Gassy fluorescent tubes

[japlytic]

A few days ago, a fluorescent tube in an emgergency light fixture would not start properly, and replacing the tube fixed this problem. Trying the original tube in another fixture produced a similar result.
There was a moderately strong purple glow at the ends of the original tube when it would not illuminate properly, indicating it was gassy. Is this true?

[kc8adu]

Re: Gassy fluorescent tubes

the cathode coating is worn out.typical eol.

[davmax]

Re: Gassy fluorescent tubes

Flourscent tubes are normally filled with gas, electrons discharged down the tube excite the gas and ultra violet is emitted this in turn causes the coating on the glass to flouresce and produce visible light. So the tube is normally gassy. Failure is normally caused by lack of electron discharge, this is usually started by heating a filament at each end so that electrons are emitted. i.e the filaments fail. The filaments only receive power before the electron discharge starts. Once the discharge (arc) is established it causes a low voltage drop across the tube that bypasses the energy going to the filaments. The discharge current is limited by a choke, therefore most of the supply voltage appears across the choke.i.e the discharge is a low impedance.

[PedroDaGr8]

Re: Gassy fluorescent tubes

A bit more explanation on fluorescent tubes.
The gas in Fluorescent tubes is Hg vapour (mercury) it emits strongly in the ultraviolet region. This UV light strikes a white powder photophor. This phosphor is what actually lights up (usually via fluorescence and phosphorescence). Most bulbs die because the emission mixture of gas in the bulb is used up via various reactions. Once the mix is used up the bulbs have a hard time maintaining the transmission. Older lamps used to use Ca5(PO4)3(F,Cl):Sb3+,Mn2+) which emit mainly yellow and bluw wavelengths of light (which look white to the eye but result in an incomplete spectrum for reflected colors, hince colors that originally match in incadescents dont match anymore), newer bulbs use additional phosphors with Europium and Terbium in them to generate a more complete spectrum.
Note: unlike incadescent lightbulbs which generate a continuous spectrum, all flourescents and LEDs emit spectrums with peaks that our eye blends. Just look at them through a polarized grating and you will see what I mean.