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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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