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Smoke Detection A smoke detector will detect most fires much more rapidly than a heat detector. This section will describe the various principles of smoke detector operation and their applications. Smoke detectors are identified by their operating
principle. Two of the operating principles are: 1) ionization As a class, smoke detectors using the ionization principle provide somewhat faster response to high energy (open-flaming) fires, since these fires produce a large number of the smaller smoke particles. As a class, smoke detectors operating on the photoelectric principle respond faster to the smoke generated by low-energy (smoldering) fires, as these fires generally produce more of the larger smoke particles. However, each type of smoke detector is subject to, and must pass, the same test fires at testing laboratories in order to be listed. Conventional smoke detectors provide a go, no-go form of detection. This means that other than alarm or no-alarm, no other information is transmitted to the fire alarm control unit. In order to provide a stable smoke detector, the system designer must ensure that the sensitivity level of the detector matches the worst environment in the facility to be protected. Some jurisdiction require that a minimum sensitivity level be used to help control the false or nuisance alarm issue. Technological improvements in microprocessor use in fire alarm systems have led to to the development of new smoke detector concepts. These new sensors use analog technology to measure the conditions in the area or space protected and transmit that information to the computer-based fire alarm control unit. This new sensor can report when it is too dirty to function properly or it is getting too sensitive due to any number of conditions in the protected space. Analog sensors provide an essentially false-alarm-free system from conditions normally found in buildings. This sensor technology also allows the system designer to adjust the sensor's sensitivity to accommodate the ambient environment or use an extra-sensitive setting to protect a high-value or mission-sensitive area. These sensors are available as photoelectric, and ionization, or combination thermal, photoelectric, and ionization units. As fire alarm systems technology advances, analog sensors will be the sensor of choice for any system application, regardless of system size. Ionization Smoke Detectors
___________________________________________________________________ smoke detector utilizing the ionization principle are usually of the spot type. An ionization smoke detector has a small amount of radioactive material that ionizes the air in the sensing chamber, rendering the air conductive and permitting a current flow through the air between tho charged electrodes. This gives the sensing chamber an effecive elecrical conductance. When smoke particles enter the ionization area, they decrease the conductance of the air by attaching to the air particles. This can be compared to a reference conductance and by disagreement an alarm is activated.
___________________________________________________________________ Air Sampling Smoke Detectors Cloud chamber smoke detection principle: A smoke detector utilizing the cloud chamber principle is usually of the sampling type. An air pump draws a sample of air fromthe protected area into a high-humidity chamber with detector. After the air sample has been raised to a high humidity, the pressure is lowered slightly. If smoke particles are present, the moisture in the air condenses on them, forming a cloud in the chamber. The density of this cloud is then measured by a photoelectric principle. The detector responds when the density is greater than a predetermined level. Continous air-sampling smoke detection: In addition to the cloud chamber smoke detection devices, there are other smoke detection devices that actively and continuously sample the air froma protected space. The air-sampling system consists of sampling pipes spaced uniformly over the ceiling, together with two supplemental pipes arranged to sample the return air exiting from the monitored space. Each one of the ceiling pipe drops is capped and has a small air sampling hole drilled in the cap to draw in a sample of air from that location. There are also sampling holes drilled in the section of the supplemental pipes that extend across the return-air grilles. This network of piping is connected to the detector/control unit where there is a fan, or aspirator, that creates a flow of air in the piping network, and this flow causes the pressure inside the pipe to be less than the local atmospheric pressure. The flow creates a slight vacuum; therefore, the piping network continually draws in air. The sampled air is drawn through a filter to the detector assemply. Inside the detector is a very intense light source that irradiates the sampled air. If there are smoke particles in the sampled air, the device, which can sense smoke particles in extremely low concentrations, will activate the first of three levels of alarm conditions. These systems are typically used in application where dollar densities are very high, such as in electronic data-processing areas and museums, or where equipment survival is paramount to continuity of operatins, such as in the communication industries.
___________________________________________________________________ Other Techniques of Smoke Detection Gas-Sensing Fire Detectors Many changes occur in the gas content of the environment during a fire. In large-scale fire tests, it has been observed that detectable level of gases are reacched after dectetable smoke levels and before detectable heat levels. One of two operating principles, i.e., the semiconductor and catalytic principle, may be used in a gas-sensing fire detector. Semiconductor Principle Fire-gas detectors of the semiconductor type respond to either oxidizing or reducing gases by creating electrical changes in the semiconductor. The subsequent conductivity changes of the semiconductor causes actuation of the detector. Catalytic Element Principle Fire-gas detectors of the catalytic kind contain a material which, in itself, remains unchanged, but which accelerates the oxidation of combustible gases. The resulting temperature rise in the element causes detector actuation.
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