How different fire detection sensors work

Photoelectric sensors

These detect large smoke particles from .05 to 10,000 microns, smoke typical of the slow smouldering fire. They operate by projecting a light source into a sensing chamber. A light receiver is positioned at some angle relative to the light source. If smoke is present in the chamber, light is reflected and refracted by smoke onto the receiver to produce a signal.

Ionization sensors

These react to particles smaller than two microns, which are characteristic of gases and fast flaming fires. Inside the detector two charged plates are separated by an air gap. To create the current, a small radioactive isotope emits high-energy alpha particles into the gap. The alpha particles knock electrons off the air molecules leaving them with a positive charge. The free electrons then attach themselves to other molecules giving them a negative charge and this movement creates a small current flow. Smoke particles attach to the ions and slow them down, affecting the current flow.

Heat sensors

These use anything from solid-state to bi-metal contacts to indicate the presence of heat when the temperature has exceeded a specific value or rate of rise. For example heat sensors would detect an alcohol fuel fire, which produces no smoke and would melt a photoelectric detector off the wall before it triggered an alarm.