UV lamps

UV (UVC) lamps (bubs) used in Aquariums and Ponds and how they work.

This post is primarily dealing with UV bulbs (or lamps which is a more correct term) emitting UVC radiation used in the Aquarium/Pond Industry, although I will discuss other UV bulb types as well since these may someday show up in the Aquatics Industry which tends to lag behind the mainstream technical, filtration, and other innovations in many ways such as in Aquarium Lighting (the SHO and T2 lights are good examples of where the Aquarium Industry has yet to embrace these high performers in overhead aquarium lights).

There are two common UVC emitting lamps types currently employed: the low pressure and the medium pressure mercury lamp.

The low pressure lamp emits its radiant energy between 250-260 NM which is where UV Sterilization is most effective with a UVC efficiency of about 38%.

The medium pressure lamp emits its radiant energy between 250 and 350 with many energy spikes in between (most notably around 320 which is more in the UVB range) with a UVC efficiency around 7%.

Please click diagram to enlarge

The low pressure lamp is clearly the better choice based on this information, however the low pressure lamp does have one flaw which the medium pressure lamp does not suffer from and that is the low pressure lamp is affected by water (or air) temperatures and operates best between 20 C (68 F) and 40 C (104 F). Generally this is not a major concern considering most aquarists operate their aquariums in well within this range. With ponds, this is still not a major concern in that most problems, including algae occur in warmer months. This problem can be also addressed with in-line heaters to pre-warm the water. Many UV Sterilizers come with quartz sleeves which form an air pocket between the lamps and water that in theory increases operating temperatures; HOWEVER my own tests have shown this improvement to be nominal at best especially when compared to a well designed direct contact UV Sterilizer. In a test with a custom 15 Watt UV Sterilizer and a 13 Watt Quartz sleeve UV Sterilizer (a well designed one at that), the increase in temperate was only 2 degrees F (I have observed up to 5 degrees depending on the units compared). This test started at 68 F and was conducted over 15 minutes, then water was removed from the units and the temperature measured. Unfortunately the quartz sleeve is accepted as a standard that sounds great in theory, but in practicality is not a true improvement in most applications (I would recommend them in some applications such as Lobster tanks, but even here a pre-heater would also be recommended).

Please see this article for much more about UV Sterilization: ULTRAVIOLET STERILIZATION (How UV sterilization works) . This article explains the benefits and myths about aquarium and pond uv sterilization


Now a potential improvement is the Amalgam High Output low pressure lamp. These lamps operate in a wider temperature range than standard low pressure lamps. These lamps are not without their problems too; even though not affected by water temperature fluctuations, these lamps can take 800 seconds to get to full power, and the warm up time is temperature dependant. The lamp life is often below the expected hours due to failure caused by the filaments becoming brittle and deposits of mercury oxide causing the ends to become black. This leads to poor heat dissipation and is the most probable failure mode. These lamps and the UV Sterilizers that contain them are also very expensive as well.

Another aspect of UV lamps that should be noted is how they function or start. Most UV lamps commonly used are Hot Cathode UV lamps. The Hot Cathode Germicidal UV Lamps are similar in their operation to the standard fluorescent lamps. The Hot Cathode lamp operates from a ballast or transformer and requires a device such as the glow switch starter to preheat the electrodes in order to start the lamp. The electrodes, located at the ends of the lamp, are tungsten filaments coated with emission material and, under normal operation, govern the life of the lamp. In view of the fact that the life of the electrodes is shortened by frequent starts, the lamp life is rated according to the number of times the lamp is started. Operation at low temperatures may result in excessive lamp blackening and rapid depreciation in ultraviolet output.

References:
http://americanaquariumproducts.com/AquariumUVSterilization.html
http://www.americanairandwater.com/lamps.htm