-smoke detection
are designed to detect the presence of smoke, typically as an indicator of fire. Commercial automatic smoke detectors were first introduced in the 1960s. Since then they have evolved into heat-based detector systems, which are considered to be more reliable than their ionization chamber counterparts for several reasons.
The sensors used in modern NIR smoke detectors work via three main principles:
i) Thermal detection is based on a change in thermal radiation emitted by heated objects compared to their normal temperature surroundings. The so-called difference pattern consists of two spatial patterns that differ from each other with respect to temperature distribution or intensity. Heat causes changes in contrast and/or color between detected objects and background;
ii) Optical detection is primarily based on the detection of light reflected by objects. Smoke particles can reflect or scatter visible and invisible (infrared) light;
iii) Electrical detection is based on the changes that occur in an electric current when smoke interferes with its flow. This interference may be caused by the high conductivity of small smoke particles, which allows them to bridge electrical contacts at certain locations.
-The most common type of heat-based detectors uses a sensor tuned to 8–14 micrometres (um). At this wavelength, thermal emissions from fires are especially intense compared with emissions at other wavelengths. By focusing on this spectral band, heat detectors do not need filters to block out unwanted signals such as sunlight, dust, etc., but instead rely entirely upon the wavelength of the source. The main disadvantage is false alarms due to accidental (non-fire) heat sources and poorly insulated electrical equipment, which produce similar emission spectra.
-Optical detectors function on stereoscopic principles: two cameras observing different angles in a single housing. Light from both cameras is combined and routed through an interference filter that passes only light with wavelengths at the emissivity peak of a fire, rejecting light from other sources including sun, skylight or room lighting. This filtered light then passes into a ratio detector circuit, where it forms the basis for triggering an alarm signal when smoke obscures enough light from one camera to create a disparity between its reading and that from the other in a certain percentage.
-Because these detectors rely on visible light, they must be carefully shielded from direct sunlight and any other sources of intense visible radiation such as flashing lights or strobe lighting. Optical detectors also excel in false alarm prevention because the sensors only alert when smoke obscures a certain percentage of the scene.
-Electric detectors use artificially generated microwave signals to detect smoke by measuring changes in transmission through an absorptive medium caused by particle build-up on a piece of surface-mounted wire mesh. These systems often use two pieces of wire mesh at 90-degree angles that change electrical properties based on changes in currents running through them due to deposited particles.
-The clean metal is a good conductor while the deposited particles cause excessive resistance which can be measured and is an indication of smoke presence. Single level smoke detectors sense the electrical resistance of the mesh; dual-level units also incorporate temperature to provide a better indication of smoke presence.