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Like most other discharge lamps, low pressure sodium types cannot be run directly on the mains electricity supply Effectively, they have no electrical resistance of their own and so the current flow would rapidly increase until the lamp was destroyed In addition, these lamps either require a high starting voltage (about 600V) which is greater than the 230V mains electricity supply, or an auxiliary circuit to provide preheating of the electrodes Full details of the kind of circuits normally employed can be found under the Control Gear section.
Rare Gas Filling
At room temperature, sodium is solid and there is insufficient vapour present for the lamp to be started at a practical voltage Filling the discharge tube with a rare gas mixture appreciably lowers this starting voltage If the lamps contained a pure rare gas, the striking voltage would still be quite high (several thousand volts), but this can be further reduced by using a mixture of two rare gases known as Penning mixtures Figure 8 shows how the striking voltage of a 180W SOX lamp varies with the percentage of argon in the neon fill gas The optimum gas mixture would have around 0.25% argon in neon Unfortunately, it is not possible to work in this range because during lamp life, the special borate glass used for the discharge tube absorbs or ‘cleans up’ some argon This is allowed for by filling the lamps with a mixture of 1% argon in neon (1.5% for the 10W and 18W sizes) Typical filling pressures are 4-8 torr for the high watts lamps, and 8-12 torr for the low watts.
Figure 8 - Effect of Argon Concentration on Lamp Striking Voltage
When a cold lamp is switched on, the control gear provides a high voltage kick This generates the required number of charge carriers inside the discharge tube for the rare gas to become conducting The voltage falls and the discharge is confined to the rare gases, as evidenced by the neon red colour of the light immediately after switch-on The discharge generates some heat and after a few minutes the sodium metal begins to melt Some of the sodium vaporises, and as this gradually fills the tube the light changes to the characteristic deep yellow colour, because sodium is ionised in preference to neon and argon.
Lamp efficacy is best when a buffer gas is used which has a low molecular weight Helium is the lowest molecular weight gas which is suitable, however it cannot be used because it diffuses out through the glass relatively quickly Neon is the next best choice, but it must be combined with a small amount of the heavier argon to form the aforementioned Penning mixture and enable the discharge to ignite at a suitably low voltage
Efficacy is also best when the gas filling pressure is low This is because the fewer atoms of rare gas that are in the tube, the less likely it is that light-generating sodium ions will knock into these and lose their energy in an atomic collision This energy loss creates heat and means lower lamp efficacy So, the lowest possible pressure of neon-argon filling might at once seem the most desirable choice But as always in lamp technology, its not quite as simple as that!
The current is supplied to the discharge by a pair of electrodes which the lampmaker has painstakingly coated with a precise amount of chemical called 'emitter' An emissive coating enables the current to be injected into the discharge space with the minimum energy loss, again to yield good lamp efficacy These electrodes necessarily run hot, and during life the coating is continually evaporating and being sputtered away When it runs out, the ballast can no longer supply enough energy to operate the lamp, and this is the end of lamp life (electrode failure) To prolong the existence of the emissive coating on the electrodes and attain a useful service life, gas fill pressures must be increased and the increased pressure exerts a force on the electrodes which reduces their rate of evaporation Unfortunately these extra gas filling atoms have to be present inside the discharge tube at all times and in normal operation they do reduce lamp efficacy due to more frequent elastic and inelastic collisions with sodium ions, but that is the price which must be paid for a long lamp life.
In the late 1970s, a Neon-Argon-Xenon mixture was introduced by Thorn Lighting in its Linear Sodium range Xenon, being a heavier atom, naturally retards the evaporation rate of the electrodes and their emitter coating Thus for the same lamp life, gas fill pressures can be reduced if xenon is employed, and that delivers enhanced lamp efficacy Not quite as much as might be expected though, because xenon, being a bigger atom, is more likely to undergo a collision with a sodium ion and make it lose its energy! But in practice only 0.2% xenon need to be employed, and this was found to enable a marginally more efficient and longer-lived lamp to be created Costing roughly 1000 times the price of argon though, xenon filling is a rather expensive solution!
Sodium Vapour Pressure
Early experiments showed that for long positive column discharge lamps, maximum lamp efficacy occurs when the sodium vapour pressure is 0.4 Pascals This occurs above molten sodium at 260ºC, and is illustrated in Figure 9 A very slight deviation from this temperature will cause a drastic decrease in light output, so it is clear that accurate temperature control of the discharge tube is vital to maintain high efficacy Because some energy is required to heat the discharge tube to this temperature, the most efficient lamp will be the one that achieves this for the lowest energy input As will be seen later, many techniques have been combined to produce a lamp having excellent thermal insulation which self-heats to the required temperature with the lowest electrical energy input.
Figure 9 - Effect of temperature variations on the luminous flux
Discharge Current Density
The lamp is also at its most efficient when run at low current density, i.e. low wattage per unit volume and length of the discharge tube This necessitates a long discharge tube of large diameter Once again the solution is not quite so simple, and in this case we have the problem that sodium vapour is opaque to its own yellow radiation Thus if the tube diameter was large, light created at the core would never reach the surface, reducing efficacy So it is necessary to make a compromise between current density and light absorption. For practical manufacture, two different tube diameters have been standardised upon - one for the low watts lamps, and a larger diameter for the high watts
The earliest sodium lamps had relatively poor thermal insulation, and in the U-shaped types a very small diameter tube indeed was required just to get the glass up to the required 260ºC wall temperature It will be seen later on that improved thermal insulation has been able to raise the wall temperature in modern lamps, then permitting the diameter of the discharge tube to be increased without light self-absorption becoming a major problem Modern lamps are generally more efficient because the improved insulation permits a larger bore tube having lower current density.
By the late 1950's tube diameter had been increased to the point where any further gain would have reduced efficacy due to self-absorption of the light Some very innovative work was then commenced by BTH Mazda, who created a range of linear sodium lamps having discharge tubes of non-circular cross-section These were much larger diameter tubes delivering low current density for optimum efficacy, but their special shaped sections ensured that the glass was closer to the discharge enabling the light to be transmitted more efficiently These unique light sources are described fully under the Linear Sodium category.
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