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If I have a screw-in fluorescent bulb with a 17 watt input, but a 60 watt output, I would count the 60 watts when calculating watts per gallon right?

You count the "actual" wattage of the fluorescent bulb; in this case, it's 17w. The 60watts is this bulb's equivalent output with an incandescent bulb- don't use this number.

Last edited by csfish at 04-Apr-2005 22:49

Hi,
Actually you do use the 60 watt value when figuring the
watts per gallon. 17 watts of power is what the bulb
consumes producing light, and because of the twists
and concentration of light in a short length, the output
is similiar to that of a 60 watt bulb.

Watts, as a measure of the amount of light are a
sort of "common man's" measure. As we grow up we all
come to know how much "light" a 15watt, 60 watt,
100 watt bulb emit. We can see the difference,
and because we use them every day it becomes a point of
reference for us. Light is better measured in lumins
with a photo electric meter. However a water proof one
that you could lay on the bottom of your tank to tell you
how bright the light was, would be very expensive.
SO, we stick with something we all know, watts.

Frank


-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--

Ok. Thanks for clearing that up for me.

Hi Kingyo,
I have to say that I strongly (and politely) disagree with Frank. If you use the "equivalent to X wattage of incandescent bulbs" rating to figure out your watts-per-gallon for your tank, you will be severely under-lit and your plants will not survive. The general guideline on all planted tank/aquarium sites on the internet and aquatic plant books is to use the actual wattage of the bulb (and ballast combination), but as Frank alluded, you must also consider other factors. These include lumen rating, spectral output, colour temperature, and perhaps colour rendering index, as well as cost. Check out Chuck Gadd's article about lighting (as well as his other excellent articles) to get a handle on planted tanks.

(I'm assuming that you're asking about watts per gallon and fluorescent lighting in regards to a planted tank.....)

http://www.csd.net/~cgadd/aqua/ah_main/mar1998.html

Last edited by csfish at 05-Apr-2005 22:47

Sorry, that was Karen Randall's article on Chuck Gadd's site...

Hi
Debate is good. I've read the article by Karen and I
believe you are addressing this part of her article:

" A standard shop light with two tubes uses 94 watts --
40 watts for each tube and 14 for the ballast. It produces
5000 lumens. Each tube (assuming Vitalites, one of the
least expensive full-spectrum tubes) costs at least $13,
and tubes fall off to about one-half the rated lumens by
the one year mark.

A two-tube fisture (using GE's SPX 50 T-8 tubes with an
electronic ballast) runs at 60 watts (32 watts per tube
and the ballast runs at 95 percent, which accounts for the
additional 4 watts). It produces 6000 lumens. Each tube
costs under $10. These tubes drop about 5 percent in lumens
in the first 100 hours of operation, but the rated lumens
are after that period. The drop off over the remaining life
of the tube (average failure time is 20,000 hours) is only
to about 80 percent."

In this selected quote, Karen is showing several things
about flourscent lighting.

The costs associated with it.
One uses 94 watts of electricity, from which you can
determine how much it would cost to run it for a year.
The other uses only 60 watts, and thus is a savings in
money.

The Burn in Rate, or Drop off Rate.
Every flourscent bulb starts off radiating light within
a specific spectrum and at a certian number of lumens.
As the phosphors age, they are not as active and both
the spectrum and the output drop off. The first 100 hours
are considered the "burn in" period, after which the
decrease levels off a constant decrease over the life
of the bulb. In the first case (Vitalites) after one year
of operation they output had decreased by 50%. On the
other hand if you spend the extra $$ and purchase the
GE SPX 50 T-8 bulbs, they drop off only 5% after 100 hours
and are then good for an average of 20,000 hours! Combine
that with the reduced power consumption of 60 watts vs 94
watts, and clearly the GE bulbs mentioned are the better
deal.

No where in that article does she say to include the
total power consumption of the bulb and the ballast in
computing the watts per gallon. The way to compute the
number of watts per gallon you have over your tank is to
total the wattages of the bulbs, and divide that total
number by the capacity of the tank.
For instance, two 96 watt compact flourscents over a
30 gallon tank = 6.4 wpg. Or, two 40 watt bulbs over
a 55 gallon tank = 1.4545wpg. The "formula" has nothing
to do with the ballasts, only with the output of the bulbs
themselves. The total power consumed creating that light
is of use only for computing the costs of operation (how
many KWH is consumed/day or /year.

The thrust of the article
is to:

Use light in the amounts of 2-3 watts per gallon (more
if necessary in tanks over 20 inches deep).

Use a Kelvin rating of lights between 5000K to 6500K
( prefer to use a more popular 6700K bulb)

Use bulbs that have the highest possible CRI vs cost

Use bulbs that come closest to natural midday sunlight

Use Full spectrum bulbs such as DAYLIGHT or SUNLIGHT labeled
bulbs instead of "warm white, or "cool white."
(The latter two are designed for reading and not for plants)

Rotate lights (replacing older bulbs) at the one year point.
Doing this keeps the quality of the light within acceptable
standards.

Keep a photo period (lights turned on) for 10-14 hours.

Keep your tank cover spotlessly clean.
Keep the water level as high as possible to reduce light
loss.

Frank

-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--

Hi All,
In answer to Kingyo's enquiry, using the "actual" wattage of a fluorescent bulb (compact or otherwise...) is just a starting point. As you pointed out Frank and in Karen Randall's article, there are many other considerations. I mention ballasts because many DIY'ers build their own lights and sometimes use a ballast combination that "overdrives" their fluorescent bulbs to create a brighter output.

For example, http://www.plantedtank.net/forums/showthread.php?t=16098&amp;highlight=ODNO