The two common smoke alarms sense different physics. An ionization chamber holds a tiny americium-241 source that keeps a small current flowing through ionized air; smoke particles attach to the ions, the current falls, and the alarm sounds. A photoelectric alarm shines a light beam past a photodiode placed off-axis, so it stays dark until particles scatter light into it. That difference produces a hard asymmetry: ionization responds sooner to the small, near-invisible particles of a fast flaming fire, while photoelectric responds sooner to the large particles of a slow smouldering fire such as a cigarette in upholstery. The gap runs in opposite directions for the two fire types and is not symmetric: in a smouldering fire photoelectric can lead by many minutes, even tens of minutes, while in a flaming fire ionization typically leads by well under a minute or two. The familiar confusion is that one technology is simply better; it is not, it is a bet on which fire you expect. After this Concept you can predict which alarm technology sees a given fire first, and explain why a home with one type only has a blind spot.
In fire protection, smoke detectors are classified by their sensing mechanism: photoelectric detectors use a light beam inside a chamber that scatters when smoke enters, triggering the alarm, while i…