page
6
is thata small electric current
flowsbetween the electrodes.
At thejunction between the
referenceand smokechambers
is thesensing electrodethat is
usedto convert variationsin
the chambercurrents intoa
voltage.
Whensmoke particles enter
the ionisationchamber,ions
becomeattached to them
withthe result that the current
flowingthrough the ionisation
chamber decreases. This
effect is greaterin the smoke
chamberthan in the reference
chamberand the imbalance
causesthe sensing electrode
to go morepositive.
The voltage on the sensing
electrode is monitored by
the sensor electronics and
is processed to produce a
signal that is translated by
the A/D converter in the
communications ASIC ready
for transmission when the
device is interrogated.
XP95 IONISATION SMOKE DETECTOR
XP95 Ionisation Smoke Detector Part Number 55000-500
OPERATING
PRINCIPLES
The XP95ionisation smoke
dete ctor has a mo ulded
self-extinguishing white
polycarbonate casewith
windresistant smoke inlets.
Stainlesssteel wiper contacts
connectthe detector to the
terminalsin the mounting
base. Inside the detectorcase
is a printedcircuit boardthat
has theionisation chamber
mounted on one side and
the address capture, signal
processingandcommunications
electronics on the other.
The ionisation chamber
systemis an inner reference
chambercontained insidean
Fig.1 Sectional view - XP95 Ionisation Smoke Detector
outersmoke chamber (Fig1).
The outersmoke chamber
has smokeinlet apertures
thatare fitted with an insect
resistantmesh.
The radioactive source
holderand the outer smoke
chamber are the positive
and negative electrodes
respectively. An Americium
241 radioactive source
mounted within the inner
referencechamber irradiates
the airin both chambers to
produce positive and
negativeions. On applyinga
voltage across these
electrodes an electricfield is
formedas shown in Fig 2.
The ionsare attracted to the
electrodeof the opposite
sign,some ions collide and
recombine,but the net result
Fig.2 DiagramshowinglinesofequipotentialfortheXP95IonisationSmokeMonitor