When the wind rushes in, electrode's privacy goes out the window

[AC/DCdude17]

X-No-Archive: Yes

When a fluorescent lamp develops a crack and loses the vacuum non
catastrophically, it often develops a clear patch in the phosphor
somewhere, usually around one of the electrodes. As the wind rushes in,
the patch grow larger and in the center, it is very clear that you can
actually see the electrode as if you're looking into a germicidal lamp.

Sometimes, the patch could be far away from where the leak is. I have
seen a clear patch near the bottom electrode of an 8' lamp that was
placed vertically on a shelf. I didn't see any crack so my guess is it
was somewhere way up. What do fluorescent lamps develop a clear patch
when the vacuum is lost?

 

[Jeff Waymouth]

Because vacuum isn't "lost". It is replaced by air. The air rushes
into the bulb through the crack, due to the difference in pressure
outside the lamp vs inside the lamp. This stream of air exerts force on
the inner glass wall of the tube, where the phospho is and "scours" it
ff the glass.

 

[jriegle]

Reminds me of the blue print machine we used to have. It took three (IIRC)
75W 4 foot UV tubes. One time one of the electrodes in a tube got so hot it
actually melted and distorted the envelope in a small area. A small hairline
crack developed and let in air causing the tube to go out.
John