Types of smoke detector and their working.

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.


Ionization smoke detectors are good general-purpose detectors which respond well to fast-burning (flaming) fires and are widely used for property protection.



Integrating ionization smoke detector


Available in the Series 60 and Series 65 ranges, this type of detector works on the same principles as the ionization 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 analog addressable systems


In XP95 systems, the integrating effect can be mimicked in both ionization 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.
Photo-electric smoke detector


Photo-electric 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 protection.



Heat detector


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 heat detectors have only one thermistor. They change to the alarm state at a preset temperature.


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 thermistor.


A heat detector may be more appropriate than a smoke detector where the environment is dirty or smoky under normal conditions. It must be recognized, however, that any heat detector will respond only when a fire is well established and generating a high heat output.



Multisensor detector


Available in the XP95A and Discovery UL ranges, this type of detector is basically a photo-electric 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 ionization detector.


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, smouldering fires.