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Re: Useful lifetime of power compact fluorescent tubes?

Joe Kopanski wrote about new flourescents being so much brighter than
old ones:
> I recently switched out the first of a pair of Custom Sealife 27W
> tubes
> after eight months use.  My plans were to alternately switch a tube
> every 8
> months so that I would always have one tube less than 8 months old
> and one
> tube more than 8 months old. This assumes a 16 month useful tube
> life.  The
> difference in light intensity between the new tube and the other 8
> month old
> tube seemed fairly dramatic to my eye.  My plants seemed to be
> enjoying a
> growth spurt since the switch.
> Is there any real data on the intensity decay of these tubes?  How
> often do
> you change a power compact tube?  Do most people run them till they
> burn
> out?  I'm now thinking that a 12 month replacement cycle (that is
> change
> alternating tubes every 6 months) is a better idea.

There is a temporary initial steep drop off in light output with

It's easy to be fooled into thinking your old bulbs are too far past
their prime when you compare them to brand new bubls.  Flourescents
lose about 10% of their brightness in the first few hundred hours or so
(preciesely when depends on the manufacture --some say the first 1,000
hours).  So a new bulb that will seem a lot brighter than an olded one
that is only a couple of months old and still has plenty of good light
to give.  Wait a couple of months, and then compare -- in fact, try
this.  After a couple of months compare the 2-month bulb to a brand new
one -- the new one should seem much brighter.

BTW, the first initial steep drop tapers off and then the "decay" is
slow and steady over the bulb's life.  In fact, were you to cycle
off/on every three hours  (the way that the test is doen for the
"average life" ratings), the filaments would very likely burn out while
the bulb still had about 70-80% of its brightness.  But with 10-12 hour
off/on cycles, the filaments will last a much longer time, and so the
bulb will keep burning while it goes below 70-90% brightness.  So trust
the manufacturer's life span ratings--at the very least, don't cut it
by more than half -- unless you have lots of stock in the bulb maker
and it makes you fell better.

Scott H.

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