hitmanx, you can just prop the light across a couple of chairs and measure the PAR out in the air, at the same distance that they would be from the substrate. That reading will be very close to what it is in the tank, in water. If anything it is a big lower than what you get in the water.
Both the 8.5in and 10.5in domes completely surround the Philips 23w 6500k cfl I am using so all available light is being reflected down. Are your par meters water proof? Could place them directly into my existing tank? I guess this is the only way I will actually know what kind of light levels I am dealing with
Yes, I did some testing with at 10.5 inch dome reflector. But, I have not compared the light output from the two sizes of reflectors. It isn't an easy thing to figure out, because the smaller dome doesn't completely surround the larger wattage CFL bulbs, making it capture less of the stray light. Intuitively, the 8.5 inch dome should give a higher PAR than the 10.5 inch one, for small CFL bulbs which don't extend out of the dome profile. But, when a bulb does extend out of the dome profile there is a big loss of light.
The higher you raise the light the larger the circle of light gets, so the lower the light per unit area gets. PAR is light per unit area. If you want to be sure to have low enough light, just raise it 2 inches instead of one inch. Then, if you still have difficulties with BBA you can either dose Excel at 2X the recommended dosage - or 2 ml per 10 gallons every day - or you can raise it another 2 inches. Without a PAR meter you can only do this by trial and error.
After 4weeks small tufts of BBA are starting to show up on dying leaves at the bottom of tank, dwarf sag, and on the anubias petite. I removed as much as I could and raise raise the light 1 inch.
Now my question is will a 10.5 inch fixture just extend the lighting footprint outwards or does it alter the available par values enough to warrant raising the light? The 8.5in is giving a spotlight effect with plants growing up and inwards.
The algae that you want to look for is black brush algae, BBA, which will be black tufts on non-growing leaves, and on hardscape. As soon as you see any, remove it, and raise the light an inch or so. Keep doing that until either you see none, or the rate at which you see it is easy to take care of.
Okay thanks for your help! I'll start there and tweak the height... what kind of things should I be looking for that would tell me to raise it? How quickly can algae bloom? This will be first tank in about 20years, so I just don't remember
I used the data from i4x4nMore, which I plotted on log-log graph paper to make extrapolating easier. I suggest starting with the light at the top of the tank, but being ready to raise it a couple of inches if necessary. I stopped making PAR meters, from boredom. I'm still looking into the possibilities of using the process described in http://www.plantedtank.net/forums/sh...d.php?t=453225 to make more of them, with less boredom.