Ionisation Smoke Detector - Fire Detectors
Fire detectors detect fire at an earlier stage and helps personnel to deal with the fire effectively using minimum resources. There are different types of fire detectors such as heat detectors, smoke detectors and flame detectors. Heat detectors monitor abnormal rise of temperature, smoke detectors monitor presence of smoke and flame detectors are activated by presence of flame at a particular frequency. On detection they give rise to an alarm as a warning, which is displayed on bridge.
Smoke detectors are widely used since they can give much earlier indication of fire, i.e. the smoke. As soon as smoke starts coming out, detector activates and alarm is sounded. A variety of smoke detectors are available. Ionization type is most commonly used.
Ionization type smoke detector consist of a radioactive source fitted at the top as shown in figure. A perforated plate called collector receives part of the radiations, also ionizes part of the surrounding atmosphere. The collector creates two region here. This increases the conductivity of the atmosphere. Now positive polarity is connected to the top part and negative polarity is connected to to the chamber cover. This complete a circuit when smoke detector is connected to its closed grid. The collector balances electric potential in the two regions it divided. Both these potential cancels and no triggering is induced normally. The presence of smoke alters the ionization of the air and electric potential of two region is unbalanced, which causes triggering of the alarm.
Sensitivity and False Alarms
Ionization smoke detectors have very good sensitivity to particle size of 5 micrometer and below. Sensitivity increases with reduction in particle size. Similarly sensitivity reduces as particle size increases. Adjustment of sensitivity, according to the conditions where detector is used, can be done from within the detector. The voltage applied to the detector is not continuous. It is pulsed and fed to the detector. Alarm will not be triggered as soon as the triggering potential is reached, instead it is activated when the triggering potential is maintained for a specified percentage of voltage pulses. This avoids false alarms caused by the detector.