SMOKE DENSITY/OBSCURATION INFORMATION
The total obscuration of the infrared beam is dependent upon the density and the width of the smoke
cloud within the beam path. As the width of the smoke cloud increases, less dense smoke is required
for the same total obscuration within the beam path. In practice, the smoke cloud is assumed to be the
entire distance of the beam path so the total obscuration required for alarm is selected by setting the
sensitivity pot as recommended in Figure 9 (Page 11).
Since the total obscuration of the beam by smoke increases with distance, the detector can be less sen-
sitive if the distance is greater. The sensitivity of the detector should be less at greater distances be-
cause the obscuration by other contaminants such as dust increases also. The sensitivity should be set
to respond to the proper smoke obscuration and also to reduce the chance of a false activation.
As shown below, if the expected smoke density is 0.5% per foot, the obscuration at 50 feet is 20% while
at 250 feet it is 73%.
30 (9m) 80 (24m) 130 (40m)
180 (56m)
230 (70m) 280
(85m) 330 (loom)
Distance Between Transmitter and Receiver
Sensitivitv Pot Setting
Total Obscuration at Alarm
2
20%
3
30%
4
40%
5
50%
6
60%
7
70%
Smoke Density vs. Beam Length
FIGURE 15
18
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