What kind of inert gas is typically in a lightbulb?
Usually a mixture of argon and nitrogen, more argon than nitrogen. I
have seen a cite or two indicating a common mixture to be 93% argon 7%
nitrogen.
Argon is preferable due to lower heat conductivity, while nitrogen
impairs formation of destructive arcs across the filament.
What happens if you let it out and operate the lightbulb without it?
In air, the filament will oxidize very rapidly, to the point of complete
failure in usually a few to several seconds.
In a vacuum, the filament evaporates more quickly, although heat
conduction from a gas is eliminated. A gas slows down evaporation of the
filament because gas atoms bounce most evaporated tungsten atoms back
towards the filament. A gas permits a higher filament temperature by
slowing evaporation. A higher filament temperature achieves a higher
percentage (although still a minority) of the filament's radiation being
visible rather than infrared.
Some filament designs have higher energy efficiency when operated at
reduced temperature in a vacuum. If the filament is a low current design
dissipating less than *roughly* 10 watts per centimeter of apparently
visible filament length (as a "rough rule" as far as I have seen), then
you get better "overall luminous efficacy" (lumens of light out per watt
in) with a vacuum and lower filament temperature than with argon-nitrogen
mixture and higher temperature.
I have seen pure argon in a very few low voltage lamps. Even in low
voltage lamps where nitrogen is not necessary to block arc formation, it
appears to me that the compromise of 93% argon 7% nitrogen compared to
pure argon makes a difference small enough for many or most manufacturers
to prefer to not have to use two different gas formulations.
There are premium fill gases, mainly ones with argon or krypton. The
larger atoms of these heavier fill gases make them more effective than
argon at slowing filament evaporation. The heavier molecular weight
reduces heat conduction losses. These premium gases are widely mentioned
to be used in flashlight lamps, whose small size reduces the expense of
these premium gases.
I have seen 120V lamps (mostly some traffic signal ones) and some
automotive ones having a krypton-nitrogen mixture. Older GE "miser"
120V incandescents 60-100 watts made close to 1980 achieved a power
consumption decrease of 5 watts with essentially no compromise in life
expectancy or light output. (More recent GE "Misers" did not have krypton
and had compromised light output.) Some automotive lamps have life
claimed to be doubled with no significant compromise in light output nor
significant increase in power cosnumption as an achievement of using
krypton.
Flashlight lamps with premium fill gases producing 2.5-3 times the light
of "ordinary" ones also consume 1.5-1.75 times as much power, and in
addition benefit from a couple "economies of scale":
a) The thicker higher power filament takes longer to be destroyed by
evaportion and can be operated at a higher temperature.
b) Percentage of input power being heat conduction loss by fill gas
decreases with thicker filaments. Reason: As filament diameter
increases, the "boundary layer" of hot gas between the filament and the
surrounding cooler gas gets thicker and its temperature gradient
decreases. As a result, heat conduction loss per unit length of
visibly apparent filament increases less than proportionately (often
hardly at all) as filament diameter is increased.
Use of a premium fill gas usually achieves a 5-25% increase in energy
efficiency if wattage, voltage and life expectancy are unchanged, with the
gains being bigger in lower wattage / lower current designs where the
efficiency of a given combination of voltage, wattage and design life
expectancy is less no matter what. (Ever notice the design life
expectancy of incandescent flashlight lamps - usually 10-30 hours?)
Halogen lamps have an inert gas (argon, krypton or xenon) plus a trace
of a halogen - traditionally iodine - but maybe in some cases a halide.
The main benefit of halogen is keeping the inside surface of the bulb
clean. Life extension from returning of evaporated tungsten to the
filament is disappointingly small because the "halogen cycle" is not good
at depositing returned tungsten to where it is most needed.
However, halogen lamps often have significant improvement in efficiency
and/or life expectancy over non-halogen ones for a couple other reasons:
1. The bulb or its "inner capsule" (in the case of ones with inner and
outer bulbs) is small and is usually made of a higher strength material
and has material thickness usually greater than usual for non-halogen
bulbs of similar size. That permits much higher fill gas pressure, which
reduces filament evaporation more.
2. The small size of this bulb or "inner capsule" increases the economic
feasibility of use of premium inert main ingredient of the fill gas.
- Don Klipstein (
[email protected])
