Re: really dim bulbs; Tungsten Halide lighting

[Robert Myers <rmyers@wvit.wvnet.edu>]

Huw, 

I don't necessarily disagree with your basic prediction, but your cited
reasons are not totally valid.

At 10:00 PM 12/7/96 -0500, you wrote:
---snip---
>The "Halogen Cycle" in the incandescent Tungsten Halide lamp consists of the
>random recovery of tungsten vapor deposited inside the quartz bulb by its
>revaporization off the hot quartz into the halogen gas (typically Bromine or
>Iodine) and redeposition on the Tungsten filament.  
>
>This provides one of the two primary advantage of this lighting system.  By
>keeping the inside of the bulb clean of Tungsten contamination, the light
>output of the bulb stays very near its full level for the entire life of the
>bulb.

True.  The tungsten deposits on the inside of the bulb will be vaporised as
WX3 (where X = Br or I) 2 W + 3 X2 -->  2 WX3 

>The second advantage, longer life, is due to the higher pressure inside the
>bulb compared to a conventional incandescent lamp.  This reduces the
>vaporization of the Tungsten in the first place.  

Wrong.  At a given filament temperature, the rate of sublimation depends
upon the pressure of Tungsten vapors (alone) in the bulb, not the total
pressure of all gasses present.  W(solid) + heat -->  W(gas) for sublimation
from the extremely hot filament and W(gas) --> W(solid) + heat for
deposition on the cooler glass surface of the bulb.

Remember Dalton's Law of Partial Pressures?  P(total) = P(1) + P(2)...  It
is the partial pressure of W [P(W)] which is the important gas pressure here.

>Longer life is not due to
>the redeposition, since that is random and does not occur where it would be
>needed, i.e. the weak spot on the filament.

This may not be correct.  The weak spot of the filament will be the
narrowest part of the filament.  Since the entire current for the filament
passes through the entire filament this weak spot, due to its smaller
diameter and therefore its higher resistance, will have the highest
temperature.  Therefore the rate of deposition of Tungsten will be highest
at those spots.  ( 2 WX3 + heat --> 2 W + 3 X2 )  This will tend to help
keep the diameter of the filament more uniform throughout its length and to
extend its life by eliminating hot spots.

---snip---

I guess this points out a basic difference between engineers and scientists.
The scientist presumes to understand the theoretical basis of a technology.
The engineer gets the technology working in practice regardless of whether
or not he fully understands the theoretical basis.  There is merit in both
approaches.  :-)  

[Hey, all you engineer-types out there - this isn't intended as flame bait.
:-)]

___
   Bob
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