Smoke, Gas & Flame Detectors - Principle of Operation
The fourth instalment of the Principles of Fire Safety series looks at smoke, gas and flame detectors. Again, research and development has continued to improve well established detection technologies and provided an array of new technologies to improve fire detection while also being less susceptible to the causes of false alarms.
Before we go any further it’s important to lay the foundation for what is fire; fire also known as combustion is a sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the by-products of combustion being; heat, smoke & electromagnetic radiation (light). Personally I think a illustration explains this chemical reaction in terms easier to understand.
It is also important to recognise that smoke is an aerosol or mixture of particulates suspended in air that comprises a collection of airborne solids, liquid particulates and gases emitted when a material undergoes combustion.
Smoke detectors are recognised as the most common method of fire detection for life safety throughout the world. There are five types of smoke detection available in Australia with the most common two point types being photo-electric and ionisation. The other three being projected (optical) beam, aspirating and video smoke detection are generally used for specialist applications.
Before we go on to explain the operation of the ionisation and photo-electric smoke detectors, it is important to explain the inherent features of the overall design of the detector enclosure. These detectors are designed to regulate the flow of air through the detector and eliminate or reduce the possibility for ingress of foreign matter and insects. These features help to reduce false alarms and improve the performance of smoke detectors.
An important consideration in selecting the appropriate type of smoke detector are the following factors; fuel, speed of growth, flame and type of smoke produced. For example ionisation smoke detectors respond well to fast flaming fires (normally associated with invisible smoke), while photo-electric smoke detectors respond well to slow smouldering fires (often associated with visible smoke).
Ionisation Smoke Detector
An ionisation type smoke detector# is the earliest form of smoke detection originally developed by Swiss physicist Walter Jaeger in 1930. The ionisation smoke detector operates on the principal that, under normal circumstances, air in a chamber is ionised by the radioactive element (Americum 241) which causes the free and equal flow of electrons between two adjacent electrodes. When smoke particles enter the chamber between the electrodes, the normal flow of electrons is interrupted causing an alarm actuation.
The ionisation chamber is very sensitive to temperature and air pressure, which is overcome by have a second reference chamber.
Ionisation smoke detectors require a very low power to operate and traditionally have been the most regularly used residential smoke alarms. Ionisation smoke detectors are most effective for invisible particles of combustion such as those found in fast flaming fires.
For applications where slow smouldering fires are likely it is normally recommended to use a photo-electric smoke detector.
Photo-electric Smoke Detector
Photo-electric smoke detectors are continuing to gain momentum as the preference due to their early fire detection in life safety applications. As the name suggests the photo-electric smoke detector is an optical device comprising a transmitter and receiver. The transmitter and receiver are mounted inside a black chamber with the transmitter and receiver in an offset arrangement. Under normal circumstances, the transmitter emits a focused light beam into the chamber. The projected light is absorbed by the black walls of the chamber and the receiver receives no light.
When visible smoke particles enter the chamber the projected light is scattered in all directions. When this scattered light is detected by the receiver it will activate an alarm.
Photo-electric smoke detectors are more effective with visible particles of combustion however with modern electronics their performance to very low quantities of smoke can be improved while maintaining a relatively consistent immunity to deceptive phenomena.
It has been historically been common to use a alternating combination of ionisation and photo-electric smoke detectors in the path of travel to an exit due to their distinct performance characteristics. A propsed alternative to this arrangement is the use of a photoelectric smoke detector combining a heat detector as a multi-sensor detector for improved performance for fast flaming fires.
Projected (optical) Beam Smoke detector
A projected beam smoke detector (beam detector) also operates using a combination focused light transmitter and light receiver. Some modern systems integrate the transmitter and receiver into a single housing, with an an opposing retro-reflective surface (prism) to return the beam to the receiver. A beam detector operates on the principal of obscuration being that an alarm state occurs when the light beam is attenuated (reduced or interrupted) by the presence of smoke.
Beam detectors are designed to operate over long distances typically up to 110 metres and require a straight uninterrupted direct line of sight. Typical uses for beam detectors include warehouses, hangers, etc with long spans where multiple point type smoke detectors would be considered impractical.
Aspirating Smoke Detectors
An aspirating smoke detector (ASD) is an air sampling device that comprises four main components;
- A network of pipes comprising one or more holes to draw sample air
- A calibrated aspirator
- A particulate filter
- A calibrated smoke sensing device
Aspirating smoke detectors generally work on the same principal as photo-electric smoke detectors. When smoke enters the sensing chamber, light is scattered by the smoke particles. This scattered light is then detected by a sensitive light receiver.
Aspirating smoke detectors are typically more sensitive to a wide range of smoke particulate size and are often used as a very early form of smoke detection. Aspirating smoke detection systems may also include features and electronics to reduce the effects of deceptive phenomena.
Video Smoke Detection
Video smoke detection (VSD) is a relatively new technology and is primarily used for asset protection. Video smoke detection comprises three main components; a video camera, a dedicated computer and specialist software for image processing. This design means that the existing building video cameras may be used to provide the source video signal for software processing.
The processing software works by identifying the telltale pluming movement of heated smoke or the obscuration of recognised features within the video frame. The software is calibrated for a standard still image. When smoke is present within the video frame or a smaller defined section of the video image, the software detects the movement of the smoke within the frame. This movement is then analysed by the software using a proprietary algorithm. If the movement is recognised by the software as smoke an alarm signal is activated.
This method of fire detection is not currently approved as a primary method of fire detection recognised by Australian Standards. This has led to the technology being sold as niche product for asset protection for large areas where traditional fire detection methods are impractical.
One of the by-products of most forms of combustion is the gas Carbon Monoxide (CO). This colourless, odourless, tasteless and highly toxic gas is produced by incomplete combustion of carbon containing compounds.
Carbon monoxide fire detectors are generally suitable for a broad range of applications where the use of traditional fixed point heat or smoke detectors may be impractical due to the presence of deceptive phenomena.
Carbon monoxide fire detectors comprise an electronic circuit and an electrochemical cell that produces a small electrical current in the presence of carbon monoxide. The electronic circuit is calibrated to a normal range of atmospheric carbon monoxide. When the concentration of carbon monoxide increases the current produced by the cell also increases which in turn creates an alarm signal.
Carbon monoxide fire detectors have a relatively short life span 5-10 years and should be maintained strictly in accordance with the manufacturer’s recommendation for optimal performance.
Flame detectors respond to the production of one or a combination of ultra-violet or infrared spectrums of electromagnetic radiation. These detectors are often used in situations where there is a potential for the rapid development of fire such as flammable liquids. These detectors comprise an electronic circuit with an electromagnetic radiation receiver. Flame detectors are actuated when they receive electromagnetic radiation from one or more defined wave lengths are received according to their design in the ultra-violet or infrared spectrum.
One of the methods to improve the performance of flame detectors and reduce the effect of deceptive phenomena and false alarms has been to combine both ultra-violet and infra-red technologies into the one system or two or three separate wavelengths in the infrared spectrum.
Multi-sensor fire detectors
Over the last 20 years there has been a dramatic improvement in the ability of manufacturers to incorporate two or more of the technologies discussed this article, This combination of technologies has occurred to; (1) improve the performance of an existing method of fire detection; and (2) in some cases to reduce or eliminate the effect of deceptive phenomena causing false alarms. Multi-sensor detectors are not a panacea and must operate according to their approval. The combination of multiple sensors using clever algorithms that may suppress short term deceptive phenomena.