In a incandescent bulb, if the glass is punctured, and then
power is supplied, the tungsten filament blows up with a
flash.Why?
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Answer Posted / nagarajan
Gases used for filling incandescent lamps must be chemically
inert, i.e. they should not react with the hot filament
(with the exception of halogen lamps). This means that they
also need to be extremely pure, as very small amounts of
some substances can have disastrous effects. For example, a
single drop of water in a gas cylinder capable of filling
half a million GLS lamps would cause every one of those
lamps to blacken and fail early! The following table shows
the gases that are inert to tungsten and make suitable
fillings:
Gas Symbol Molecular Weight Thermal Conductivity % in
Atmosphere
Hydrogen H2 2 0.1805 N/A
Helium He 4 0.1513 0.0005
Neon Ne 20 0.0491 0.0015
Nitrogen N2 28 0.02583 78.03
Argon Ar 40 0.01772 0.94
Krypton Kr 84 0.00943 0.0001
Xenon Xe 131 0.00565 0.000009
The molecular weight is the weight of one atom or molecule
of the gas. The heavier atoms are more effective in reducing
filament evaporation, making the lamp last longer. It can be
seen that they also feature lower thermal conductivity,
reducing gas losses so the filament will run hotter and
produce light more efficiently.
Consequently for incandescent lamps it would appear that
xenon would be the best choice of fill gas. But this is only
present in very small quantities in the atmosphere, which
makes it expensive. Krypton is about ten times cheaper but
still too scarce, so we generally use argon which is roughly
500 times cheaper. However the first gas-filled lamps had to
use nitrogen, because at the turn of the century there were
no argon distillation factories in existence. When argon
lamps were eventually introduced, it was found that its
lower electrical ionisation potential resulted in problems
of arcing between the lead wires. This was overcome by the
admixture of a small amount of nitrogen, and an 85:15
argon:nitrogen mixture is common in European lamps. Lower
voltage American lamps can get away with a 95:5 gas and thus
are slightly more efficient. The fill pressure is about 0.8
atmospheres which normally rises to just above atmospheric
pressure when the lamp is in operation.
By the 1930s the price of krypton had reduced considerably
as new distillation plants were opened, and in 1936 Tungsram
of Hungary put the first krypton-filled GLS lamp on the
market. To reduce the required volume of this expensive gas,
a new glass bulb shape was designed. The result was a lamp
having a mushroom shaped envelope, which is still produced
today. The Krypton Mushroom lamps usually offer a
combination of about 5% more light and 50% longer life than
ordinary argon-filled bulbs. Alternatively an energy-saving
range of lamps producing the same light but with 10% lower
wattage is available. It is also standard practice to employ
krypton in high efficiency miniature lamps, because the
benefits of krypton are much more pronounced for lower
voltage types. Miner’s cap lamps are an important
application, since a smaller battery can provide the same
light level. Even xenon is sometimes found in today’s
miniature halogen capsules where a very high efficacy or
long life is called for, especially in automotive and
projection lamps.
The only other gas which has been commercially employed in
incandescent lamps is hydrogen. It is characterised by a
very high thermal conductivity making it inefficient for
general lighting, but it has been employed in special signal
lamps because it cools the filament more rapidly between
pulses. The faster nigrescence time permits higher speed
signalling by light pulses. The table below shows the main
applications of the available lamp filling gases:
Gas Type Applications
Hydrogen Rapid-Nigrescence lamps for high speed signalling
(obsolete).
Neon Low pressure sodium, Neon glow indicators, Linear Neon,
Penning-start sodium.
Nitrogen Very large incandescent, Studio Halogen, Mercury &
Metal Halide lamp outers.
Argon Incandescent, Halogen, Linear Fluorescent, Mercury and
Metal Halide arc tubes.
Krypton Small halogen, Krypton Miners, Special automotive,
Energy saving fluorescent.
Xenon High performance halogen, High pressure sodium arc
tube, Flashtubes, Xenon Arc.
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