Date: 12/29/95 07:40:17 AM
To: Aquarium FAQ list 
Subject: Ultraviolet Sterilizers / draft

Here is a draft of an addition to the filter FAQ for UVS.
I tried in vain to keep it short, it is 2k long.

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Ultraviolet Sterilizers

In especially sensitive aquaria, infections with parasites, fungus, bacteria 
and virus can cause serious problems.  High intensity ultraviolet light 
destroys the DNA in living cells and can be an effective means to control 
living pathogens.  This type of water treatment is most beneficial for 
breeders (as it can help control infections of incubating eggs), for 
centralized multi-tank filtration (to control the spread of disease between 
tanks), and for delicate and/or costly setups such as large tanks and reef 
systems (as an safety measure).  Most aquarists who practice prudent 
quarantine procedures for newly acquired fish don’t need a UV Sterilizer.

To be effective, UV Sterilization (UVS) must expose the germs to high enough 
light intensity for a long enough period of time.  Martin Moe cites 35,000 
to 100,000 microwatts per second per square centimeter as the norm.  The 
most effective UV light is the high energy UV(C) light roughly at the 
wavelength of 250 Angstrom.  Common problems which can reduce the kill rate 
of the germs are: 

1) Allowing the water to flow too fast past the UV light.

2) Light blockage due to a build up of salt deposits or 
   bacterial slim on the bulb.

3) Fading of the light due to age of the bulb (which 
   typically have a six month life.)

The same property of this light that kills germs can damage your eyes, and 
special care must be taken to avoid direct or indirect eye contact with this 
light.  The UV(C) light does not penetrate water very well, so to be 
effective, UV Sterilizers commonly position the UV bulb close to the water 
which also can pose a risk of electrical shock should the bulb break, etc..

There are three types of UV Sterilizers:

1) Tray type. (Typically homemade) with UV bulbs suspended in a reflecting 
fixture over a shallow tray of slow flowing water.  Benefits; easily 
cleaned, can be cheap, can be made large enough for commercial applications.  
Problems; safety risks to your eyes, too large and awkward for many home 
uses.

2) Tube type, wet bulb.  Tube types have the benefit of exposing all sides 
of the fluorescent tube to water with no reflector.  The water passes 
directly past the bulb which is mounted in a waterproof tube.  Benefits; 
cheap, compact & effective.  Problems; difficult to clean the slime 
accumulations from the bulb, safety risks due to electrical shock.

3) Tube type, dry bulb.  Similar to above, but the UV tube is surrounded by 
a quartz tube [glass blocks UV(C) light] insulating it from the water.  
These are more expensive and probably safer.  Changing the light bulb is 
more easy, and dry bulb tube types can have a internal device to wipe slime 
from the quartz tube.