IONIZATION SMOKE DETECTOR

A typical ionization chamber consists of two electrically charged plates and radioactive source (typically Americium 241) for ionizing the air between the plates .The radioactive source emits alpha particles that collide with the air molecules and dislodge their electrons as molecules loser electrons ,they become positively charged ion. As other molecules gain electrons, they become negatively charged ions. Equal numbers of positive ions and negative ions are created. The positively charged ions are attracted to the negatively charged electrical plate, while the negatively charged ions are attracted to the positively charged p[late. this creates a small ionization current that can be measured by electronic circuitry connected to the plates.(normal condition in the detector)

Particles of combustion are much larger than the ionized air molecules. As particles of combustion enter an ionization chamber, ionized molecules collide and combine with them. Some particles become positively charged & some become negatively charged. As these relatively large particles continue to combine with many other ions, they become recombination centers, and the total number of ionized particles in the chamber is reduced. This reduction in ionized particles results in decrease in the chamber current i.e. sensed by the electronic circuitry monitoring the chamber. When the current is reduced by a predetermined amount, a threshold is crossed and alarm condition is established.