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Date:	Tue, 22 Jan 2002 23:02:12 PST7
From:	"Benedict G. Archer" <bgarcher@gte.net >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: Monitor Glitch

Content Type: text/plain

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Peter Skye wrote:
> Benedict G. Archer wrote:
> >
> > Incandescent light bulb life is a function of time
> > on and operating temperature. Lifetime measured
> > in time on is the same whether on continuously
> > or cycled on/off at ~1 Hz or any slower rate.
>
> Ben, I'm curious about the study you're referring to. The studies I've
> read (although I can't cite any of them) and my personal experience both
> indicate that incandescent bulbs are most likely to fail at power-on.
> The studies said it was because of thermal shock.
>
> For example, I've changed the 75w bulb in my desk lamp several times --
> and each time the failure has been when I turned it on. A lamp has
> never suddenly "gone out" while I'm using it. (I have had incandescent
> lamps do that, but rarely.)
>
> I'm always willing to learn. Can you point me at a study that shows
> these lamps aren't more likely to fail at power-on?
>
> (Tangent: I extended the life of the incandescent equipment lamps (such
> as vu meter lamps) in the studios by running them at low voltage,
> typically 10%-20% lower than their "spec" voltage. Neumann record
> cutting lathes dropped the voltage even more -- iirc they ran 12v lamps
> at 9v which is a 25% drop. Architectural lighting is sometimes run at a
> lower voltage to reduce the maintenance cost of constant relamping.
> Theme park special effect lighting is sometimes run at a lowered voltage
> for the same reason. I've seen manufacturer charts on lamp life vs.
> voltage applied as a % of spec voltage and the change is both non-linear
> and quite dramatic.)
>
> (Another tangent: The rated life expectancy hours are printed on the
> box that the 120v incandescent lamp comes in. (Low voltage incandescent
> lamps also have life expectancies.) I've seen 650 hours to 1300 hour
> life expectancies for 60w-100w bulbs; the cheaper bulbs usually have
> lower life expectancies. Not a big cost factor since most of the cost
> of operating the bulb is the electricity consumed. 1000 hours at 100w
> is 100kw-hr, and at 16 cents/kw-hr that's $16 for the electricity -- and
> only $1 for the bulb.)
>
> - Peter
>
Peter, Try this link. http://www.misty.com/people/don/bulb1.html#how

It doesn't have the whole story, but interesting information on running
lights at reduced voltages and more links.

The information I was recalling is from long ago too, and I can't cite a
specific study. But filament bulbs fail when the filament breaks, and
the filament breaks, assuming no mechanical shocks or jars, because
filament material has evaporated leading to filament thinning. The
evaporation is not perfectly uniform leading to uneven thinning.
Because a thinner spot has more resistance it runs hotter and evaporates
faster leading to an already thin region on the filament thinning faster
than less thin places on the filament. Bulbs often fail when just when
turned on because heating is proportional to the square of the current
and a cold filament has lower resistance and momentarily draws more
current until it reaches operating temperature. But, at least as I
recall, it's the evaporation of the filament that leads to failure, and
evaporation only occurs when the bulb is powered. The complete story is
undoubtedly more complicated, and my recollection about lifetime being
unaffected by turning on and off may require some qualification. If the
concern is cost, your own analysis shows that the cost of the energy
used is greater than the cost of bulb replacement. But cost or energy
usage may not always be the main consideration. I don't know how the
analysis would work out for a fluorescent tube or a crt monitor.

Ben

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