Smoke Alarm User Manual

Installation, Operation and Maintenance Manual
Issue 1 Page 2-7
February 2001
2.3.1 Detector Head
Located within the Detector, the detector head is a highly sensitive detector utilising a laser, focussed to a
narrow beam. The laser beam exits a sample chamber into an attached beam dump area which monitors
the laser intensity. A receive optics assembly is mounted on the sample chamber with its optical axis
normal to the laser beam axis (see Figure 2.4).
A particle-size discriminator inhibits the detector from responding to large particles, thus preventing
occasional large particles (dust, pollen, soot, etc.) from generating an alarm. As the smoke particulate
passes into the sampling element a sensor electronically counts each particle. Particle counting HSSD
devices are much more sensitive to the prevailing concentration of small particulate (the tell tale sign of
early combustion) than conventional devices. Although larger particulate such as dust does not
significantly affect the signal recorded (number of particles) from either a background or a prevailing
smoke environment. Most particulate with a diameter greater than 10Pm is electronically recognised and
thus not added to the smoke count register. This ensures a true and clear measurement of the absolute
real time concentration. Electronic circuitry within the detector head ensures that no change in sensitivity
occurs with changes in air flow. The Detector is calibrated so that it will not respond to particle sizes
smaller than 0.01 or greater than 10 microns. The intensity of the laser is monitored by a photocell. If the
intensity of the laser begins to vary from the factory set level, a feedback circuit automatically
compensates the laser drive circuit accordingly. This circuit arrangement virtually eliminates any change
in laser energy output which in turn keeps the detector sensitivity stable.
An airflow measuring circuit monitors the flow of air through the detector. Should the air flow vary or stop,
a status signal is transmitted to the termination board.
One of the features of the detector head signal processing that provides the dynamic sensitivity range.
The sensitivity range can be adjusted via LaserNET software to meet with system requirements. A
processor located on the signal processing board controls communications between the detector head
and the termination board. Information recorded by the detector head is continually communicated to the
termination board.
Two types of detector head are available; the standard detector head which has a dynamic sensitivity
range of 0.005% to 1%/m (0. 0015% to 0.3%/ft) and an ultra sensitive detector head specifically designed
for the clean-room applications. The ultra sensitive detector head incorporates an avalanche photon
detector and has a dynamic sensitivity range of 0.001% to 0.1%m (0.0003% to 0.03%/ft) for use in very
clean applications. Detector sensitivity can be programmed to accommodate varying background levels.