3
MAN3037-2
XP95 PRODUCT GUIDE
5
Ionisation Smoke Detector
5.1
Operating Principles:
The XP95 ionisation smoke detector has a moulded self-
extinguishing white polycarbonate case with wind resistant smoke
inlets. Stainless steel wiper contacts connect the detector to the
terminals in the mounting base. Inside the detector case is a printed
circuit board that has the ionisation chamber mounted on one side
and the address capture, signal processing and communications
electronics on the other.
The ionisation chamber system is an inner reference chamber
contained inside an outer smoke chamber (Fig 1). The outer smoke
chamber has smoke inlet apertures that are fitted with an insect
resistant mesh.
The radioactive source holder and the outer smoke chamber are the positive and negative electrodes
respectively. An Americium 241 radioactive source mounted within the inner reference chamber irradiates
the air in both chambers to produce positive and negative ions. On applying a voltage across these
electrodes an electric field is formed as shown in Fig 2. The ions are attracted to the electrode of the
opposite sign, some ions collide and recombine, but the net result is that a small electric current
flows between the electrodes. At the junction between the reference and smoke chambers is the sensing
electrode that is used to convert variations in the chamber currents into a voltage.
When smoke particles enter the ionisation chamber, ions become attached to them with the result that the
current flowing through the ionisation chamber decreases. This effect is greater in the smoke chamber than
in the reference chamber and the imbalance causes the sensing electrode to go more positive.
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.
Figure 1 – Sectional View – XP95 Ionisation Smoke Chamber
