10
MAN3038-2
DISCOVERY PRODUCT GUIDE
7
Ionisation Smoke Detector
7.1
Operating Principles:
The Discovery Ionisation Smoke Detector uses the same outer case
as the optical smoke detector and is distinguished by the red
indicator LEDs. Inside the case is a printed circuit board which has
the ionisation chamber mounted on one side and the signal
processing and communications electronics on the other.
The ionisation chamber consists of a reference chamber contained
inside a smoke chamber (Fig. 4). The outer smoke chamber has
inlet apertures fitted with insect resistant mesh.
At the junction between reference and smoke chambers, the sensing electrode converts variations in
chamber current into voltage changes.
When smoke particles enter the ionisation chamber, ions become attached to them with the result that the
current flowing through the chamber decreases. This effect is greater in the smoke chamber than in the
reference chamber, and the imbalance causes the sensing electrode to become more positive.
The analogue voltage at the sensor electrode is converted to a digital format which is processed to provide
an analogue value for transmission to the control panel when the device is polled.
The Discovery Ionisation Detector, like all ionisation detectors, has some sensitivity to air movement (wind).
The extent to which the analogue value will change depends on the wind speed and on the orientation of the
detector relative to the wind direction. Relatively small changes in wind direction can cause significant
changes in analogue value.
For wind speeds up to 1m/s (200ft/min) the change in analogue value will not exceed 5 counts. Continuous
operation in wind speeds greater than 2m/s (400ft/min) is not recommended. However, wind speeds up to
10m/s (2000ft/min) can be tolerated for short periods and will not under any conditions increase the
probability of false alarms.
Ionisation smoke detectors are supplied in individual packing with a red lid serving as a dust cover which can
be left in place after fitting to prevent ingress of dust and dirt until commissioning of the system takes place.
At this point the covers must be removed.
Figure 4 – Sectional View – Discovery Ionisation Smoke Chamber
