I'm trying to refine my DIY PAR meter, which uses a cheap lux meter as the starting point. One of my goals is to have a waterproof sensor, so it can be used in water filled aquariums. My design has evolved to the point where I am casting the parts with epoxy resin, containing either black or white pigment - black for the housing, white for the diffuser. At this point it works great, better than any I have made before, except
when I check it in water it reads about half what it should read, based on the reading from an Apogee PAR meter. This happens with even 1/16 inch of water over the top of the sensor!
I tried coating the epoxy diffuser with clear nail polish (acrylic), but that had no effect at all. I tried putting a piece of plastic wrap (saran) over the diffuser, held in place with a rubber band. It read correctly that time
. I repeated this 3 times after trying other things, and it worked correctly every time. I tried a shiny finish on the diffuser and a matte finish on the acrylic nail polish, and there was no difference.
So, I'm stumped.
Suggestions as to why this is happening, or how to get around it are very, very welcome.