensor part of the meter. Isn't that sensor just the photodiode mounted in something to hold it with the cosine diffuser over it? Can I buy another photodiode, the one mistergreen uses for his PAR meter, and package it, and use it in place of the sensor that came with my luxmeter? Obviously I don't know much about how these are hooked up.

If I did this, would it work?
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Old 06-18-2012, 03:11 AM   #30
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It won't necessarily be a match. The voltage output may vary from diode to diode so the readings will be off even though they read the same spectrum.
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