Ionisation smoke alarm is based on the principle of ion flow within a chamber.  An alarm condition
occurs when the ion flow is affected by smoke particles.  This type of alarm is most effective to detect
small particles of smoke.

Photoelectric smoke alarm is based on the principle of optical detection. It is also known as the
"scattered" light principle. An alarm condition occurs when smoke particles enter a light path and
some of the light is "scattered" by reflection and refraction onto a sensor. This type of alarm is best
for areas where dense smoke may occur, such as in ductwork.
 
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