I recently converted a 20H co2 tank with 48 watss of t5 lighting to an excel tank.
I took out one bulb leaving me with 1.2 wpg. Is this on the low side for an excel tank ?

If I used 2 bulbs and raised the fixture I could get more light and better coverage .

If I use the inverse square law will this give me a ballpark idea of how high to raise the lights ? I believe the inverse square law applies to a point of light and assumes light is traveling through air rather than water and tank lids.

Can I use this approach as a start point to calculate how much to raise the lights or is there some other better method ? If 1.2 wpg of T5 lighting is adequate then maybe it's not worth the bother and wasted light.
I .

I assume your 20H tank is 17 inches tall vs about 13 inches for a standard 20 gallon tank. Is that right. Based on that the light intensity at the substrate level, assuming the depth of the substrate is about the same as the height of the bulb above the water, is between about 13/17 (75%) or (13/17)squared (60%) of the intensity the light would give on a standard tank. That means you now effectively have between 1.4 and 1.8 watts per gallon, as far as carpet plants are concerned. I think you could leave both bulbs in use, and have about the right amount of light without raising it. Of course if the T5 fixture uses good individual reflectors for each bulb, the effective intensity will be about 1/3 higher, or about 2 to 2.4 watts per gallon, which I think would be too much for an Excel only tank. Then raising the light fixture about 6 inches would drop the intensity back to the effective watts per gallon of 1.4 to 1.8.

Linear bulbs like T5's should lose intensity at closer to one over the distance from the bulb, than 1 over the distance squared. But, as you get further away from the bulb the relationship gets closer to the inverse square one.

All of this means, I think, if those bulbs have individual reflectors, you should raise the fixture about 5 to 6 inches. (Isn't it a good thing this isn't rocket science?)

Vaughn,

My tank is 16" deep. The light is a twin tube current fixture with a parabolic aluminum reflector. It's decent but not AHS quality.

I did the following calculations on my tank. Distance from bulb to substrate is 14". If I use the square of the distance then if I move the fixture up 2" then I have about 75% of the intensity compared to the fixture located on the tank.

If a linear light source falls off closer to the ratio and not the square of the distance then I raise the light up 4" to get 75% of the intensity.

I simply used the general wpg rule and said I now have about 1.8 wpg. I don't know how much light is actually falling on the substrate so I did not know what else to do.

I'm not clear why you compared the 20H to a standard 20. A standard 20 would be a useful comparison if we new how much light was being transmitted to the botton of a standard tank. Were you thinking that the wpg rule applied more accurately to a standard tank of "standard" depth and then adjusting for a deeper tank ?

Henry

The watts per gallon "rule" seems to apply reasonably to standard shaped tanks that are 20 gallon or more in volume. So, that's why I used a standard 20 gallon tank depth as a comparison. 2.4 watts per gallon on such a standard tank would be high light intensity if the bulbs are PC bulbs with a reflector as good as the AH Supply reflector. That is roughly based on my experience. Starting from there: your tank is about 14 inches from bulb to substrate. A 13 inch deep, standard tank would have the bulbs about 3 inches closer to the substrate. So, using the linear drop off of light from tubular bulbs, you should have the equavalent of 11/14 times 2.4 watts per gallon, or 1.9 watts per gallon. I would consider that moderate light intensity, except that your T5 bulbs are brighter than AH Supply PC bulbs. Based on that I would guess you have the equivalent of around 2 watts per gallon or a bit more. Raising the lights to get that to the equivalent of 1.8 watts per gallon would mean raising them to 2/1.8 times 14 inches, or 15.5 inches, so you might want to try raising them 1.5 inches.

This is nothing but crude estimating, but at least it tells you that you don't want to raise them a foot. And, it gives a feeling for the logical amount to raise them.

Things like this always remind me of back when I was working for NASA. One time one of my fellow engineers, with more experience and education than me derived an equation for how much heating a certain shape object would experience in a very hot high mach number jet of air. But, he needed some data to verify his equation. I did a test in a device I was trying to develop into a usable piece of testing equipment for just that kind of testing. I managed to get about one second of data, with not much accuracy, and gave the numbers to him. He fitted his equation, on log log paper to that single data point and wrote a paper about his method. I kept trying to tell him he couldn't do that, but he just said, "but I did". That was when I discovered the magic of log log graphing data - you can prove anything that way. That is what this playing with light intensity is like.

Ok Vaughn I follow. I understand these are crude measurements. I just want to get to a start point that gets me into the ballpark. Then I'll see how the plants and algae do and adjust as needed. Thanks for the info.