Can a SDoper explain how can a neon sign burn out? Surely, the neon does not get consumed, correct? (If so, how can a noble gas form into anything else?) Along these lines, how does a fluorescent bulb burn out? Isn’t just UV light simply exciting a phosphorus compound on the wall of the tube? Isn’t the UV created by electrical excitment of an inert gas?
So, Dopers, please give me the Straight Dope on these two types of lights?
In neon lights, I believe the gas leaks out of the tube.
I think in both cases, it’s just gradual leaking that does them in.
Faulty electrodes can also cause both neon and flourescent light bulbs to fail.
The (or one of, as the case may be) transformer can also go bad. If you’re not a glass worker it’s much easier to fix then a leaking tube.
A neon artist friend of mine says signs go bad for either having a leak and the gas simply escapes (btw, neon is just one of the gases that can be used) or another, much less common problem is “poisoning” caused by foreign material (moisture, dirt, etc) left in the tubing when it’s pumped out, or contaminated gas. eg: welding-grade gas was used instead of sign-grade, which is on the order of 99.995% pure.
Usually, contaminated gas or tubing will be immediately apparent.
The leaks are usually at the electrodes, where the metal portion is bonded to the glass.
Transformers can go bad as well, either from age or being undersized and unable to supply enough current needed by however many feet of tubing are in the display.
The neon CAN’T leak out, the tube is under vacuum. Air leaks in! :rolleyes:
Fluorescents burn out for the same reason incandescents do, the filament fails.
Secondary failures due to ballasts, transformers and other peripherals.
The gas is at low pressure, not “under vacuum”, since a vacuum means no gas at all. 
Fluorescent lights do not have filaments, just mercury vapor (and other stuff), electrodes, and a fluorescent coating on the inside of the glass.
Wrong. Standard four-pin fluorescent tubes DO have filaments in them that are used to vaporize the mercury when the bulb is started cold, then are switched out of the circuit by the starter once the bulb has ignited. Compact fluorescent bulbs and certain rapid-start types that use an electronic ballast do not have filaments or starters, and these only have two pins or contacts.
While a very strong vacuum is used to pull (one hopes) every last atom of air out of the tubing, the final configuration is to have the gas at a pressure of around 10 Torr.
Compared to atmospheric pressure, this is very low pressure - sea-level atmospheric pressure is 760 Torr.
You are quibbling about semantics. What exactly is the pressure in a neon sign tube? It is a few millibars! That is a vacuum, maybe not a high vacuum but it is a vaciuum nonetheless and is the pressure of the rare gas which ‘fills’ the tube.
AHHSOOOO! Then just why are there two pins at each end of the common tube type fluorescent and if there is no continuity at either end (due to filament burnout) why does the tube not establish a plasma. :rolleyes:
The filaments and the high voltage surge of the ballast are essential to establishing the plasma that causes the phosphor to emit the visibile photons, i.e. light.