Determining The Sensitivity - Siemens FDA241 Planning, Mounting

4.11 Determining the sensitivity

Building Technologies
Fire Safety
The sensitivity for the parameter set alarm threshold is set on the aspirating smoke
detector. This is the level of sensitivity used to measure the airflow that is made up
of air from all the aspirating holes and the end caps.
The sensitivity at an individual aspirating hole is determined according to how
much air it contributes to the total airflow. The sensitivity of the individual aspirating
hole can be calculated.
Example of how to calculate the sensitivity at an aspirating hole:
The aspirating smoke detector has a sensitivity of 0.1 %/m Obs.
The aspirating hole highlighted by the arrow contributes 5 % of the air that makes
up the total airflow.
Therefore, the sensitivity of this aspirating hole can be calculated as follows:
6.5 6.2 6 5.8
0.1 % /m Obs
Figure 12: Aspirating holes and how much air they contribute to the total airflow
a Sensitivity of the aspirating smoke detector (example: 0.1 %/m Obs)
b Percentage of air contributed to total airflow (example: 5 %)
Obs Obscuration
Result: The highlighted aspirating hole has a sensitivity of 2 %/m Obs.
Collective effect
If smoke only comes from one aspirating hole, the aspirating smoke detector
detects this according to the calculated sensitivity. (In the example, the sensitivity
of the smoke detection system at the highlighted aspirating hole is 2 %/m Obs.) In
the event of a fire, smoke usually comes from multiple aspirating holes, which
increases the sensitivity of the smoke detection system. If smoke is coming from all
the aspirating holes, the sensitivity of the smoke detection system matches the
sensitivity of the aspirating smoke detector. (In the example, the smoke detection
system has a sensitivity of 0.1 %/m Obs). This is known as the 'collective effect'.
[%]
5.7 5.5 5.4 5.2
a
0.1 % /m Obs
= 100 %
b 5 %
Planning
Determining the sensitivity
5.1 5 4.9 4.8 4.7
= 2.0 % /m Obs
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