High Performance Optical Technology Comes of Age
Effective smoke detector technology can significantly influence the outcome of a fire emergency. With users trusting people’s lives first and foremost, as well as valuable company assets, to smoke detectors, it is paramount that devices can be relied upon. Also key when selecting fire detection technology is to assess the different types of detector available and identify the right option for the right application. From the 1950s, ion chamber smoke detectors (ICSDs) had a virtual monopoly in smoke detection applications until the development of optical chamber smoke detectors (OCSDs) in the 1970s. The rise of optical chamber technology, and the impact of radiation and underperformance of ICSDs, led to OCSDs becoming the detection system of choice.
Further innovations have since been introduced, most notably the combination of optical and heat sensing in one high performance optical (HPO) detector. The development of this technology, alongside additional testing and analysis of both ICSD and OCSD technologies, poses further questions regarding the use of ionisation smoke detectors. Here, Phil Cunliffe, Senior Product Manager EMEA at Tyco Fire Protection Products, discusses why HPO detection is the recommended choice for a wide range of applications.
Ionisation vs. optical
ICSDs use an ionisation chamber and radioactive material to detect smoke. This radiation is contained between two electrically charged plates and ionises the air, causing current to flow between the plates. The flow of ions is disrupted when smoke enters the chamber, reducing the flow of current and activating the alarm. ICSDs are more sensitive to the small smoke particles produced by flaming fires, enabling early detection of the fire before the smoke gets too thick. This high level of sensitivity was a key factor in ICSDs becoming the principal contributor in the development of automatic fire detection and alarm systems throughout the 50s and 60s. This eventually led to an increase in the safety of occupants of buildings in case of fire and the decrease in property losses.
Although the ability of ICSDs to quickly detect fast burning fires and fires that produce lighter smoke particles has long been recognised, some aspects of their performance have led to their decline. This has even resulted in a number of UK and European manufacturers ceasing to offer them to system designers and installers. There is increased awareness that ICSDs do not perform well in detecting smouldering fires and/or fires that produce heavy and dark smoke particles – a major risk when responding to fires involving modern furniture.
ICSDs have proved susceptible to a wide range of false alarm sources due to being highly responsive to the invisible, small particles of smoke produced by many ‘normal’ events. The straightforward design of ICSDs that has made them a workhorse of the fire detection technologist has also been a limiting factor in adapting them for this false alarm risk. Finally, the mounting burden of national and international regulations regarding radioactive sources has made it increasingly costly for companies to manufacture ICSDs and ship them to installation sites.
These drawbacks have led to increased favouring of optical chamber (also known as photoelectric) smoke detectors. These devices function by aiming a light source into a sensing chamber at an angle away from the sensor. Normally, the light passes in front of the detector in a straight line. When smoke enters the optical chamber, light is scattered by the smoke particles and reflected onto the sensor, which triggers the alarm. OCSDs are more sensitive to larger smoke particles so respond quickly to slow-burning fires, such as smouldering soft furniture and overheated electrical equipment, making them the detector of choice for this type of fire risk. Less likely to be triggered accidentally, OCSDs also work effectively in areas that could be susceptible to false alarms, such as kitchens. Plus, they contain no radioactive material, making them a safer option.
However, OCSDs come with some shortcomings of their own, notably their poor response to clear flaming fires and their sensitivity to dust and insect infestation, which can cause a high number of false alarms, particularly in rural areas during the summer season. These limitations have been largely overcome by a new class of optical detector.
The development of HPO detector technology
Introduced by Tyco Fire Protection Products, HPO detectors were one of the first smoke detectors to exploit the possibility of using temperature increase in combination with the small amount of optical scatter produced during flaming fires. The devices combine optical and heat detector technology to react to a range of fire types, including slow smouldering fires and open flames – previously only easily detected by ion chamber detectors. Adopted by the UK as well as global manufacturers, HPO technology has been a key contributor in providing safer detection of a wide range of smoke, while reducing the false alarm risks previously associated with ICSDs.
