I may be wrong, but I believe that only incandescent bulb life is shortened
by turning it off and on due to the surge of electricity passing through the
filament. This surge weakens the filament by vaporizing pieces of the
filament. During normal usage, samll pieces of the filament detatch and
float freely in the bulb, but they are drawn back to the filament to settle
on it. It is the vaporization of the small pieces of the filament that
shortens the life of an incandescent bulb. I don't believe the same is true
for any flourescing type lamp, but I don't know as much about those types.

"Bruce Geist" wrote in message
...
Basically, you are only culling the population of algae spores and
bacteria
to a level, presumably, that is more like an open stream or lake. All
tank
water is never sterile at any one moment in time, since algae and bacteria
are constantly re-growing.

To control a specific organism or class of organisms, use the lowest
lethal
dose for that organism or class of organism, meaning you turn your flow
rate
up so that water passing though is exposed to the right level of
radiation,
and no more. That way, you are killing algae at the quickest possible
rate. The flow rate is effectively your control on the dosage you are
delivering. So, if algae is your target, I'd turn your flow rate to about
200 gallons per hour. This will deliver (I am estimating here) about
50,000
micro watt-seconds / cm^2 of radiation to the water passing through your
sterilizer.

Here is the shaky part; I am not sure exactly what dose will kill all
algae.
So be aware, I am making a swag here. I do know that Chlorella Vulgaris,
a
kind of algae, is killed by 45,000 micro watt-seconds / cm^2 of UV.
Therefore, I am recommending a flow rate that I believe will cause your
36
watt sterilizer to deliver the 50 K micro watt-seconds / cm^2 dosage. I
do
not actually have intensity data for your particular sterilizer, though I
have requested it from the manufacturer (because I am doing some
mathematical modeling for an article I am writing..) I have intensity
data
for a 30 and 64 watt UV set-up, so I am interpolating a result here.
However, the dosage estimate is really an estimate! Sorry I cannot be
more
precise.

Now its your choice whether to leave on the sterilizer or cycle it. I do
not believe the statement that cycling a UV lamp shortens its life
significantly. Maybe it does, but I find it hard to believe. In fact,
the
lamp manufacturer defines its useful life in terms of hours on, not number
of cycles. If you leave the thing on all the time, the lamp will last
about
6 months. If you use it only during the day, my expectation is that it
will
last a year. Since your target is algae, maybe its enough to sterilize
when
the algae is multiplying during your tank's lighted period.

Do some experiments, and see if you perceive a difference in water quality
and/or algae proliferation when cycling and not cycling. And then let me
know what you conclude! I'd really like to know what your experience
turns
out to be...

-Bruce

http://www.wideopenwest.com/~brucegeist

"Christopher" wrote in message
rthlink.net...
Thats a very detailed explanation, THANKS!
have a couple questions though:
if the primary reason I'm using the steralizer is to kill algae spores
so
my
tank stays clean is it smart to only have the steralizer running for X
hours
a day? Cant the spores be released and land and settle on the glass and
leaves during the off period? My second question is how much life of
the
bulb is saved when you run it for 8 hours a day? I know that the turning
on
of the bulb takes away from bulb life, but would the fact that its only
run
for 1/3 of the time save bulb life in the long term? I am concerned
since
36W PC florescents are $50 (not exactly cheap)

"Bruce Geist" wrote in message
...

Actually, the 9.2 factor will give you 99.99 % throughput through the
sterilizer. To get 99.9% throughput, the factor is 6.9.
See http://www.wideopenwest.com/~brucege...ver/index.html
for details.

One strategy is to set the flow rate through the sterilizer so that
one
pass
kills target organisms you wish to kill. If you set the flow rate
slow
enough that everything you want to kill is killed, then run your
sterilizer
long enough to "turnover" the tank water once or twice a day,
and then shut the sterilizer off.

Basically, the more slowly you put water through the sterilizer, the
more
UV
exposure every bit of water passing through the sterilizer receives.
If
you have a 36 watt sterilizer with about a 3 inch diameter chamber for
the
water, and your flow rate is about 100 gallons per hour, then you are
probably irradiating your water at about a 100 microwatt-seconds dose,
which
is
enough to kill most bacteria, algae, and many protozoa. (This is only
a
crude estimate; I am working on developing some assessment tools on my
website which will provide more specific advice..) If you are using a
lower
wattage sterilizer, or a lower diameter sterilizer, then your flow
rate
will
need to be lower still to achieve the same dose.

In the article I wrote cited above, I show that when you have run
99.9%
of
the water through the sterilizer at least one time, in actuality 99.2%
of
water has passed through 2 or more times, and 96.8% will have passed
through
the sterilizer 3 or more times! I recommend 6.9*(tank size in
gallons)/(flow rate in gallons) to be your target turnover time. If
your
flow rate is chosen with some care, you need not run your sterilizer
all
the time. You will save bulb life by running your sterilizer through
only
one or two
turn-over intervals per day.

I hope you enjoy the article on turnover time; this is the first time
I
have
published the web address. Remember that you can skip to the
conclusion
and get the formula to use. I am in process of collecting data for a
suite
of articles on the topic UV sterilizers. Be watching this news group
or
my
web site; I am waiting on data from a manufacturer. -Bruce Geist

"Christopher" wrote in message
rthlink.net...
is it wise to run the UV steralizer 24 hours a day? assuming that
you
only
desire a 99.9% steralization rate you don't have to run the
steralizer
that
much (depending upon gph through the steralizer and tank size).
I've
read:

9.2 * (tank size / gph through steralizer) = hours a day UV
steralizer
must
be on to attain 99.9% steralization rate (assuming uniform mixing).
This
said my calculation is at most 7 hours a day, should I just run it
for
8
hours a day and save my bulb life and power bill?

Should I run it 24 hours a day to achieve 99.999% steralization or
for
8
hours a day?