Bulletin 82

GERMICIDAL LAMPS

GERMICIDAL LAMPS

 Germicidal lamps are very prevalent in all phases of research and industry, mainly because of their abiotic effect.  They are used in culture media preparation systems to sterilize the air in biological hoods, and in water treatment systems to purify and destroy toxic substances in water.  These include bacteria, odors, viruses, microbes, totally complexed cyanides and phenols or phenol derivatives, polychlorinated biphenyls (PCB), insecticides and pesticides, ammonia, fatty acids, glycerols, glycines, acetic acid, ethanol and sulfides.  Much of this treatment results in an oxyphotolysis to carbon dioxide, water and non-toxic salts with the metal contaminants being changed to filterable oxides.  Recovery of valuable metals such as chromium and nickel is achieved from steel plant waste water as well as iron oxides.

 In many cases, the result is pyrogen-free water with objectionable odors removed and containing no dangerous chemical by-products.

 With drinking water, this usually means the elimination of chlorine, chlorinated organics and odors; whereas, industrial effluents must conform to strict water quality standards relating to the toxicity to the aquatic life environment.

 Operating rooms, isolation areas, supply rooms, kitchens, patients' rooms, and other nursing care facilities make extensive use of germicidal lamps to maintain a low bacterial count in the air.  Other examples of beneficial air sterilization are food product plants, poultry brooder rooms, laying houses, incubators and hatcheries, stables, pens and veterinary hospitals.  In fact, germicidal lamps are used in any environment where there is need to minimize the problem of bacterial contamination whether it be with people, products, animals, water, or air.

 Sanitation cabinets for reusable clothing and equipment, such as goggles, spectacles, sterile instruments, and hard hats, must also be maintained with irradiance levels above a specified limit prescribed by the U.S. Public Health Service, to minimize the number of organisms transmitted.

TYPES OF GERMICIDAL LAMPS

 1.      Hot Cathode Germicidal Lamps

 The hot cathode lamps are identical in electrical characteristics to the standard preheat design fluorescent lamp.  They may be operated on a typical preheat circuit which employs a glow switch starter and choke, or they may be operated on starterless circuits such as quick start or trigger start.  The electrodes, located at the ends of the lamp, are tungsten filaments coated with emission material and, under normal conditions, govern the life of the lamp.  In view of the fact that the life of the electrode is shortened by frequent starts, the lamp life is rated according to the number of times the lamp is started, or the burning cycle.  Operation at refrigerator temperatures may result in excessive bulb blackening and rapid depreciation in ultraviolet output.  Starting of the hot cathode lamp at low temperature is sometimes unreliable and may require special equipment.

 2.       Cold Cathode Germicidal Lamp

 The cold cathode lamp, instead of having tungsten filaments for electrodes, has sturdy cylindrical electrodes, and the lamp is started instantly by means of a high voltage rather than by a glow switch starter.  In view of the fact that the electrodes seldom wear out, the lamp life is usually governed by the ultraviolet transmission of the glass, rather than the electrode life and frequency of starts.  The lamp may be operated in refrigerator temperatures without causing excessive bulb blackening and resulting loss in ultraviolet output.  The high voltage assures dependable, instant starting, even at freezing temperatures.

 3.      Slimline Germicidal Lamp

 The electrical characteristics of the Slimline germicidal lamp are similar to those of the Slimline fluorescent lamp.  The lamp is the instant start type utilizing high voltage for starting instead of a glow switch starter to preheat the filament electrodes.  Although the lamp starts "cold" by means of  shock starting, it operates with the electrodes hot.  Lamp life, as with the standard hot cathode lamp, is governed by the life of the electrodes.  It is possible to operate the G36T6 lamp at four different currents, 120, 200, 300, 420 milliamperes, which results in four levels of ultraviolet output.

 The Slimline lamp is recommended chiefly for air ducts where high intensities of ultraviolet are required.  It is well adapted for water treatment, conveyor belts, and similar applications where a high intensity is needed in a limited space.

 In view of the high output of the lamp, extra precautions must be taken to prevent injury to products and personnel, either from direct or reflected ultraviolet rays.

OUTPUT THROUGHOUT LIFE

 The output of germicidal lamps slowly decreases throughout life, for the glass tube gradually loses its ability to transmit the short wavelength of ultraviolet.  (In case of the Slimline lamp, the life of the lamp is also governed by the frequency of starts, while this is not the case with the cold cathode lamp.)  The ultraviolet output also decreases if the line voltage is reduced.  Temperature and air movement will cause a decrease in lamp output, as described in the following paragraph.

TEMPERATURE

 Ultraviolet lamps operate most efficiently at room temperature, the lamp output being rated at an ambient temperature of 80^o F.  However, as with all gaseous discharge lamps, the output of the lamp will diminish as the temperature increases or decreases from this optimum point.  For instance, the output of a lamp at 40^o F ambient temperature is only about two-thirds of the output at 80^o F.   Drafts of air passing over the lamp, or its submersion in liquids, will cause a considerable decrease in output because they cool the tube below its normal burning temperature.  For this reason, especially at lower temperatures such as in commercial refrigerators, lamps should be shielded from air currents.  High velocities of air and low temperature may tend to shorten the useful life of lamp tubes.

 The Slimline lamp tubes, operating at 420 milliamperes, react somewhat differently from other lamps.  At this high current rating, the operating temperature of the lamps is higher than is the case with other lamps. Therefore, movements of air will serve to cool the bulb to a temperature that allows a greater percent of the maximum ultraviolet output than would be the case with a lamp having a lower operating temperature.

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