Air in the dual sensing chamber is irradiated to produce ions that travel
to the positive and
negative electrodes, creating a current flow. As smoke enters the outer chamber, the flow drops and the voltage
increases. The detector will then enters the alarm state and the indicator LED lights up.
An intergating ionisation detector, suitable for use in areas where transient high levels
of smoke may be expected, is also available.
Ionisation detectors have 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.
The sensing part of the detector consists of two
chambers - an open, outer chamber and a semisealed
reference chamber within. Mounted in
the reference chamber is a low activity
radioactive foil of Americium 241 which
enables current to flow between the inner and
outer chambers when the detector is powered
up. As smoke enters the detector, it causes a
reduction of the current flow in the outer chamber
and hence an increase in voltage measured at the
junction between the two chambers. The voltage
increase is monitored by the electronic circuitry
which triggers the detector into the alarm state at
a preset threshold. An externally visible red LED
lights up when the detector changes to alarm
state.
An Integrating Ionisation detector, suitable for
use in areas where transient levels of smoke
may be expected, is also available.
