Ampac FIRERAY 100R, Product Manual

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FIRERAY PRODUCT GUIDE
Page 3
2.2 Operation Principles
The Optical Beam Smoke Detector works on the principle of light obscuration. The photosensitive
element of the Optical Beam Smoke Detector sees light produced by the Transmitter in a normal
condition. The Receiver is calibrated to a preset sensitivity level based on a percentage of total
obscuration. AMPAC’s range of Optical Beam Smoke Detectors offers various sensitivity settings to
choose from.
Unlike point type photoelectric smoke detectors, Optical Beam Smoke Detectors are generally less
sensitive to the colour of the smoke. Therefore an Optical Beam Smoke Detector may be well suited
to applications unsuitable for point type photoelectric detectors, such as applications where the
anticipated fire would produce black smoke. Optical Beam Smoke Detectors do require visible
smoke and therefore may not be as sensitive as Ion detectors in some applications.
Optical Beam Smoke Detectors are sensitive to the cumulative obscuration presented by a smoke
field. A combination of smoke density and the linear distance of the smoke field create this
cumulative obscuration across the projected light beam. Cumulative obscuration is therefore a
measure of the percentage of light blockage.
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Note: Condensation can be considered as an obscuration phenomenon.
Since the sudden and total obscuration of the light beam is not a typical smoke signature, the Optical
Beam Smoke Detector will see this a fault condition, not as an alarm. The threshold is typically set
by the manufacturer at a specified sensitivity level. This minimises the possibility of unwanted alarm
due to the blockage of the beam by a solid object, such as a sign or a ladder being inadvertently
placed in the beam path.
Very small, slow changes in the obscuration of the light source are also not typical of a smoke
signature. These changes occur because of environmental conditions such as dust or dirt
accumulation on the Optical Beam Smoke Detector’s optical assemblies or on the reflector surface.
Small, slow changes may also occur due to building movement.
Changes caused by environmental conditions are typically compensated for by an automatic gain
control (AGC). When the Optical Beam Smoke Detector is first turned on and put through its setup
program, it assumes the light signal level at that time as a reference point for normal condition.
As the quality of the light signal degrades over time, the AGC will compensate for this change. The
rate of compensation is limited to ensure that Optical Beam Smoke Detector will maintain its set
sensitivity and will also still be sensitive to smouldering or slow building fires.
When the AGC can no longer compensate for the loss of signal (as with accumulation of dust or dirt
or building movement) the Optical Beam Smoke Detector will signal a fault condition, so that
maintenance can take place.
With a motorised Optical Beam Smoke Detector, it is also possible to steer and reposition the unit to
retain good alignment after building movement.