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Altitude and Light Bulbs
Name: Don G.
Status: Other
Age: 30s
Location: N/A
Country: N/A
Date: April 2002
Question:
This is a light bulb question. My company uses a type of
specialized bulb to illuminate a plasma video display. We use the same
type of bulb at two different locations, one at sea level and one at 7200
feet above sea level. The bulbs used at 7200 feet burn out much faster
than the bulbs used at sea level (in about 1/3 of the time).
They are left on constantly at both locations and the power source has
been monitored for any deviations, but none have been observed.
Could high altitude have anything to do with the high failure rate?
Replies:
Don -
Just a thought... The more dense air at sea level would dissipate more heat
through convective currents. Have you monitored the temperature of the bulbs to
see if the high altitude bulb runs hotter? Interestingly... assuming you are
using a filament bulb... a hotter filament would have an increased resistance
and therefore draw less current and perhaps reducing the heat production. It is
hard to know which effect might be preeminent.
It is also possible that these factors could cause the current to cycle thereby
varying the temperature of the filament. In a study that was done by the US Army
many years ago it was noted that the most important factor in the life expectancy
of a filament in a vacuum tube was the variation in temperature. The more the
variation, the shorter the life. If you were to monitor the temperature and
current of your bulb, you might find if this could be the case. Should this
prove to be the case, a better regulated power supply could then possibly
solve
your dilemma.
Larry Krengel
I would expect that the short life is due to overheating due to the
poor heat transfer to air at the low air pressure at high altitude.
You could check this by putting a fan on some of the balls to see if
it extends their life.
Since atmospheric pressure at 7200 feet altitude is about 25% less than
at sea level, I would expect convective cooling to be 25% less, since
the number of air molecules striking the bulb will be 25% fewer. This
will mean that the bulb and so the filament will be hotter. The
lifetime of a filament is an extremely sensitive function of its
temperature.
Best, Dick Plano
This is a tough question without knowing the details of the configuration,
power, etc. The altitude could have an effect, however. Although the thermal
conduction of an ideal gas is approximately independent of pressure, the
heat transferred by CONVECTION would not be. It is possible that the reduced
pressure at 7200 ft. above sea level reduces the heat loss of the lamps,
which in turn causes the lamps to burn at a higher temperature with a
concomitant lessening of their life.
Vince Calder
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