Product guide

page
12
without the presence of smoke,
if sustained for 20 seconds.
The processing algorithms in
modes 1 to 4 incorporate
drift compensation.
The characteristics of the
five response modes are
summarised below.
Mode 1 has very high smoke
sensitivity combined with
high temperature sensitivity.
This gives a high overall
sensitivity to both smouldering
and flaming fires.
Mode 2 has a smoke
sensitivity similar to that of a
normal optical smoke
detector but has no response
to temperature. This mode is
therefore equivalent to a
standard optical detector. It
is suitable for applications in
which wide temperature
changes occur under normal
conditions.
Mode 3 has moderate smoke
sensitivity combined with a
moderate sensitivity to heat.
This combination is
considered the optimum for
most general applications
since it offers good response
to both smouldering and
flaming fires.
Mode 4 has lower than
normal smoke sensitivity
combined with high heat
sensitivity. This makes it
suitable for applications in
DISCOVERY MULTISENSOR DETECTOR
Discovery Multisensor Detector ▲ Part Number 58000-700
OPERATING
PRINCIPLES
The Discovery multisensor
detector contains an optical
smoke sensor and a
thermistor temperature
sensor whose outputs are
combined to give the final
analogue value. The way in
which the signals from the
two sensors are combined
depends on the response
mode selected. The five
modes provide response
behaviour which incorporates
pure heat detection, pure
smoke detection and a
combination of both. The
multisensor is therefore
useful over the widest range
of applications.
The multisensor construction
is similar to that of the
optical detector but uses a
different lid and optical
mouldings to accommodate
Fig.5 Sectional view - Discovery Multisensor Detector
PCB
Twin Alarm LEDs
RFI Shield
External
moulding
Chamber cut
away to reveal
optical bench
Thermistor
Optical chamber
the thermistor temperature
sensor. The sectional view
(Fig 5) shows the
arrangement of the optical
chamber and the thermistor.
The signals from the optical
smoke sensing element and
the temperature sensor are
independent, and represent
the smoke level and the air
temperature respectively in
the vicinity of the detector.
The detector’s micro-
controller processes the two
signals according to the
mode selected. When the
detector is operating as a
multisensor (i.e. modes 1, 3
and 4) the temperature
signal processing extracts
only rate-of-rise information
for combination with the
optical signal. In these
modes the detector will not
respond to a slow
temperature increase – even
if the temperature reaches a
high level. A large sudden
change in temperature can,
however, cause an alarm
which a certain amount of
fumes or smoke is
considered normal.
Mode 5 has no smoke
sensitivity at all, but gives a
pure heat detector response
meeting the response time
requirements for a Class A1
detector in the draft European
standard EN54
–
5:2000. In
this mode the detector will
respond to slowly changing
temperatures and has a
“fixed temperature” alarm
threshold at 58°C. The
analogue value in this mode
will give the approximate
air temperature over the
range 15°C to 55°C.
In mode 5, the smoke sensor
is still active though it does
not contribute to the analogue
signal. As a consequence, if
the detector is used in a
dirty or smoky environment
the optical sensor drift flag
may be activated in the
heat-only mode.
Note: in situ testing of the
multisensor detector should
be carried out as for smoke
detectors in response modes
1-4 and for heat detectors in
response modes 5.
Design Note: if the
multisensor is to be used in
mode 5, heat detector
spacing/coverage should be
applied.