High Pressure Sodium Lamps

   High Pressure Sodium Lamps
            A High Pressure Sodium Lamp (HPS) consists of translucent ceramic arc-tube     
      constructed of polycrystalline alumina (PCA), which is enclosed in a clear 
      or coated evacuated hard glass outer bulb shell. HPS lamps have a vacuum 
      inside the outter bulb shell, or jacket, to isolate the arc tube from 
      changes in ambient temperature. Clear bulb shells permit good optical 
      control of the light source, where coated bulb shells diffuse the light 
      source. The polycrystalline alumina material used in High Pressure Sodium 
      arc-tubes differs from the material used in other HID light sources due to 
      the extreme demands of high pressure and temperature (2,500°-3,000°F) of 
      the sodium arc-stream. Arc tubes are filled with the exact amount of arc 
      metal (commonly called amalgam) which is required for lamp operation. 
      Polycrystalline alumina tube materials do no lend themselves to the molten 
      sealing method used in the construction of mercury vapor and metal halide 
      arc tubes. Instead, polycrystalline alumina end caps using either a 
      wire-out end seal or a compound (shrink-fit and cemented) end seal are 
      epoxied or glued to the tube body using silicone glass. Each tube end cap 
      contains an electrode. The sodium-mercury amalgam and starting gases are 
      replaced inside the arc tube before it is sealed closed. Unlike mercury 
      vapor and metal halide lamps, HPS lamps are excess amalgam lamps. 

This means there is more sodium and mercury arc metal placed inside the tube 
      than can be vaporized during starting and operation. The amount of amalgam 
      that vaporizes depends on the total energy in the arc and the temperature 
      of the amalgam. If the lamps becomes too hot, too much amalgam will 
      vaporize, and operating voltage will increase. As the lamp ages, the 
      amalgam is actually consumed in very small amounts, thus a surplus must be 
      added to the arc-tube when it is manufactured.

REDUCING  INPUT VOLTAGE  and  temperature  increase Bulb life 

      When HPS lamps were first introduced, the amount of amalgam not held in a 
      vaporized state remained condensed in an external reservoir located in the 
      coolest part of the lamp. If the lamp was vibrated by winds or passing 
      traffic, amalgam from the reservoir would splash down onto the arc tube, 
      causing a thermal shock that would extinguish the lamp. The lamp would 
      then go through its start-up process and cycling would occur. 
      Experience has shown that during the first 20 minutes or so of HPS lamp 
      operation, the lamp voltage might rise or fall from start to start or even 
      during continuous operation as varying amounts of amalgam enter the arc 
      stream. Most HID lamps use a wire support frame to protect, cushion, and 
      align the arc tube in the center of the bulb. The design and placement of 
      this support frame is particularly important in HPS lamps because it can 
      affect the temperature of the arc tube and end caps. As we have seen, arc 
      tube temperature has a direct effect on the amount of amalgam vaporized. 
      

      Average Lamp Life      

Figure 1. Survival Curve for
High Pressure Sodium L    

      Lamp operating temperature 
      Ballast characteristics 
      Ballast supply voltage 
      Operating hours per start 
      Depending on the type of High Pressure Sodium lamp, most lamps can be 
      expected to yield an average rated life of 24,000 hours. In streetlighting 
      service, 24,000 hours equates to 5 years and 9 months of dusk to dawn 
      operation. It is extremely important to note that all lamp manufacturers 
      tend to glance-over the fact that the 24,000 hour figure is an average 
      lamp life expectancy for a group of lamps sampled over a 24,000 hour 
      period of time. 

      Figure 1 demonstrates the Survival Curve of a group of 
      lamps sampled over a 24,000 hour period. Note that by the time the 24,000 
      hours elapse, only 65% of the sample group are operating. In other words, 
      in a theoretical sample of 2,000 lamps, only 13,000 lamps (65%) in that 
      group will be burning at the end of the 24,000 hour period. In looking at 
      Figure 1 it is also noteworthy that at 40% of 24,000 (9,600 hours or 2 
      years and 4 months of streetlight service), 10% of the group will fail.