In normal ambient conditions, the HPO detector behaves like a normal optical detector. When it detects a rapid rise in temperature, its sensitivity increases and the presence of smoke confirms a fire condition, which is transmitted as an alarm. The ingenious design of Tyco’s HPO detectors, based on the well-established false alarm resistance of Tyco’s optical detector design, and their improved signal processing significantly reduce the effect of dust and eliminate entirely the false alarms cause by small insects.
A thermistor in the electronic assembly senses the temperature rise and provides information to the detection algorithm software on the dynamic changes in temperature which may occur during a fire. The sensitivity of the smoke sensor therefore rises proportionally with dynamic increases in temperature until the pre-determined alarm response threshold is reached.
The reliability of fire detection in buildings can be greatly improved with detectors that are able to give a more uniform response to the range of fires and environments which can occur. The way the fire itself starts can be an influential factor in the development of the fire – from a non-extinguished cigarette, an electrical fault or an arson attack, for instance. Plus, the environment in which the fire develops may vary due to circumstances present at the time, for example, temperature, ventilation or air circulation, open or closed windows and doors. Fires develop differently depending on the nature of the fuel; natural and synthetic furniture, for example, will burn at different rates. Detectors must therefore deliver exceptional reliability to respond to this unpredictability.
For over 20 years, Tyco Fire Protection Products’ HPO detector has provided equally good response to fast developing, clear burning flaming fires as slow developing, smouldering fires. The technology is able to analyse not just the kind of smoke present but also other environmental parameters that occur during a fire condition, such as changes in temperature. An added benefit is improved immunity to false alarm sources compared with the previously used detectors.
Response characteristics of the HPO detector, compared to those of ICSDs and OCSDs, in a series of Test Fires specified in the EN 54 (and ISO 7240) series have illustrated this. The alarm threshold responses showed the HPO detector to give the most consistent response across all four Test Fires, in comparison with the other two types of smoke detectors. By using the rise in temperature, it also produced a response equivalent to or better than that of the ICSDs in the flaming fires TF4 and TF5.
Over the last two decades, several standards have been developed for HPO detectors to assess their performance against a set of commonly agreed requirements. The European Insurance Committee (CEA) was the first organisation to address the need for a standard relating to HPO smoke detectors, and published prevention specification CEA 4021, Requirements and test methods for multi-sensor detectors, in 1999. Updated in 2003, this responds to smoke and heat, and smoke detectors with more than one smoke sensor. ISO followed in 2004 with the publication of ISO 7240-15, the international standard for Point type fire detectors using scattered light, transmitted light or ionisation sensors in combination with a heat sensor. This much awaited standard was adopted in the UK without changes and published as BS ISO 7240-15. Tyco’s HPO detectors comply with and have been certified against these standards. More recently, the much-awaited EN 54 part 29 standard was approved in 2015. Entitled ‘Multi-sensor fire detectors – Point detectors using a combination of smoke and heat sensors’, this will supersede all conflicting national standards from January 2017.
A holistic solution
ICSDs have been the mainstay of the fire detection industry for several decades and have undoubtedly saved many lives and much asset loss. While their detection performance for flaming fires is not in question, there is a growing awareness of the problems associated with the use of radioactive sources, and the potential for false alarms from ‘normal’ events. The availability of low cost, reliable OCSDs has done much to shift the emphasis away from ICSDs, but the performance of optical detectors in flaming fires has always limited their acceptance by the industry.
The introduction of HPO smoke detector technology has extended the fire detection capability of OCSDs to provide good performance in both flaming and smouldering fires, making them an ideal solution for fire risks from modern materials in residential, commercial and industrial premises. Offering an effective non-radioactive alternative to ionisation smoke detectors, this advanced type of smoke detection technology enables users to reduce false alarms without any real sacrifice in detection capabilities.