VESDA
®
LaserSCANNER Product Manual
3
The Scanning Function
The LaserSCANNER is designed to sample air from different sectors and to identify through a
scanning process the sector reporting presence of smoke.
Scanning
During normal operation the scanner valves remain open and the LaserSCANNER draws air
from all the pipes. When smoke is detected at a higher level than the scan threshold, for a period
exceeding the configured scan delay (0 to 10 seconds), the detector performs a fast scan. During
the fast scan operation the LaserSCANNER samples air from each sector (pipe) separately by
controlling the scanner valves in the inlet manifold. The fast scan sequence for each pipe is
approximately 4 seconds.
The sector reporting the highest level of smoke above the alert threshold is designated as the
first alarm sector. The display modules assigned to the detector reports this by illuminating the
first alarm and alert LEDs, and displaying the corresponding pipe number on the numeric screen.
Having identified the first alarm sector, the LaserSCANNER performs a slow scan using
intelligent sequencing. The sector with the highest level of smoke (the first alarm sector) is
scanned first. Each of the other sectors is scanned one at a time. Intelligent sequencing scans
the first alarm sector in every alternate scan (e.g. if all the pipes are in use and pipe 1
corresponds to the first alarm sector then intelligent sequencing will scan Pipe
1,2,1,3,1,4,1,2......). The time spent sampling air in each sector depends upon the configured
sector time.
Adaptive scan threshold
The scan threshold is the configured threshold at which the detector commences scanning once
smoke has been detected at the threshold. The LaserSCANNER performs an adaptive scan
process to automatically set scan thresholds at the optimized level. The adaptive scan threshold
process is explained below:
•
Typically separate scan thresholds are set for day and night periods.
•
Scan thresholds are initially set to the lowest day or night alert threshold divided by the
number of pipes. If the threshold is set by AutoLEARN, the day or Night Alert Threshold is
divided by 2.
•
The scan thresholds are then adapted automatically to eliminate unnecessary and
excessive scan cycles.
-
Adapting the scan threshold upwards -
When the detected smoke level is lower than
the configured Alert Threshold, the scan threshold increases upwards by 0.005% obs/m
0.0016% obs/ft.) with every fast scan cycle. It will continue to do so until the scan threshold
is above the ambient smoke level or it equals the Alert Threshold. The scan threshold
cannot exceed the Alert Threshold
-
Adapting the scan threshold downwards -
At every changeover time (from Day to
night and from night to day) the detector determines if it is necessary to adapt the scan
threshold downwards. Thus, if the scan threshold has been adjusted upwards, then in the
following 24 hour period the detector begins a process of gradually lowering the scan
threshold downwards to an optimum setting. The detector maintains a record of the
maximum smoke readings measured in each day (or night) period. At the start of the next
day (or night) period the scan threshold is automatically scaled downwards by calculating
the difference between the current scan threshold and the maximum smoke reading for the
previous day (or night) period.