The LWF Blog
Fire Safety for Facilities Management Personnel – Fire Detection & Fire Alarms – Part 187February 6, 2023 11:50 am
Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 186, LWF discussed the types of detector and what circumstances they are best suited for use in. In part 187, we begin to look at smoke detectors.
The two main types of point smoke detector are ionization chamber smoke detectors and optical scattering smoke detectors.
An ionization chamber detector contains a radioactive source which ionizes the space within the chamber’s air, allowing a small current to pass between two electrodes. Smoke particles from a fire enter the chamber and interfere with the ion transport which leads to ion-electron recombination and reduces the current between the electrodes. This interruption is what triggers the alarm condition.
An optical detector contains a light emitting diode and a receiver. The principle is based on light scattering. The presence of smoke scatters the light from the transmitter towards the receiver. However, in principle, obscuration of light by smoke could be used, as in the case of beam detectors.
Line-type smoke detectors, also known as beam detectors, use a beam of light, commonly in the infrared part of the spectrum. A transmitter unit and a receiver unit may be mounted on opposing walls up to 100 metres apart in areas of large volume. In a smaller area, the transmitter and receiver may be contained within the same housing and the beam is reflected off a relatively small passive reflector. Some types of line smoke detector also respond to thermal turbulence, making them combined heat and smoke detectors.
Smoke detectors tend to be more sensitive than heat detectors, which means they are able to respond more quickly to a fire. However, they are also more likely to cause false alarms. The overlap in the range of particle sizes to which optical and ionization chamber detectors are sensitive means that either type is suitable for general applications.
Ionization chamber detectors are sensitive to very small, invisible particles produced in rapid flaming and clean burning fires. Optical detectors are less sensitive to the invisible particles, but are more sensitive than ionization-chamber detectors to the larger particles issuing from slow smouldering, or from smoke that has ‘aged’. Optical detectors are more likely to produce false alarms from tobacco smoke and steam, whereas ionization chamber detectors are more likely to give false alarms in areas where cooking of any kind may take place (burnt toast being a prime example of a trigger). An ionization chamber detector may also give false alarms if installed in an area of high air flow.
In part 188 of this series, LWF will continue to discuss smoke detectors. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.
Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 35 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.