VESDA by Xtralis Introduction to the VESDA System Design Manual
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3 Introduction to Obscuration
Smoke obscuration is the effect of smoke on visibility. It is the most widely accepted measure for expressing
the concentration of smoke and refers to the percentage of light that is blocked, or obscured over a specified
length (meter/foot). Increase in obscuration levels decreases visibility, hence an obscuration of 10% obs/m
(3.13% obs/ft.) will reduce visibility to 90%.
Typically pure oxygen has an obscuration rate of 0% obs/m (0% obs/ft.). At 0.01% obs/m (0.003% obs/ft.)
visibility levels are in the order of approximately 40 Km.
Incipient smoke, invisible to the naked eye in ordinary conditions has an obscuration range of 0.1% obs/m to
1% obs/m (0.03125% obs/ft. to 0.3125% obs/ft.). The figure below shows the relationship between increase in
obscuration and decrease in visibility. Obscuration is used to define the sensitivity level of a smoke detector.
The alarm sensitivity range for VESDA detectors can begin at 0.005% obs/m (0.0015% obs/ft.), allowing
detection of smoke at the incipient stage from overheating wires or equipment.
3.3%/m
1%/ft
6.6%/m
2%/ft
10%/m
3.125%/ft
13%/m
4%/ft
EXIT EXIT EXIT EXIT
Figure 3-1: Relationship between increase in obscuration and decrease in visibility
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