Evaporation rate of light bulb filaments exposed to air

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Discussion Overview

The discussion revolves around the evaporation rate of light bulb filaments, specifically tungsten filaments, when exposed to air. Participants explore the implications of this exposure on the filament's longevity and behavior at high temperatures, comparing it to conditions in inert gas environments.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the time it would take for tungsten filaments to evaporate when exposed to ambient air at temperatures of 1000C-1500C.
  • Another participant suggests that igniters for model rockets, which use lightbulb filaments exposed to air, demonstrate that the process is essentially instantaneous.
  • Some participants assert that tungsten oxidizes quickly in air, indicating that the filament burns rather than evaporates under such conditions.
  • A participant references a Wikipedia entry stating that tungsten evaporates during normal operation in an inert gas atmosphere, but emphasizes that the conditions in air are chemically more aggressive.
  • A side note mentions that in halogen lamps, evaporated tungsten is captured and redeposited onto the filament, which may extend its life.

Areas of Agreement / Disagreement

Participants generally agree that tungsten filaments oxidize quickly in air, but there is disagreement regarding the relevance of evaporation in this context. The discussion remains unresolved regarding the specific time frame for filament decay under the stated conditions.

Contextual Notes

Participants note the difference in behavior of tungsten filaments in inert gas versus air, highlighting the chemical aggressiveness of air. There are also references to the operational conditions of filaments in different types of lamps, which may affect their longevity.

tuloste
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I know that incandescent bulbs are filled with some inert gas to reduce the rate at which the filament decays. If the filament was to be exposed to ambient air, how long before it evaporated? How long can a mesh of light bulb filaments (or other material with as high a melting point that can be woven to threads of similar width) be kept at a temperature of some 1000C-1500C under such conditions?
 
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I don't know exact numbers, but the igniters for model rockets and homemade explosives are essentially lightbulb filaments exposed to air and electricity. The process is, for all practical purposes, instantaneous.
 
The filaments are tungsten metal, which oxidize essentially instantaneously, as Danger points out. Evaporation does not come into it, the hot tungsten just burns.
Do note the old Coleman lamps used a glowing wick open to the air, but the wick was heated by the burning of the lamp fuel and the light created by the hot rare Earth oxides held in the wick.
 
etudiant said:
The filaments are tungsten metal, which oxidize essentially instantaneously, as Danger points out. Evaporation does not come into it, the hot tungsten just burns.

“During ordinary operation, the tungsten of the filament evaporates; hotter, more-efficient filaments evaporate faster.”
http://en.wikipedia.org/wiki/Incandescent_light_bulb
 
Bobbywhy said:
“During ordinary operation, the tungsten of the filament evaporates; hotter, more-efficient filaments evaporate faster.”
http://en.wikipedia.org/wiki/Incandescent_light_bulb

During ordinary operation = glowing white hot in the inert gas atmosphere held inside a light bulb.
The OP was asking about operation in the air, which is much more aggressive chemically.
 
Yes, etudiant, you are right about what the OP was asking. Excuse me, I jumped the gun.
Bobbywhy
 
A side-note that might be of interest to OP:
In a halogen lamp, the evaporated tungsten is "captured" by the iodine or bromine and redeposited onto the filament, thus extending its life.
 

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