The Planted Tank Forum - View Single Post - Convert Lux meter to PAR meter?
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post #127 of (permalink) Old 09-22-2012, 07:04 PM Thread Starter
Planted Tank Guru
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Join Date: Dec 2005
Location: Sacramento, CA
Posts: 20,304
OK, the way we learn is from our mistakes! Today I tried my new waterproof sensor in the noon sun. It doesn't work well. The acrylic half sphere on the end is much too sensitive to the angle to the light. I could rock the sensor back and forth towards the sun and a little off from towards the sun, and it made a big difference in the reading, ranging from about 450 to 1500 with only maybe 20-30 degrees of movement. That makes it unusable if any accuracy is needed.

If I had listened to mistergreen I wouldn't have used the half sphere, but used just a flat disc as the measuring end of the sensor. One other bad effect of the half sphere is the magnification it gives, which just adds to the difficulty in reducing the reading by a factor of 60 to get it to read PAR instead of lux. I now think that it would work best to get a white acrylic 1/8" thick end for the sensor. But, I don't really need a waterproof sensor, so I may just let this go. (But, I may also get the PAR fever again and try one more time later!)

EDIT: Well the fever came back in about 3 hours! I realized I might be able to hacksaw off the top of the sensor, and reuse the rest of it with a flat top. So, I successfully removed the top, found the guts all in good shape, and then searched again for a better arrangement of filters. I now have it back together with new filters, a flat acrylic top, painted white with nail polish on the inside and lightly sanded to eliminate most reflection from the top.

When I did the final assembly and glued the top on, I accidentally left one filter out, so it reads high now, 35 instead of 25. I think I can glue another flat top on it, with another filter sandwitched between them to get back to the right reading. That will be tomorrow's project.


Last edited by Hoppy; 09-23-2012 at 02:56 AM. Reason: Added more info
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