How do Apollo fire detectors work?
Ionisation (ionization) smoke detector
The sensing part of the detector consists of two chambers - an
open, outer chamber and a semi-sealed 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, particles become attached to the ions, causing a
reduction in 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.
Ionisation smoke detectors are good general-purpose
detectors which respond well to fast-burning (flaming) fires and
are widely used for property protection.
Integrating ionisation smoke detector
Available in the Series 60 and Series 65 ranges, this type of
detector works on the same principles as the ionisation smoke
detector, but has modified signal processing circuitry which allows
an alarm threshold to be present for up to 20 seconds without
initiating an alarm.
This type of detector is suitable for use in areas where
transient high levels of smoke may be expected.
Integrating effect in analogue addressable systems
In XP95 systems, the integrating effect can be mimicked in both
ionisation and optical smoke detectors by adjusting the control
equipment software to cause an appropriate delay. Discovery smoke
detectors should be set to Mode 2 or Mode 4 to achieve this effect.
Click here to find out more.
Optical (photo-electric) smoke detector
Optical smoke detectors incorporate a pulsing infra-red LED
located in a chamber within the housing of the detector. The
chamber is designed to exclude light from any external source. At
an angle to the LED is a photo-diode which normally does not
register the column of light emitted by the LED. In the event of
smoke from a fire entering the chamber, the light pulse from the
LED will be scattered and hence registered by the photo-diode. If
the photo-diode "sees" smoke on the two following pulses, the
detector changes into alarm state and the indicator LED lights up.
The detector housing is identical to that of the ionisation
detector but has an indicator LED which is clear in quiescent state
but produces red light in alarm.
Optical smoke detectors respond particularly well to
slow-burning (smouldering) fires. They are widely used for life
Most conventional heat detectors (all Series 60 and Series 65
& AlarmSense A1R, BR and CR) operate by using a matched pair of
thermistors to sense heat. One thermistor is exposed to the ambient
temperature, the other is sealed. In normal conditions the two
thermistors register similar temperatures, but, on the development
of a fire, the temperature recorded by the exposed thermistor will
increase rapidly, resulting in an imbalance of the thermistors,
causing the detector to change into alarm state. Rate-of-rise
detectors are designed to detect a fire as the temperature
increases, but they also have a fixed upper limit at which the
detector will go into alarm if the rate of temperature increase has
been too slow to trigger the detector earlier.
The Series 65 and AlarmSense CS (static response) heat detectors
have only one thermistor. They change to the alarm state at a
Externally, the heat detectors are distinguishable from the
smoke detectors by having wide openings to the surrounding
atmosphere to allow good movement of air around the external
A heat detector may be more appropriate than a smoke
detector where the environment is dirty or smoky under normal
conditions. It must be recognised, however, that any heat detector
will respond only when a fire is well established and generating a
high heat output.
Available in the XP95 and Discovery ranges, this type of
detector is basically an optical smoke detector - so it will
respond well to smouldering fires. The addition of a heat sensing
element allows the multi-sensor to give a response to fast burning
(flaming) fires which is comparable to that of an ionisation
Multisensor detectors are general purpose detectors which
respond well to a wide range of fires.
Carbon Monoxide (CO) detector
Carbon Monoxide (CO) is a poisonous gas produced by combustion
and a CO fire detector is used to indicate the outbreak of a fire
by sensing the level of CO in the air. The detector has an
electrochemical cell which senses CO, but not smoke or other
combustion products. The cells do not require much power, so the
detector can be made electrically compatible with ordinary smoke
and heat detectors. Apollo manufacturers a CO detector as part of
the Discovery range.
CO detectors are particuarly good at detecting deep-seated,
To find out more, visit the CO Detectors page.
A beam detector is designed to protect large, open spaces and is
made up of three main parts: the transmitter, which projects a beam
of infra-red light; the receiver which registers the light and
produces an electrical signal; and the interface, which processes
the signal and generates alarm or fault signals. When a fire
develops, smoke particles obstruct the beam of light and, once a
pre-set threshold has been exceeded, the detector will go into
To find out more, visit the XP95 Beam Detector page.
A flame detector is designed to detect either ultraviolet (UV)
or infra-red (IR) radiation emitted by a fire. The XP95 Dual IR
flame detector is sensitive to low-frequency, flickering infra-red
radiation. This means that the detector can operate even if the
lens is contaminated by a layer of oil, dust, water vapour or
Flame detectors are effective in protecting areas where flaming
fires may be expected.
To find out more, visit the XP95 Dual IR
Flame Detector page.
Relative performance of Apollo detectors in test fire.
For definitions of terms used within the fire detection
industry, visit the Glossary.