66 



RADIATION BIOLOGY 



Unusual enclosure or extremes of air temperature, such as in refrigerators, 

 ovens, and air ducts, will reduce the ultraviolet (jutput of the germicidal 

 tube to the same extent as the light output of a similar fluorescent lamp. 

 The reduction is about 10 per cent at 50° and 100°F, 20 per cent at 40° 

 and 1 10°F, and 30 per cent at 35° and 120°F. 



DEPRECIATION 



In common with fluorescent lamps, bactericidal tubes depreciate 

 rapidly during the first 100 hr of operation. This is considered a part of 

 the manufacturing process, and commercial lamps are given an initial 

 rating as if at \0d hr of normal operation. In Fig. 2-11 the approximate 



120 



1000 



2000 



LIFE, hr 



3000 4000 



1 — I — I — I — I — I — I — I — I — I — I — I — I — \ — I — I — I — I — I — I — I — I — I — r 



5000 6000 7000 8000 

 120 



10 20 30 40 50 60 70 80 90 100 

 PERCENTAGE KILL BY UNIFORM EXPOSURES 



Fig. 2-11. Depreciation, life, and bactericidal-effectiveness curves of typical low- 

 pressure ultraviolet sources. 



depreciation of low-pressure sources is shown. Those in high-transmis- 

 sion glasses start from 20 to 25 per cent above the 100-hr rating, whereas 

 those in f used-quartz glass or in similar Vycor glass start from only 

 slightly above the 100-hr rating, considered as 100 per cent in Fig. 2-11. 



The depreciation rate, and therefore the effective life, is affected by the 

 length of the operating periods; the shorter the average operation per 

 start, the faster the depreciation and the shorter the effective life. In 

 general, such lamps fail to start and operate normally at about the end of 

 their effective life, i.e., at about 60 per cent of their 100-hr rating. Lamps 

 with so-called "cold" electrodes provide some exception to these rules, 

 but they still have the basic depreciation limitations of constant opera- 

 tion. They may continue to start and operate normally after their out- 

 put has dropped to ineffective levels. 



It should be noted that ultraviolet killing is an exponential rather than 

 linear function of ultraviolet intensity. The curve at the right of Fig. 

 2-1 1 provides a typical illustration of the relatively small amount of 

 change in ultraviolet killing that may result from a large depreciation. 



