Which type of detector can be reset only after a clear smoke condition?

A beam detector operates by using a light beam, typically infrared, to detect smoke. When smoke enters the path of the beam, it absorbs or scatters the light, triggering the alarm. The critical aspect of beam detectors is that they can only be reset once the smoke condition is resolved, meaning the smoke must be cleared from the detection area. This feature is particularly important in environments where smoke may indicate serious fire conditions, as it ensures that alarm systems remain active during a dangerous situation until the area is safe.

The other types of detectors function differently. Spot detectors generally use a thermal sensor to detect changes in temperature, which can be reset based merely on temperature recovery rather than elimination of smoke. Fixed-temperature heat detectors respond when a preset temperature is reached but do not require a clear smoke condition for resetting. Gas detectors measure the presence of specific gases; the need for resetting is often based on gas concentration rather than smoke presence. This distinction highlights the unique operational logic of the beam detector in fire safety systems.