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As a sodium lamp ages, the initial sodium distribution is very rarely maintained When sodium lamps are constructed with an integral vacuum outer bulb temperature gradients are set up along the discharge tube Power losses are greater at the tube ends near the cathodes, and so these regions of the tube are hottest The regions behind the cathodes are cold because there is no discharge there The U-bend end of the lamp is also cold because there is no IR coating or glass sleeve around it If precautions are not taken to eliminate these cold spots, the sodium migrates from hot areas to cold.
The situation can be improved by grading the thickness of the IR coating along the tube length The coating is thicker near the bend so that insulation is improved there In addition, the tube diameter at the bend is critical As that area of the tube runs cold it is standard practice to slightly constrict the glass there This brings the glass closer to the discharge and increases the wall temperature
The biggest problem arises if the sodium migrates behind the electrodes This point of the glass tube is where the sodium resistance is weakest and chemical attack can occur readily This will often cause the glass to crack and is a common cause of premature failures in sodium lamps For this reason, sodium lamps should never be run with the cap down as this encourages sodium to migrate to the cooler areas behind the electrodes
Sodium Depletion
If sodium migration is so severe that a portion of the discharge tube becomes depleted of sodium vapour, the rare gasfilling takes over the discharge at that point Since this is more difficult to ionise, the lamp current density increases here and efficacy begins to diminish In addition, this region produces almost no light, further reducing efficacy If the rare gasfilling pressure is low, say 6 torr (as in high watts lamps), this part of the tube will run cooler than the rest, encouraging sodium to migrate back to that point and solving the problem In low watts lamps, higher gasfilling pressures (8 to 9 torr) are necessary and under these conditions sodium migration may not be self-correcting since the sodium-depleted discharge region may overheat, exacerbating the migration problem The sodium can all be distilled to one end of the lamp leaving behind just the rare gas discharge Such lamps are known as red burners and are still occasionally seen in modern streetlighting installations.
A further snag occurs when a zone becomes deficient in sodium - the argon in the gasfilling becomes ionised These argon ions are attracted to a negatively charged sheath beside the discharge tube wall, and if they impact the wall with sufficient force they will become embedded in the glass Argon cleanup by the glass changes the Penning gas mixture causing the lamp striking voltage to rise Eventually the striking voltage will rise above that supplied by the electrical control gear, and the lamp will not start This is the end of lamp life (Gas Failure).
Sodium Reservoirs
Some of these problems can be overcome by distributing the sodium evenly throughout the discharge tube so that it is less likely for some areas to become depleted In addition, sodium vapour does not diffuse along the tube easily as it has to travel against the electric current As a result, each droplet of sodium supplies vapour mostly to its immediate surroundings It is therefore beneficial to distribute droplets of sodium around the discharge tube, so that all areas have sufficient sodium vapour It is for this reason that each lamp contains about 500,000 times more sodium than is actually required as vapour in the discharge
Philips and GE lamps have the sodium fairly evenly distributed along the discharge tube length during production, but in practice it rarely stays like that Since 1958 Philips has located the sodium in small dimples in the discharge tube which are naturally colder, helping to maintain an even sodium distribution A dimpled SOX lamp is shown in Figure 26 In practice some of the sodium still migrates out of these dimples after several thousand hours but they do have a number of benefits Firstly it allows the lamp to run up about 50% more rapidly, and secondly it helps prevent wattage rise throughout life, which occurs as the sodium migrates towards the bend and depletes other areas Osram and Thorn have both made and sold lamps having dimpled discharge tubes on and off at various times, frequently employing whole discharge tubes sourced in from Philips for trial purposes, but neither of them felt that they were successful enough to justify modifying their own production
Figure 26 - A SOX Lamp Having Dimples Moulded into the Discharge Tube
The only time where special geometries have been completely successful in maintaining the original sodium distribution was in the Linear Sodium lamp Here the 'horns' of the crescent-shape discharge tube were tapered inwards so that capillary action held the molten sodium inside the narrow channels In the Thorn discharge tubes having a clover-leaf cross section, elongated v-shaped dimples were formed at the apex of each leaf, affectionately known as 'greenhouses' because of their similarity in shape to a greenhouse roof Because of the more uniform temperature distribution in SLI lamps the sodium tended to stay where it was originally placed.
Another technique, employed by Osram in the late 1950's with its SOI Integral lamps, was to form holes in the heat-reflecting sleeves at regular intervals The idea was that more heat would be lost at these localities and the marginally cooler wall might serve to retain some sodium here The idea was unsuccessful, but for historical interest, an example of this test lamp can be seenhere.
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