🚀 go-pugleaf

On 10/04/2012 02:57 PM, George Herold wrote:
> On Oct 4, 12:43 pm, Jeff Liebermann<je...@cruzio.com>  wrote:
>> On Thu, 04 Oct 2012 10:03:21 -0400, Phil Hobbs
>>
>>
>>
>>
>>
>> <pcdhSpamMeSensel...@electrooptical.net>  wrote:
>>> On 10/03/2012 09:41 PM, Jeff Liebermann wrote:
>>>> On Wed, 03 Oct 2012 10:32:57 -0400, Phil Hobbs
>>>> <pcdhSpamMeSensel...@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/MC3...>
>>>> 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_filamen...>
>>     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     je...@cruzio.com
>> 150 Felker St #D    http://www.LearnByDestroying.com
>> Santa Cruz CA 95060http://802.11junk.com
>> Skype: JeffLiebermann     AE6KS    831-336-2558- Hide quoted text -
>>
>> - Show quoted text -
>
> Hi Jeff, Phil.   First I know nothing about incandescent bulbs.
> But how about this as a model of why turning bulbs on and off might
> cause them to fail sooner.
>
> 1.) I think we all observe that bulbs tend to blow when you turn them
> on.
> (unless you knock the lamp over or something.)
>
> 2.) I assume that the failure is mostly due to the thinner �hot spots�
> on the filament.   Thinner regions heat up faster (higher resistance
> with equal current).
>
> 3.) Now even if the thinner region doesn�t blow, it still gets hotter
> and loses a bit more tungsten than the rest of the filament.   (For
> that small amount of time that it�s turning on.)  But still this means
> that turning on the bulb causes the thin region to become a bit
> thinner.
>
> And that�s it.  Repeated on and off means that the thin region has a
> higher average temperature than the thick part of the filament.  It
> evaporates faster and fails sooner.
>
> George H.

If the effect is real, that sounds like a good candidate for a 
mechanism.   Certainly you'd expect that to be important right near the 
end of the bulb's life, so maybe it's important throughout.

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