Difference between Optical and Ionisation smoke alarms

Ionisation Alarms where should they be used?
Ionisation type sensors are particularly sensitive to the almost invisible smoke produced by fast flaming fires.This makes them more liable to false alarm due to cooking fumes if sited in a hallway close to a kitchen. Ionisation alarms are less vulnerable to false alarms caused by dense tobacco smoke,excessive dust and insect ingress. The BS 5839: Pt.6: 2004 Standard recommends that ionisation alarms should not be used in hallways and landings,where there is a risk of false alarms caused by cooking fumes.


  1. Inside the sensor chamber is a minute (safe) radioactive element that ionises the air within.This causes a small current to flow in the chamber and this will remain constant for the life of the alarm unless smoke particles enter.
  2. When smoke enters the sensor chamber,the balance of the current is disturbed.
  3. This is detected by the electronics in the alarm circuitry and a signal is sent to the Integrated Circuit (IC).
  4. This causes the alarm sounder to operate.

Optical Alarms where should they be used?
Optical sensors are more responsive to

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