Name two common types of automatic smoke detectors and how they operate.

• A. Ionization detectors detect large particles; Photoelectric detectors detect smoke via heat
• B. Ionization detectors detect large aerosols from flaming fires; Photoelectric detectors detect smoke via scattered light
• C. Ionization detectors detect small aerosols from flaming fires; Photoelectric detectors detect scattered light
• D. Ionization detectors detect smoke via scattered light; Photoelectric detectors detect small aerosols

Answer: (C)

Two common automatic smoke detectors rely on different sensing methods: ionization and photoelectric. Ionization detectors use a small amount of radioactive material to create ions in a air chamber, which allows a steady current to flow between two plates. When smoke particles enter the chamber, they attach to the ions and reduce the current, triggering the alarm. This makes them fast to respond to flaming fires that produce many tiny aerosol particles.

Photoelectric detectors shine a light inside a chamber and sense smoke by how it scatters or redirects the light toward a sensor. When smoke is present, the particles scatter enough light into the detector (or otherwise affect the light path) to trigger the alarm. This type is especially responsive to larger smoke particles typical of smoldering, non-flaming fires.

The best description matches that ionization detectors detect small aerosols from flaming fires, while photoelectric detectors detect scattered light. The other statements mix up the detection mechanisms or particle sizes, which doesn’t align with how these detectors actually operate.