A UV lamp is not an ordinary light bulb. One of the most misunderstood things about a UV water treatment system is why the lamp should be changed even though it still appears to be working. Why is it that the useful life is typically defined as 9,000 hours, or about one year of continual use?
The problem arises from the fact that for most of us, our understanding is heavily influenced by our experience with incandescent lights. These are your garden variety bulbs, used in most household light fixtures, it is not necessary to change or replace this bulb until it quite literally burns out.
An incandescent light bulb works by passing electricity through a tungsten filament. This generates heat that causes the tungsten to glow. That glow is the visible light spectrum, so we see the light. The tungsten filament is deteriorated with use and eventually it becomes so thin that it quite literally snaps, and no longer lights.
A UV lamp, however, operates on a different principle, more like a fluorescent light. In this case, the electricity is still passed through a tungsten filament which heats up, but that energy “excites” the mercury vapor contained in the lamp. (Not to worry, only a very small amount of mercury is used, typically <10 mg, and it’s safely contained.) In this case, it is the mercury vapor that glows and, emits ultraviolet (UV) light. This is exactly what’s needed to inactivate any pathogens in your water supply. As you may know, UV light is not visible to the naked eye and is, in fact, quite damaging, so never look directly at a UV lamp while it’s on. If your system has a view port, you may see a blue glow from the lamp, but that’s not a result of the UV rays. Remember, UV light cannot be seen by the human eye. This glow is simply a by-product of the lamp’s operation. The useful life of the UV lamp is determined by the rate at which the mercury is consumed. That very small amount of mercury contained in the lamp will provide sufficient UV rays to disinfect the water for about one year. After that, although the lamp appears to be working, there is not enough UV produced to safely drink the water. Changing the lamp on time is critical to ensuring your safety. That’s why many UV systems have a timer that will count down from 365 days and then sound an alarm to let you know it’s time for a lamp change.