On 10/03/2012 09:41 PM, Jeff Liebermann wrote:
On Wed, 03 Oct 2012 10:32:57 -0400, Phil Hobbs
wrote:
I don't know of any data that supports this common idea, but I'd be
interested in reading about it if anybody's actually done the experiment
carefully.
It's an accelerated life test. The deration curve of the incandescent
light bulb is well known and assumed to be
(Vapplied/Vdesign)^-12 to ^-16 * Life at design voltage
http://www.welchallyn.com/documents/Lighting/OEM_Halogen_Lighting/MC3544HPX_Catalog_2_11_09.pdf
See Fig 5 on Pg 5 for the graph. Nobody wants to wait 1000 hours for
a bulb to blow. So, they increase the applied voltage, which
dramatically decreases the lifetime down to reasonable test times.
Using a rack of bulbs, they obtain an average (or median) lifetime at
the higher voltage. Then, they work backwards on the curve to
estimate what it would be at the design voltage.
You can't run an accelerated life test when the exponent isn't known
more accurately than 12 to 16.
When I was specifying lamps for a direction finder for the USCG, I had
to deal with minimum lifetime specs. I asked the vendor (Dialight)
how they tested their T-1 3/4 bulbs and was told that they did an
accelerated lifetime test on a few bulbs from each lot to insure
adequate lifetime along with the usual sampled 1.5% AQL failure test.
Electromigration is a smaller effect in an AC bulb, since
the leading order effect cancels.
Yep. As I understand it (possible wrong), AC filaments break in the
middle, mostly from vibration flexing.
I don't think so, because there's no mechanism for that, as I said. The
wire is fully annealed at all times, so there's no possibility of
progressive fatigue failure.
I suspect that the notion that cycling is hard on bulbs comes from the
way that the bulb often fails at turn-on, when the thinnest hot spot
vapourizes before the rest of the filament has a chance to come up to
temperature and reduce the inrush current.
Yep. See my comments on the relatively high failure rate on the
40watt theater marquee lamps due to cycling. The same lamps in the
lobby and foyer were not cycled and seemed to last forever.
I was actually disagreeing with you. There are lots of possible reasons
for the marquee lights failing prematurely. I'm not a tungsten expert
myself, so I'd be very interested in seeing actual data that shows a
dramatic shortening of life due to cycling. I'm not saying it's
impossible, just that I haven't seen any such data.
The tungsten in the lamp is run within a few hundred kelvins of its
melting point, so it's always in the fully annealed state, which ought
to mean that there are no metal fatigue mechanisms operating, just
material migration due to sublimation.
Yep, but different failure mode. When the extremely thin layer of
tungsten plating evaporates, the light becomes dimmer. Below some
brightness level, it is considered to have failed. However, most such
tungsten coated filaments fail due to corrosion of the base steel
alloy wire which is exposed to the internal gases inside the bulb
after the tungsten evaporates. The gases (mostly nitrogen and some
argon) are inert, but there's a little water vapor outgassing from
heating the glass envelope, which eventually corrodes the filament.
Other failure modes are hot spots and notches caused by manufacturing
variations and tungsten evaporation.
The filament isn't tungsten-plated, it's pure tungsten or a low alloy.
The brightness drop comes from tungsten condensing on the envelope.
And the connecting wire isn't plain steel, it's generally Dumet,
http://www.jlcelectromet.com/dumetwire.htm
which is a 42% Ni steel with OFHC copper or nickel plating.
You're making a lot of that up. I'd still like to see
carefully-collected data.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net