🚀 go-pugleaf

On Thu, 04 Oct 2012 10:03:21 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 10/03/2012 09:41 PM, Jeff Liebermann wrote:
>> On Wed, 03 Oct 2012 10:32:57 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net>  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.

True, but I believe that's the range expected from different types of
light bulbs (nitrogen filled, halogen, vaccuum), and not the range
expected for a given device.  I suspect that more accurate exponent
value could be empirically determined for a given device, and later
used only for that device.

>> 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.

<http://en.wikipedia.org/wiki/Incandescent_light_bulb#Reducing_filament_evaporation>
   One of the problems of the standard electric light bulb is
   evaporation of the filament. Small variations in resistivity 
   along the filament cause "hot spots" to form at points of 
   higher resistivity; a variation of diameter of only 1% will 
   cause a 25% reduction in service life. The hot spots evaporate
   faster than the rest of the filament, increasing resistance 
   at that point a positive feedback that ends in the familiar 
   tiny gap in an otherwise healthy-looking filament.

Note the photo of the filament with a break in the middle.  When I was
quite young, I would break burnt out AC light bulbs to see what was
inside.  If the filament was intact, the break was always somewhere
near the middle.  If a piece broke off, one end of the broken piece
was usually near the middle.  In later years, I would look at the
remains of DC panel lights (usually type 47 for old Motorola radios)
and noted that the breaks were always near the supporting terminals,
probably due to metal migration.

>>> 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.

So much for my anecdotal data.  My theater marquee experience was in
about 1966.  The theater actually did keep records so that they could
stock enough replacement bulbs, but I don't have copies of any of
that.  

I tried Googling for similar repetative on-off tests and didn't find
anything.  If I have time, I'll try again.  I must admit that the lack
of test data does look suspicious.  Perhaps sending the idea to
Mythbusters and have them runs a test?

>The filament isn't tungsten-plated, it's pure tungsten or a low alloy. 
>The brightness drop comes from tungsten condensing on the envelope.

Oops.  I thought it was plated.

>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.

No, not fabricated.  It's my reliance on my memory in an area that I'm
not familiar with.  I tried Googling for the wire used, couldn't find
much, and made a bad guess.  The plating came from somehow getting
thorium coated tungsten wire used in vacuum tubes mixed up with light
bulbs.  Sorry for the errors and muddle.

>Cheers
>Phil Hobbs

-- 
Jeff Liebermann     jeffl@cruzio.com
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