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UV Sterilizer Bulb

Do UV bulbs degrade with time and use?

Most UV manufacturers boast that their lamps use bulbs that degrade
more slowly than other brands.

UV bulbs send an arc of electricity through a vapor of mercury causing
it to emit photons.  The frequency of the photons depends on the gases
in the bulb, the temperature of the bulb, and other things.  Generally,
a small percentage of a low pressure UV bulb's energy is expended in
the useful UV range.  For a 15 watt lamp, I recall the amount being
about 6 watts of actual useful UV light and the rest are photons at
other frequencies outside that UV range, including all the way down at
infra red (i.e., heat).

This percentage of useful UV supposedly degrades over time even though
no phosphors are used in a UV to alter the frequency of the photons. 
In ordinary florescent lamps, the the phosphors basically slow down the
photon towards the "normal" visible range (actually they absorb the
photons and then emit other photons at slower frequencies).

The amount of useful UV, rate of decay, and the amount of exposure that
is necessary in an aquarium, for example to kill protozoa in the water,
is subject to some debate -- for example, does a repeated exposure
means a lower dose each time is adequate?  How long does it take for a
given pump to put all of the water in a given tank through a UV lamp. 
The best formula I have seen assumes the return water mixes perfectly
with the other water in the tank without any interference--a perfectly
unrealistic assumption.  So the useful life of bulbs at given wattages
and given water flow rates varies as reported from one manufacturer to
the next.

Scott H.

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