Incipient Stage Detectors

Combustion produces microscopic particles when a fire is just starting. Ionization-type particulate detectors (Fig. 46-1) are designed to detect these particles by noticing a reduction in the electrical current flow and to set off an alarm. Ionization-type detectors work best indoors where there is stagnant air, where the air velocity is low, and where there is little visible smoke with large particles. Ionization-type detectors respond best to fast-burning flaming fires, which need a fast response and produce less smoke. They should not be installed on warm or hot ceilings, or in kitchens, bakeries, workshops with open flames or burners, or where there are concentrated engine exhaust fumes. They need periodic cleaning to remove dust, and regular recalibration. Because the incipient stage of a fire also changes the gas content of the air, gas sensing fire detectors are often used along with particulate detectors. Incipient stage detectors cover between 14 square meters (150 square ft) and 84 square meters (900 square ft), depending on the type of detector and the situation.

Wilson cloud chamber type detectors are sensitive to microscopic particles in the early stages of a fire but insensitive to dust. They use continuous air sampling and give few false alarms. Wilson cloud chamber detectors require piping and are expensive in small installations. The price becomes competitive when over 30 detection points are needed. These detectors are used in high-value installations like museums, data processing spaces, libraries, clean rooms, and facility control rooms.

Photoelectric smoke detector for smoldering stage

Ionization particulate detector

Photoelectric smoke detector for smoldering stage

Figure 46-1 Automatic fire detectors.

Ionization particulate detector

Ionization particulate detector with microprocessor adjusts sensitivity to environmental conditions, for incipient stage detection

Figure 46-1 Automatic fire detectors.

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