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Video on planned obsolescence.

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tooki:

--- Quote from: SilverSolder on April 04, 2021, 07:41:19 pm ---If a filament is suspended in a perfect vacuum, do we agree that the only way it can lose heat is by radiation?  (IR + light)

--- End quote ---
I guess so, maybe. But it doesn’t matter, since incandescent light bulbs don’t use vacuums. The envelopes are filled with inert gas at roughly a bit below ambient pressure, such that the glass envelope isn’t under any real pressure. If they were under vacuum, the glass envelope would have to be much more robust, plus it makes the manufacturing far more complex. (As vacuum tube manufacturing shows us!) Instead of pulling a high vacuum they just shove a hose in and pump in the inert gas, which pushes out the air.* Since it’s at ambient pressure, it can then be “lazily” sealed off without the complexity of having to maintain high vacuum during pinch-off.

*Edit: hmm, I double checked in response to replies, and indeed the videos I found showed that they pumped out the air and then added the inert gas fill. But I know I saw the flushing-the-air-out-with-inert-gas method for manufacturing something, and it’s gonna bug the hell out of me that I can’t remember what it is!


--- Quote from: SilverSolder on April 08, 2021, 12:22:52 pm ---I was thinking about the implications of a coiled filament.  It seems to me that by coiling the filament, it cannot radiate from half its surface (the IR radiation on the inside of the coil is balanced by the IR radiation coming in from the opposite side).  So a coiled filament will run hotter than the same length of filament that has not been coiled.  -  Which in turn means that a coiled filament could be made from thicker wire, and might therefore last longer...  ?




--- End quote ---
Thats not what a filament looks like. It’s a much, much more open coil, and again, it’s a coil of coiled wire, totally changing the radiation formula: nowhere near half is being reflected back in. It does make a difference, and indeed it’s part of why we do it.

The <1” (2.5cm) of filament in a bulb is actually around 20” (50cm) of coiled coiled wire.


wraper:

--- Quote from: tooki on April 08, 2021, 04:24:10 pm ---If they were under vacuum, the glass envelope would have to be much more robust, plus it makes the manufacturing far more complex. (As vacuum tube manufacturing shows us!) Instead of pulling a high vacuum they just shove a hose in and pump in the inert gas, which pushes out the air. Since it’s at ambient pressure, it can then be “lazily” sealed off without the complexity of having to maintain high vacuum during pinch-off.

--- End quote ---
Not true. Air is pumped out thus creating vacuum inside and only then inert gas is filled.

james_s:

--- Quote from: tooki on April 08, 2021, 04:24:10 pm ---I guess so, maybe. But it doesn’t matter, since incandescent light bulbs don’t use vacuums. The envelopes are filled with inert gas at roughly ambient pressure, such that the glass envelope isn’t under any real pressure. If they were under vacuum, the glass envelope would have to be much more robust, plus it makes the manufacturing far more complex. (As vacuum tube manufacturing shows us!) Instead of pulling a high vacuum they just shove a hose in and pump in the inert gas, which pushes out the air. Since it’s at ambient pressure, it can then be “lazily” sealed off without the complexity of having to maintain high vacuum during pinch-off.

--- End quote ---

Lots of bulbs do use vacuums, typically below about 40 watts, this is because as the wattage goes down and the filament is thinner, the convective losses of a gas fill become much more pronounced. The pressure is not an issue at all, vacuum tubes for example are obviously vacuum filled and the envelopes on those are not any thicker than light bulbs. It actually isn't as simple as just "shoving a hose in", it's necessary to draw a fairly hard vacuum and bake out impurities before backfilling with inert gas. Getting lazy with this process is one reason a lot of cheap bulbs don't last very long. The inert gas fill is done for only one reason, to reduce the evaporation of tungsten by increasing the pressure inside the bulb, it doesn't save any manufacturing effort.

MikeK:
And Bill Hammack needs to make more videos, dammit.

SilverSolder:

According to the reference posted by @Wolfram (perfect handle for this topic!), http://lamptech.co.uk/Documents/IN%20Operation.htm ,  the losses due to gas convection is only 20%, most of the energy goes out of the bulb via radiation.

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