Detectors 200
Detectors 200 : Particle Detectors: (4.26) Smoke Detector: (2) Design: (2. 2) Ionization: An ionization smoke detector uses a radioisotope such as americium-241 to produce ionization in air; a difference due to smoke is detected and an alarm is generated. Ionization detectors are more sensitive to the flaming stage of fires than optical detectors, while optical detectors are more sensitive to fires in the early smouldering stage. The radioactive isotope americium-241 in the smoke detector emits ionizing radiation in the form of alpha particles into an ionization chamber (which is open to the air) and a sealed reference chamber. The air molecules in the chamber become ionized and these ions allow the passage of a small electric currentbetween charged electrodes placed in the chamber. If any smoke particles pass into the chamber the ions will attach to the particles and so will be less able to carry the current. An electronic circuit detects the current drop, and sounds the alarm. The reference chamber cancels effects due to air pressure, temperature, or the aging of the source. Other parts of the circuitry monitor the battery (where used) and sound an intermittent warning when the battery nears exhaustion. A self-test circuit simulates an imbalance in the ionization chamber and verifies the function of power supply, electronics, and alarm device. The standby power draw of an ionization smoke detector is so low that a small battery can provide power for months or years, making the unit independent of AC power supply or external wiring; however, batteries require regular test and replacement
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