At what stage does an ionization detector typically detect fire?

An ionization detector is designed to respond quickly to the early signs of combustion, particularly during the incipient stage of a fire. This stage is characterized by the initial development of fire when it is just starting to smolder and produce smoke but has not yet burst into flames. Ionization detectors operate using a small amount of radioactive material to ionize the air in the sensing chamber. When smoke from a smoldering fire enters the chamber, it disrupts the ionization process, causing a drop in current and triggering the alarm. This ability makes ionization detectors particularly effective for detecting fast-flaming fires or fires that produce little visible smoke at their onset.

In contrast, other stages such as the fully involved and alert stages represent later phases of fire development where conditions are more severe and smoke is usually more prevalent, potentially limiting the effectiveness of an ionization detector. The smoldering stage, while it may seem relevant, is typically regarded as part of the incipient phase, thus reinforcing the appropriateness of the incipient stage as the correct context for ionization detection.