for the future
Ionisation Chamber Smoke Detectors
respond very quickly to smoke composed of
very small particles. They respond appreciably slower to the dense smoke composed of
When smoke is introduced
into the sampling chamber, the potential of the collector increases.
If the collector potential
increase exceeds a
preset level, the detector signals an alarm condition to the control equipment.
detectors are normally used where both fire and material values are
low or where detectors
cannot be used in the system. Heat detectors can provide an
acceptable though less sensitive alternative.
types are available which are the Rate of Rise detector, the Fixed
the Line Type detector.
Optical Beam Detectors
beam detectors consist of a Transmitter and a Receiver. They are
placed 10 to 100 metres apart.
type of detector is specifically designed for large open-type areas
Where the installation of
detectors would be difficult. The
transmitter unit projects a infra-red light beam directly at the
receiver unit. The receiver unit converts the received light
beam into a signal which is
continuously monitored by the detector. If fire breaks out in
an area protected by these
detectors, smoke particles rising upward interrupt or partly
deflect the light beam thus
reducing the strength of beam received by the receiver unit.
includes a small pump which draws samples of the room air through a
tube into a
element. The detector provides a number of outputs, each of which
relate to a different density of
smoke contained in the air being sampled. It is normal
practice to monitor at least two outputs
from each detector, one can indicate that smoke is present in
the air and the
other that fire is present.
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