PAR Meter Data

I got first crack at the local plant club PAR meter, so yesterday I made a simple plastic holder for it so I could easily hold it in position in the tank. It is just an acrylic rod, with a piece of acrylic tube glued to the end, with the rod bent in the oven.


Today I took some readings on my 45 gallon tank:


The tank has a DIY light fixture housing a 2 x 55 watt AH Supply light kit, with GE9325K bulbs.

My data is:


I am surprised and very pleased by the evenness of the lighting over the substrate, and the intensity is about what I was aiming for when I raised the light. This is the tank with the RFUG filter, with CO2 enriched water from the canister filter going into the inlet, so it flows upward under the plants. The plants are all growing slowly, with only one bulb installed. Today I installed the second bulb and will leave it installed.

 

Today I did some testing on my 10 gallon tank, which has a Home Depot Hampton Bay 27 watt PC desk light, heavily modified into an aquarium light fixture. They look like:


Continued on next post:

 

The 10 gallon tank has grown a little GDA on the glass, so first I tested it with the glass "dirty". Then I cleaned off the GDA and did one more test to see if the improved reflection off the glass adds to the intensity. It did, but the water was not yet clear, since I did a water change too. Tomorrow I will retest to see if the intensity goes even higher. The difference between the dirty glass number and the clean glass number shows the minimum boost we get from reflected light off the front and back glass. Minimum, because the glass wasn't really all that "dirty".

Here is the data, in tabular form and plotted on log-log paper:


This, I find absolutely fascinating! For this light fixture, the light intensity truly does drop with the inverse square of the distance to the bulbs, and it doesn't matter if the distance increase is in the water or in the air above the tank. If this can be repeated with other light fixtures it points the way for estimating how far we need to raise a light fixture to reduce the intensity at the substrate the amount we want. And, it helps in estimating the effect of tank depth on light intensity.

 

I just repeated the measurement at the center at 1inch above the substrate, and it is now 79, after the water cleared up from cleaning the glass, and doing a 50% water change yesterday. So, with slightly fogged glass from GDA I had 70 micro mols per sq. m per sec, and with clean glass I had 79. That means at reflection off the front and back glass increases the intensity by at least 12%, probably more. This intrigues me. So, I just cut a couple of pieces of clear acrylic sheet to 9" x 12", and spray painted one side with flat black paint. When they dry thoroughly I will slip one down against the front glass and one against the back glass to see how that reduces the intensity.

If the light reflection off the glass were not significant, a 4 foot long fixture would produce the same intensity right under the center of it in any size tank of the same depth. A 55 gallon tank or a 75 gallon would both have the same light intensity from that light.

 

With flat black acrylic panels at the front and back of the tank, the intensity was 69 micromols per sq. m. per second, and with the panels removed the intensity was 81, just about the same as I measured this morning. I turned the panels around so the shiny side was towards the water, and the reading went to 72 (I'm not sure what that means.)

Taking the numbers at face value, that means the intensity was increased by about 17% by reflection from the front and rear glass. And, it suggests that even a haze of GDA on the glass drops the light intensity by almost as much as having the inside of the glass painted flat black. Of course the accuracy of these measurements is such that the 17% could be 12%, but the effect is definitely there.

 

Even with my linear tube AH Supply setup I got much better consistency with light over the entire substrate when I raised the fixture. So, no argument about that. The little 10 gallon fixture is one I made just to prove to myself that I could do it, making use of an otherwise wasted desk lamp. It would probably work very well on a 5-7 gallon, more nearly cube tank, but isn't good on the 10 gallon - the ends of the tank are really poorly lighted. After I finish experimenting with the 10 gallon tank again, I expect to try to sell the light fixture and possibly the tank.