How Does a Smoke Detector Work?
There are two basic kinds of smoke detectors: ionization detectors and photoelectric detectors. A smoke alarm uses one or both methods, sometimes plus a heat detector, to warn of a fire. The devices may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.
One type has a light sensitive cell inside. It also has a beam of light which shines into a small chamber. When smoke enters the chamber, it reflects the beam of light toward the cell. When the light strikes the cell, the alarm is set off.
The other type, an ionization detector, works differently. This device contains a tiny bit of radioactive material. This material allows the air molecules to carry a small electric current. When smoke enters, it interrupts the flow of electricity, triggering the alarm.
Both ionization and photoelectric detectors are effective smoke sensors. Ionization detectors respond more quickly to flaming fires with smaller combustion particles; photoelectric detectors respond more quickly to smoldering fires. In either type of detector, steam or high humidity can lead to condensation on the circuit board and sensor, causing the alarm to sound.
Ionization detectors are less expensive than photoelectric detectors, but some users purposely disable them because they are more likely to sound an alarm from normal cooking due to their sensitivity to minute smoke particles.
However, ionization detectors have a degree of built-in security not inherent to photoelectric detectors. When the battery starts to fail in an ionization detector, the ion current falls and the alarm sounds, warning that it is time to change the battery before the detector becomes ineffective. Back-up batteries may be used for photoelectric detectors.