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There are four principal failure mechanisms which determine the life of a modern low pressure sodium lamp, detailed below in order of significance:
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End of electrode life The emissive coating becomes exhausted
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End of gas life The argon content of the gas filling is consumed
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Rectification May occur as a result of electrode failure.
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Premature failures Caused by cracking, electrolysis effects etc.
End of Electrode Life
This is the mechanism by which most SOX lamps reach the end of their life Quite simply, it occurs when all of the emissive coating over the surface of the electrodes has been consumed, and the voltage required by the discharge exceeds what the control gear is capable of supplying The precise time period which elapses before this condition is reached is depending primarily on the quantity of emitter which is applied to each electrode during production Clearly it is impossible to pick up precisely the same emitter weight on every electrode, and the natural variation in emitter weight leads to the variation in lamp life.
Switching frequency also plays a significant role here SOX lamps are designed for one switching per 24 hours Because the load placed on the electrodes is very high during ignition, and it causes significant amounts of electrode material to be sputtered off, the more frequently it occurs the shorter will be the life of the lamp.
End of Gas Life
One drawback of borate glasses, which must be used in the fabrication of SOX discharge tubes, is that they tend to have a high affinity for argon ions Most SOX lamps employ 1 to 1.5% argon in the neon gasfilling, this serving to considerably reduce the voltage required to strike the discharge to reasonable levels However all the way throughout lamp life the glass will be absorbing small amounts of the argon In some lamps, especially the high wattage types and in particular the 180W lamp, a situation can arise where the argon gasfilling will all be consumed before the electrodes reach their end of life.
The problem is also more noticeable in lamps where an even sodium distribution is not maintained In the sodium depleted regions, the neon-argon gasfilling becomes ionised to a greater extent and under these conditions the rate of argon absorption is accelerated The lamp will 'fail' when the voltage required to strike the discharge exceeds the open circuit voltage of the ballast.
Rectification
This is a process by which the lamp begins to acts as a rectifier, converting the AC current it is fed with into a DC current A lamp might begin to do this as a result of the electron emission characteristics of one electrode changing with respect to the other Most commonly this simply results from one electrode running out of its emissive coating before the other However the same effect can occur if sodium metal comes into contact with one of the electrodes or its lead wires - this is another reason why the lead wires are glass-sleeved, to minimise the risk of contact with liquid sodium.
Once rectification begins, much higher currents than normal will flow in both the lamp and the windings of the ballast In earlier designs of lamps the high current rectification could continue for many hours During this process the temperature of the ballast increases owing to the high current it passes, and this can lead to the breakdown of insulation between the windings, allowing a higher current again to flow Eventually the ballast would burn out under such high loading Modern lamps are all now equipped with a fusible monel wire inside the cap which will fail if rectification occurs, thus saving the ballast from the otherwise inevitable destruction.
Premature Failure
This is perhaps the most troublesome kind of failure because it cannot be predicted, and occurs relatively early in lamp life Generally premature failures happen as a result of manufacturing defects which have, until recently, been difficult to eliminate The precise failure mechanisms tend to be different for each manufacturer, representing the different production techniques that are employed.
Electrolysis effects perhaps account for the most significant number of premature failures These occur as the result of a high electric field being set up across a glass component, which can lead to decomposition and cracking of the glass Alternatively since the sodium within the discharge tube is ionised, it can easily be drawn into the glass and crack it if a strong enough electric field is present.
The formation of these electric fields can nearly all be traced back to the presence of the barium getter in the outer jacket Since this film is electrically conductive, if it comes into contact with any internal wires then an electric field will be set up between the glass substrate onto which the barium is deposited, and any wires passing through the glass Decomposition of the glass will occur leading to cracking and an air leak This can occur both for the pinch-seals of the discharge tube, and the pinch seal of the outer envelope
If the infra-red reflective coating is in contact with the barium getter, this will also become charged since it is a good electrical conductor Then if the top support of the discharge tube at the U-bend is fabricated of metal and touches the discharge tube, it can set up an electric field in the U-bend and tipoff area Sodium can be drawn into the glass here which may eventually cause cracking and discharge tube leakage.
All of these effects have been eliminated in the new generation of SOX PSG lamps recently introduced by Philips Since the barium getter coating has been eliminated and replaced with a solid getter pellet, the electric fields that sometimes used to occur across some glass parts can no longer exist Consequently the vast majority of premature failures are avoided in this new lamp design, and service life is determined only by electrode and gas life The SOX PSG lamp therefore offers considerable maintenance savings, since in theory no spot-replacements should be necessary during the first 6000 hours of lamp life.
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