[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Erik's lighting results

>Subject: Erik's lighting results
>I agree with Frank that Erik's survey results are thought-provoking and an
>interesting contribution to the hobby.  I have some observations and

Ditto from me. Good job Eric. A real thought provoker.

>	First, the difference between the data from Amano's books and the
>Usenet data is striking.  I've only read one of Amano's three volumes and
>my recollection is that the smaller tanks feature a lot of stem plants
>with very dense plantings that shade each other, and that the larger tanks
>feature more moderate and low-light plants, with relatively little
>shading.  If my recollection is true and his other books are consistent,
>could the trend in Amano's data be a product of his unique style?

I'd agree with Rogers observation. Just looking at the Usenet data, It seems
that the 2 watts/gal to 4 watts/gal better fit the data than the "estimated 
surface area" (Volume to the 2/3 power).  As Roger pointed out, Amano data
may be skewed to his style of planting.

>	Second, when I visualize a trend through the four data points from
>commercial fixtures it parallels the trend in Amano's data, but is offset
>downward by a factor of about 4.  Commercial fixtures I've seen are
>usually single-tube lights, equipped with whatever tube is close to but
>shorter than the tank the fixture will fit on.  So the trend (if there
>really is one) is determined mostly by the *length* of the tank and the
>wattage of standard fluorescent tubes, not by area or by volume.  You
>might create the trend in Amano's data just by using the largest 4
>commercial fixtures (or a fixture with 4 fluorescent tubes) that fit over
>a tank.

Again, another very good point. With fluorescent lighting, you can only put
in bulbs that can fit over the top (surface area) of the tank. Commercial fixtures
would be skewed to the cheapest solution that would light just across the length
of the tank.  (i.e. One tube that fits most of the length of the tank.)

Perhaps there's a better equation to fit the data.  Eric, is it possible to
plot Watts versus actual surface of the corresponding tank where the data
came from?  It seems to me that estimate surface area by taking the volume to
the 2/3 power applies if all the tanks have about the same ratios of length:width:height.

Where's this all heading? Well, here's my 2 cents worth of contribution:

0. Assume that the lighting intensity is always less than the plants light saturation
levels.  (Light saturation levels for most aquatic plants is around 10% of full sunlight.)
(A lot of lighting [i.e. mucho watts] is needed to reach this lighting level.)

Also, assume that plants need about 12 hours of little or no light for their rest period.

1. A plant's mass (size & weight) is proportional to it's leaf surface area.

2. A plant's growth rate is proportional to the surface area of it's leaves.

3. A plant's growth rate is proportional on the amount (intensity) light shining on it's 

Therefore, the taller a plant gets, the more total amount of light the plants needs.
As the plants leaves grow wider, and the plant spread out across the gravel, the more
total light the plant(s) will need.

Also, a plant will grow in height and spreads out until the growth becomes limited by the
available light.  Which is another way of saying that a plant stops growing if it doesn't
get enough light.

4. The volume of space that a plant occupies is proportional to the mass (size) of that plant.

5. The total amount of light is proportional to the intensity (wattage) of the light bulbs and
   how long the lights are on.

Therefore, the final maximum amount of all plant mass (size/weight) is proportional to the
tanks volume and lighting watts.  (With light duration being fixed.)

And, also, the growth rate is proportional to watts and how much space is available.

The experience of the Usenet community is that 2-4 Watts/gallons give adequate growth for the
types of plants of interest. Some types of plants need more light than others.

6. For fluorescent lighting, the number of light bulbs that you can use is proportional the 
tank's top surface area.  (The situation is quite a bit different for MH lighting. In MH 
lighting, the light is concentrated over a much smaller area compare to fluorescent.  It's also
fairly easy to reach the light saturation levels with MH lighting, which complicates any
analysis such as this.)

Therefore, for fluorescent lighting, growth rate is also proportional the tanks top surface area.
However, this rule is just a limitation of fluorescent lamp technology. Plants really just care
about light intensity & duration. Of course light quality (PAR etc...) also has a roll.


IMHO, either watts per gallon or watts per surface area will give you an estimate on how
much light you need. For fluorescent lighting, certain size tanks lend itself to putting more
lamps on top of the tanks.  How many tubes you need depends on the type of plants you want to grow
and how often you want to trim them.

Comments would be appreciated.  To the logical purist, I was a little foot loose and fancy free on
the above derivation.  I could have been even more formal which would have made it even more
unreadable.  Of course, any statement is only as good as the assumptions that the statement is
based.  So, see if you agree with my assumptions.


While we are on the subject of proposing fluorescent lighting rules of thumb.  Here are mine:

For fluorescent lighting that extends across the length of the tank (that are 16 to 24 inches
      deep), 2-3 watts per gallon is recommended. (Depending on the type of plants you want to grow
               and how often you want to trim them.)
      1 tube every 3 inches of tank width gives about 3 watts/gallon.
      1 tube every 4 inches of tank width gives about 2 watts/gallon.

Standard pet shop lighting, (which is about 1 tube for every 12 inches tank width,)
                             gives about 3/4 watt/gallon.


For 20 Gallon High tank (12 and 1/2 inches wide and length of 24 inches):
                    4 tubes (15 watts x 4) gives you about 3 watts/gallon.
                    3 tubes (15 watts x 4) gives you about 2 watts/gallon.
For 55 Gallon tank (12 and 1/2 inches wide and length of 48 inches): 
                    4 tubes (40 watts x 4) gives you about 3 watts/gallon.
                    3 tubes (40 watts x 3) gives you about 2 watts/gallon.
For 90 Gallon tank (18 inches wide and length of 48 inches): 
                    6 tubes will give you about 3 watts/gallon.
                    4 tubes will give you about 2 watts/gallon.

Exceeding 3 watts/gallon using normal T12 fluorescent lighting becomes difficult. Heat build up and
reflector efficiency/tube spacing become problems. T8 bulbs and electronic ballast can stretch this
rule a little, but Metal Halide (MH), VHO, "HO T8" or compact fluorescent lighting become more 
practical at lighting levels above 3 watts per gallon.  It is possible to successfully grow certain 
type of plants at 3/4 watts/gallon, but your going to have problems finding those type of plants at
your local fish shop. Tanks deeper than 24 inches have problems of being difficult to maintain 
(Unless you have 3 foot long arms) and fluorescent lighting doesn't have enough punch (light intensity)
to get through the taller plant leaves that will tend to shade the shorter plants.

Ron Wozniak  Allentown PA, USA
rjwozniak at lucent_com
AGA member