Bosch D296 Installation Manual page 6

6 en | System overview
Since the sudden and total obscuration of the light beam is not a typical smoke signature, the
detector sees this as a trouble condition, not an alarm. This threshold is at a sensitivity level
that exceeds 90% to 95% total obscuration . This minimizes the possibility of an unwanted
alarm due to the blockage of the beam by a solid object such as a sign or ladder inadvertently
placed in the beam path.
Very small, slow changes in the quality of the light source are not typical of a smoke signature.
Such changes can occur because of environmental conditions such as dust and dirt
accumulation on the transmitter , the receiver's optical assemblies, or both. Generally, the
automatic environmental compensation circuit compensates for these changes. When you
initially turn on the detector and run the setup program, it assumes the light signal level at
that time as a reference point for a normal condition. Over time, the quality of the light signal
degrades (perhaps due to dust), and the environmental compensation circuit compensates for
this change. The compensation rate is limited, ensuring the detector remains sensitive to slow
or smoldering fires. When the automatic environmental compensation circuit no longer
compensates for signal loss (as with an excessive accumulation of dirt), the detector signals a
trouble condition.
The receiver indicates a trouble condition if the beam strength increases more than 20%. An
incorrectly aligned transmitter and receiver or a partially blocked beam can cause a trouble
when the transmitter and receiver are installed.
Each transmitter sends an invisible infrared beam of a specific frequency and intensity. Each
receiver detects and measures the beam's intensity. See the following figure.
Figure 2.1: Infrared Transmission
As smoke obscures the beam, the receiver senses a decrease in the signal strength and
measures that decrease. The receiver compares the signal level with two preset thresholds:
–
–
If the signal falls below the alarm threshold for the programmed alarm period, the receiver
signals an alarm. See the following figure.
2017.03 | 7.0 | F.01U.068.899
1 Transmitter
2 Beam
an alarm threshold that is set using the sensitivity switch and
a trouble threshold that is preset at approximately 10%
3 Receiver
Installation manual
Long-range beam smoke detectors
Bosch Security Systems, Inc.