So for all you uber-geeks out there here's the brain teaser of the day, and by no means do I have the answer which is why I bring it up. Anyways:
Greenwich Aquaria uses 400 watt Radiums set 12 inches above a 36 inch deep aquarium. I am trying 250 watt XM 20k bulbs set 8 inches above my tank which is 24 inches deep. Given the PAR readings from Sanjay's website, I'd like to estimate the ratio of PAR at 18 inches depth in the Greenwich tank (30 inches from the bulb) versus 8 inches depth in my tank (16 inches from the bulb). Intuitively I am assuming that PAR from a MH will drop off in proportion to the square of the radius, but when I try to convert that to a theoretical chart showing the expected percentage reduction in PAR for every 4 inches below the surface I get something like
surface = 1
4 inches below = - 50%
8 inches below = - 80%
16 inches below = - 90 %
and that doesn't make sense to me when I look at MH lighting charts showing PAR measurements for various depths. My estimate seems to drop off much faster than actual measurements.
I realize there are a myriad other factors that will get involved such as water clarity, bulb age, reflector, etc etc, but like the title says I'm just trying to SWAG this for fun as I intend to purchase an Apogee light meter soon and compare the light in my tank versus Greenwich Aquaria using raw data, and
I think it would be interesting to create a chart for estimating the PAR at various depths in a tank given a certain bulb/ballast configuration
So the question is:
Should PAR drop in proportion to the square of the radius and is that radius properly measured in meters and how do I convert that to a chart estimating the percentage reduction in PAR?
A hearty thanks to any that take the time to answer
Joe
P.S. I have already changed to 4 x 250 watt XM 20k bulbs for reasons I plan on discussing in a future thread after trying this setup for 90 days, but just for grins I'll throw in the pictures of 4 corals I took for comparison 3 months from now. Wish me luck!
