A first approximation method for smoke detector placement based on design fire size, critical velocity, and detector aerosol entry lag time

An analog light-scattering-type smoke detector was tested in a wind tunnel at various low velocities. The air flow in the wind tunnel contained an aerosol concentration that resulted in a high ambient optical density, simulating smoke well above threshold optical detector density. The objective of this research was to determine the lag time to alarm, Δt, associated with difficulty of smoke entry into a detector. A “critical velocity” was identified for the smoke detector, below which the lag time increased exponentially with decreasing velocity. Increased lag time results in the detector responding unacceptably late—or not at all—even when ambient obscuration is well above limits defined in UL standard tests.