An ionisation using a single radioactive source is shown diagrammatically in figure above. The ionization smoke detector detector has a chamber that is open to the air and is divided into two regions by a perforated electrode known as the collector. A small radioactive source ionises the air in these two regions and the electrical potential between them is balanced on the collector. When smoke enters the chamber, the balance is disturbed by an amount dependent on the smoke density. This change forms he basis of the analogue output to the trigger circuit.
Sensitivity and False Alarms
Ionization smoke detector responds best to invisible (below 5 micro meter diameter) particles. The sensitivity can be varied by adjusting the threshold value or varying the configuration of the closed chamber characteristics. Their sensitivity is very high for particles of one micrometer and below but falls off with increase in particle size.
Susceptibility to false alarms is decreased by the use of pulse type detectors in which the voltage is applied in pulses and the alarm trigger is operated only after the threshold potential has been maintained during a specified percentage of pulses.
“Fire Safety at Sea” by Dr. James Cowley